THE COST and EFFICIENCY of EARTHING on LOW- and MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS by L

Paper No. 930 621.316.99:621.315.1 SUPPLY SECTION

THE COST AND EFFICIENCY OF EARTHING ON LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS By L. GOSLAND, B.Sc, Member. {The paper was received 20th September, 1949. // was read before a joint meeting of the SUPPLY AND UTILIZATION SECTIONS 22nd February, the SOUTHERN CENTRE 29th March, and the NORTH-WESTERN SUPPLY GROUP 25th April, 1950.) Value SUMMARY adopted After describing the scope of protective earthing and the various = Length of typical line, miles 2* methods available, the paper sets out a method by which the amount (0-5 for single- of protection afforded by each method may be evaluated from a phase knowledge of the incidence of those types of fault which contribute protective to the risk of shock. The cost of applying each method is established, multiple and risks are calculated on the basis of available information of the earthing) incidence of faults. The systems are then compared on the basis of 1 000/n = Number of earth-leakage circuit-breakers 50* the amount of protection afforded for a given expenditure. Recom- developing faults per year per 1 000 mendations are made for the alleviation of the conditions under which earth-leakage circuit-breakers con- protective multiple earthing may be utilized in Great Britain. nected. 365/1 = Period for which earth-leakage circuit- 42* breaker fault persists. rSm, pm — Resistance of earth electrode per consumer with protective multiple earthing. r'sm, pm — Resistance, per consumer, of earth elec- (1) LIST OF SYMBOLS trodes at consumer's installation. Value r'sE ~ Average resistance, per consumer, of adopted earth electrodes at consumer's instal- 1 000a = Number of appliances developing earth 25* lation, with separate earth-wire. faults per year per 1 000 appliances. Rsw = Resistance, per consumer, representing 3656 = Periodf for which an appliance fault 0-5* load per consumer at any time, with persists before being corrected if no protective multiple earthing. protective gear operates. RB = Resistance representing balanced load 1 000c = Number of earth continuity conductors beyond break in neutral on polyphase (including earthing leads), per 1 000 20* system with protective multiple earth- consumers, that develop open-circuits 60§ ing. in the course of a year (see Section 6). Ru = Resistance representing unbalanced load 365rf= Period for which open-circuit of earth 365J on one phase. continuity conductor persists. ro = Resistance of consumer's earth electrode 1 000c5£ = Number of service earth-wires, per 1 000 1* (including earth continuity conductor). consumers, that develop open-circuits 0-I|[ FJJ = Proportion of all appliance faults that See Fig. 2 in the course of a year (see Section 6). are dangerous with an intact earthing 365dsE ~ Period for which open-circuit of service 365* system, for a given value of r/j. earth-wire persists. F'D = Proportion of all appliance faults that are See Fig. 2 1 OO0c'smtPm^ = Number of service neutrals, per 1 000 1* dangerous with broken service earth- consumers, that develop open-circuits 0-11| wire on separate earth-wire system, in the course of a year, with protective for given rsE- multiple earthing. Fe = Proportion of all appliance faults that Say 0-95 — Period for which open-circuit persists on 0-5* are dangerous with a faulty earth- service neutral. leakage circuit-breaker system. = Number of appliances connected per 50* FSE= Proportion of all appliances on a line See mile of line. that cause danger on the occurrence of Table 2 ; = Number of breakages of separate line 2-5** a fault on any one appliance in con- earth-wire, per 1 000 miles of line per junction with a broken line earth-wire. year. Fsm = Proportion of all consumers on a line in See 365gsE = Period for which open-circuit persists on 30* danger from broken line neutrals. Fig. 6 separate line earth-wire, in the absence Fpm = Average proportion of all consumers on See of shock or other effect. a line in danger, with protective mul- Fig. 6 Number of breakages of line neutral 2-5** tiple earthing, with neutral and one or alone, per 1 000 miles of line per year, two phase conductors broken, and with protective multiple earthing. fuses not blown on remainder. 1 OOOfpm — Number of breakages of line neutral and Iff T= Fraction of time average consumer takes See one or two phase conductors, with load. Fig. 5 fuses not blown on remaining phase 7" = Fraction of time during which danger See conductors, per 1 000 miles of line per exists on an installation on a system Fig. 7 year, with protective multiple earthing. with protective multiple earthing when *, pm — Period for which open-circuits on line service neutral is broken, having re- neutral persist before attention is called gard to load connected and resistance (e.g. by failure of supply). of consumer's earth electrode, if any. p = Soil resistivity, ohm-cm. Mr. Gosland is with the British Electrical and Allied Industries Research Association. //= Current which will blow a fuse in one * Table 4. t All times in days. minute. t Assumed values. § Deduced from Table S. Z = Fault impedance. II Anticipates future experience (see Section 7.1). II Subscripts sm, pm, refer to single- and polyphase systems with protective multiple rr = Resistance of earth electrode at trans- earthing. former neutral. •* Table 4 and Section 14.5. ft Table 5 and Section 14.3. • Table 4. [563] 564 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON /-) and r2 -- Resistances to earth at the two sides of a than 40 volts can be said to be absolute, and independent of the broken earthwire or neutral. resistance at the transformer neutral. E-~- Supply voltage. V= Voltage drop across an impedance. The protection of Post Office lines and crossings must be so arranged that, if a phase conductor approaches the protected (2) INTRODUCTION zone, it must first touch another conductor and cause a current The risk of shock from the accidental electrification of to operate the series protective elements (fuses or circuit-breakers) accessible conductors that are not normally alive can be avoided of the supply. The interests of the Post Office are safeguarded by a variety of methods, e.g. direct earthing, multiply-earthed by the conditions printed in P.O. publication No. A.80.B; the neutral, neutralization, or earth-leakage circuit-breakers. Fre- arrangement of the power conductors in such a way that the quently, when supply is by overhead line in a region of high neutral conductor, which is permanently earthed at the supply soil-resistivity, absolute safety appears to be economically transformer, lies directly between the telephone conductors and impracticable by any method, and choice of method must be the insulated power conductors, is considered a sufficient safe- governed by the relative costs of ensuring a given standard of guard. safety. With different methods, however, the costs are divided Where the guarding conductor is the earthed neutral, no differently between supply undertakings and consumer, and, in current flows in the earth electrode, and protection is again any event, evaluation of the protection obtained by a given independent of the resistance of the earth at the transformer. If expenditure is difficult. An approach to a quantitative com- the guarding conductor is connected to the neutral, the only parison of risks has been made by Taylor.1 The present paper other essential condition is that the neutral shall not normally describes available methods of protective earthing in a typical differ in potential from earth, in case it should fail and approach overhead-line distribution system, and evaluates the cost and the protected zone. Where the guarding conductor is separately degree of protection associated with each, in terms of factors earthed, current must flow in the earth to operate the series pro- that can be separately assessed. tective elements, and considerations as in consumers' installations Probably no precise selection will ever be possible; choice apply, but since series protective elements at the supply will have must, in the end, rest on judgment. It is hoped that the paper higher operating currents than those at consumers' installations, may help by enabling judgment to be applied to a number of earth electrodes of lower resistance are necessary, unless there independent factors, rather than to a result that depends on these is fitted at the supply an earth-leakage trip which can discriminate factors in a complicated manner. The paper does not directly against leakage currents up to the maximum that can flow in consider fire risks, which appear to demand further study of the any consumer's installation. earth continuity conductor. In substations, different considerations apply for large substations and pole-mounted transformers. In the former, pro- (3) SCOPE OF PROTECTIVE EARTHING AND METHODS tection to personnel is provided by the bonding of all metalwork AVAILABLE and is independent of the value of the substation earth resistance: Protection is required in substations, in domestic and industrial in the latter, however, conditions analogous to those in a con- installations,2 for Post Office telephone lines and installations, sumer's installation apply, and the risk of shock due to a fault for rural installations,3 and along roads and at road crossings. to earthed metalwork elsewhere on the system increases with The five general methods of protection are as follows. increase of the neutral earthing resistance. Rural installations are covered by No. 14 of the Overhead (3.1) Direct Earthing Line Regulations (E1.C.53). This may be taken to demand, on On this system, the general mass of the earth is considered as wooden poles, the use of the neutral (earthed) conductor as a a conductor of negligible resistance, which is connected to the guard against breakage of line conductors, and, on metal poles, system neutral, and an earth connection is made at each of the protection against leakage either by an earthed wire (not the types of installation mentioned (see Fig. la). neutral) running from pole to pole and connected to the poles, It is important to understand the implications of this system. or by a form of double insulation. In domestic and industrial installations, each will have its own over-current protection, and protection against faults to earth (3.2) Direct Earthing to Separate Earth-Wire, Singly Earthed depends on whether the sum of the earth electrode resistances at Where soil resistivity is high, a method analogous to that the transformer and at the consumer's installation is low enough described in Section 3.1 can be applied, with a separate earth- to permit the fault current, if a phase conductor comes into wire run throughout the system and earthed at a single point contact with an earthed enclosure, to blow the fuses on the con- [see Fig. l(b)(i)]. With a continuous earth-wire, the only risks sumer's premises. This is recognized by Regulation 1005(a) of are due to a low-impedance fault between a phase conductor The Institution's Regulations for the Electrical Equipment of and earth, which may cause a high voltage-drop along the earth Buildings, although in this there is no indication of the time in wire, or to the touching of a phase conductor and an earthed which the fuse shall operate. Since there is no fixed relation conductor not connected to the earth wire. In the former case, between the supply impedance, the resistance of the earth at the the over-current protection will keep the duration of the risk transformer neutral, and the resistance of that at the consumer's very small; in the latter, the risk is negligible if the resistance to installation, the voltage between the consumer's protected metal- earth at the single earth point is kept lower than the resistance work and the general mass of the earth is indeterminate during the of any accidental earth. Further risk arises if the earth wire is blowing of the fuse, and protection against excess voltage depends interrupted: if this happens, any fault to protected metalwork on the short duration of voltage. For any given value of the beyond the break constitutes a hazard at all appliances on the consumer's earth resistance, and any specific fault, increase of same side of the break. The value of the earth resistance is only the earth resistance at the transformer can only decrease the important in relation to faults to metalwork not connected to voltage on the consumer's protected metalwork, though it can the earth wire. increase the time for which the fault persists, and if the product of the consumer's earth resistance and the 1-min fusing current of (3.3) Direct Earthing to Separate Earth-Wire, Multiply Earthed his largest fuse is less than, say, 40 volts, the protection against The risks present with the method described in Section 3.2 are the risk that earthed metalwork may rise to a potential of more reduced if the earth wire is multiply earthed [see Fig. 1(6)00]. LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS 565

Transformer Line Consumer Line (3.5) Multiple-Earth Neutral .zz s (a) Direct earthing Ph The risk noted under Section 3.4 can be reduced if the same principle is used, but with the neutral conductor earthed at each end of the line, or at intervals along the line, or both. With the neutral intact, the voltage on any protected equipment is (b)(i) Separate earth wire, singly earthed Ph limited to a fraction of the voltage drop along the neutral, and this becomes large only for faults of such low impedance that over-current protection removes the danger after a short time. E Risk arises, however, when the neutral conductor is interrupted. Q-S- (b)(i 1) Separate earth wire, multiply earthed Ph to oi Multiple earthing may have a variety of meanings. What is meant here by multiply-earthed neutral is the connection to earth of an l.v. neutral conductor at a number of points, with (c)(i) Neutralization Ph the connection of the metalwork of appliances to the neutral. To call it "multiple earthing" or "multiple-earth neutral" is inadequate and confusing, but at the same time an accurate (c)(ii) Nulling" description such as "multiple earthing of the neutral with con- Ph nection of frameworks to the neutral" is too long. The conditions for use of a multiply-earthed neutral are in some cases governed by regulation, e.g. in the Standards Asso- (c)(Hi) M.E.N.(Victoria, Australia) ciation of Australia Wiring Rules (No. CC1, 1946). In Great Britain, use of systems of this type is dependent on the consent of the Postmaster-General.

(O(iv) P.M£. (Present British) Ph (3.5.1) "Nullung." Where a system of the above type is used with the provision of earth electrodes on the line neutral, but not necessarily at ( and rj) must permit fuse to blow on line/frame fault. each consumer, and (d) at such additional points, as equally (c)(i) System is so designed that neutral cannot break without main fuse blowing. spaced as possible, as are necessary to bring the total resistance (c)(ii) Swiss regulations provide that the resistance of any earth connection shall not exceed 20 ohms. Neutral to be earthed at transformer and every between the neutral conductor and the general mass of the earth 500 m along line. No overall resistance specified. German regulations are to not more than 2 ohms. The earth electrodes (a) must have more onerous. a resistance not greater than 4 ohms, individual electrodes under (c)(iii) K,n is specially provided, it must be at least 4 ft of 3-in (internal diameter) pipe. >'»m provided if necessary to ensure even distribution of connections and to (b) and (d) must not exceed 10 ohms each, and the form of reduce overall resistance to 10 ohms. electrodes (c) is specified (see Electricity Commissioners' multiple- Overall resistance to earth of neutral must not exceed 10 ohms, (c)(iv) r = 4 ohms. earthing approval under the Electricity Supply Regulations, nn = 10 ohms each, one at end of line and others as equally spaced as 1937). The earth at the supply may be omitted when a trans- possible, as necessary to reduce overall resistance to 2 ohms. former feeds two or more lines in parallel. m - 6-ft rod electrode or equivalent. ()() = 200-500 ohms. In the remainder of the paper, "protective multiple earthing" W)(ii) i> to water system if it exists. '',• - specially provided electrode. is used to cover all systems of the type in which the metalwork re-r't not less than 100 ohms or 6 ft separation. In all systems rj> must permit h.v. fuses to blow in case of fault between of an appliance is connected to the neutral, which in turn is con- windings of transformer. nected to earth at intervals. Distinction is made, when necessary, (3.4) Neutralization between the use of electrodes on the line and at consumers' installations. A system has been described4 in which the neutral point of the supply is earthed to a low-resistance earth, and the metalwork of each installation to be protected is connected to the (3.6) Voltage-Operated Earth-Leakage Circuit-Breakers neutral [see Fig. l(c)(i)]. This introduces the risk that an open- All methods previously discussed have relied for protection circuit on the neutral conductor exposes installations beyond it against danger from earth leakage on low-resistance return paths to a voltage approaching phase voltage, even when no fault for leakage currents, with disconnection by over-current pro- exists on any consumer's installation. This might be avoided tection when the leakage current is very heavy, but, where series by the provision of guards, etc., so that, if only a neutral broke, circuit-breakers are possible, direct protection against excess it would be certain to make contact with a live wire, and so blow voltage on exposed non-current-carrying metalwork can be pro- the fuse and disconnect the faulty system. This method of provided. With earth-leakage circuit-breakers,5 all exposed metal- tection is not, however, the same as that known on the Con- work in an installation is connected to a common earth-continuity tinent as "nullung" (see Section 3.5.1) and it has not come into conductor, which is insulated from earth and connected to an general use. earth electrode through the trip coil of a circuit-breaker in the 566 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON service connection, which operates to interrupt the supply if the as being of the order of 2 000 ohms (the minimum measured), a voltage on the protected metalwork exceeds a predetermined voltage of the order of 10-15 volts might be dangerous, but value. This method of use, the most common in Great Britain, danger is very unlikely below 20-40 volts. From the evidence is illustrated in Fig. l(d)(i). The method used in Australia is of shock therapy, very high transient voltages can be supported, illustrated in Fig. l(d)(ii), and other methods are possible. On and even if convulsions follow recovery can be complete. Thus, an overhead line system it appears to be necessary to protect the short duration gives some margin on the values mentioned.! protective equipment against the effects of lightning. The figure of 40 volts has some status as a "danger voltage" since it has been accepted as an upper limit in The Institution's (3.7) Current-Operated Earth-Leakage Circuit-Breakers Regulations for the Electrical Equipment of Buildings (Reg. 1006): Protection can also be applied by the use of a circuit-breaker accordingly, while it is recognized that current is the major factor operating on current balance, with an earth electrode (which associated with shock risk, it is assumed in the present paper may be of high resistance if the current balance is sensitive) that those conditions are dangerous in which a body of resistance connected to non-current-carrying metalwork. This system has 2 000 ohms is subjected to a potential of 40 volts (i.e. a current of 20 mA). Thus, for 250-volt operation, all appliances with an not been applied to domestic installations to any great extent, 4 and is not further discussed in the paper. impedance to earth of less than 10 ohms can be considered faulty. A limiting impedance of 104 ohms for faulty appliances is concordant with the fact that voltage-operated earth-leakage (4) EVALUATION OF HAZARD circuit-breakers designed in accordance with B.S. 842 must (4.1) General Considerations not trip at 15 mA. This implies that an appliance with an It is desirable to establish some quantitative basis of estimating impedance to earth of 16 000 ohms is not faulty. the degree of safety secured by a given expenditure on any of In fact, 2 000 ohms is a low estimate of a typical "body the systems of earthing described. The most important condi- resistance," since such a value is obtained only under conditions tions giving rise to risk with the different systems are set out in which, while the best electrically, are the most conducive to Table 1. It will be seen that in most cases risk is dependent shock. A resistance of 5 000-20 000 ohms is probably a better

Table 1

CONDITIONS GIVING THE POSSIBILITY OF DANGER WITH DIFFERENT SYSTEMS OF PROTECTIVE EARTHING

Type of fault

Earthing system Faulty Broken Broken Resistance of Broken line Broken line Broken earth-leakage earth wire service earth- normal earth earth-wire neutral service circuit-breaker wire electrode neutral system

Direct earthing A > Neutralization A B R'

Protective multiple earthing (single-phase) A II 1 B B 1

Protective multiple earthing (polyphase) A * * B B' Separate earth-wire A A * > Earth-leakage circuit-breaker A 1 1 1 1 1 1 A

A: In coincidence with faulty appliance in installation. B: In coincidence with load (single-phase) or unbalanced load (polyphase) beyond the point of break. B': Dependent on consumer's own load only. A': In coincidence with faulty appliance on any installation beyond break. • It is assumed in all cases, except that of direct earthing, that the effective resistance of the normal earth-electrode system is so low that no danger arises when an appliance fault is unaccompanied by a fault on the protective system.

on the incidence of faulty appliances, in addition to other factors, estimate, especially if hand-to-foot conditions are considered, and it is necessary to discuss terms in which this may be evaluated. and these high resistances, combined with a danger voltage of 40 volts, would mean a dangerous current of the order of 10 mA. (4.2) Faulty Appliances A figure of the order of 2 000 ohms, however, is necessary if acceptance of a dangerous voltage of 40 volts, and a non-faulty An appliance may be considered faulty, if, when it is energized appliance of 16 000 ohms impedance, are to be retained. A con- from a system with earthed neutral and the case of the appliance siderable increase in the assumed value of body resistance would, is earthed, current flows in the earthing conductor. For sim- however, have little effect on the figuresderived , arguments used, plicity the fault is treated as a simple impedance. A permissible or conclusions reached in the present paper. lower limit of impedance for appliances that are not to be considered faulty can be determined in relation to those conditions which are known to be dangerous to human life. Clearly there (4.3) Fault Impedance of Faulty Appliances can be no precise knowledge on this subject. Although what are classed as faults may have impedances 4 The threshold of sensation is 0-1-1 mA, and depends upon from 0 to 10 ohms, not all faults are dangerous in all circum- whether the contact is sustained or intermittent, and on the stances, and it is necessary to consider what is likely to be the capacitance of the current, as well as upon physiological condi- distribution of impedances among all faults. It is commonly tions.6. 7 Above 1 mA the sensation is unpleasant, and is likely believed that perhaps 90% of all faults are due to direct contact to be painful at 4-5 mA.6 From 10-20 mA the likelihood of t A further margin comes from the fact that the path of the current through the sustained injury is present. If the body resistance is accepted body is not necessarily the most unfavourable. LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS 567 of some portion of a winding with the frame of an appliance, extreme difficulty. It raises the rather difficult question of what and on this basis it can be assumed that the impedances of 90% eventually happens to persistent faults. of all faults are uniformly distributed between 0 and, say, Of the three possibilities considered, the least unsatisfactory 100 ohms, whilst the impedances of the remaining 10% are appears to be (b), although estimates of duration can hardly be uniformly distributed between 0 and 104 ohms. based on any firm evidence. To simplify discussion, it is assumed throughout the paper that each consumer has only one (4.4) Incidence of Faulty Appliances appliance; account is taken of the fact that each consumer may There appear to be three ways in which the incidence of have more than one, in the estimation of the incidence of faulty appliances might be stated: appliance faults. (a) At any time a certain proportion of all appliances are faulty. (4.5) Evaluation of Hazard with Direct Earthing (b) Appliances develop faults at the rate of so many per (4.5.1) Resistance of Earth Electrode. appliance per year, and the faults persist for times that have a Suppose that, on a given supply system using direct earthing, given distribution. a consumer's earth electrode resistance of rD ohms is mandatory, (c) Appliances develop faults at the rate of so many per and that rD exceeds the value below which absolute safety from appliance per year, and these faults persist until their presence risk of shock from an appliance fault is obtained. Let 1 000a is indicated by the operation of protective equipment or by be the number of appliances developing faults, per 1 000 ap- complaint of shock. pliances per year, and let 3656 days be the average duration of None of these statements is completely satisfactory. State- appliance faults from the time they develop until the time they ment (a) cannot, for obvious reasons, be applied to a system are rectified (it is assumed that most appliance faults are dis- using earth-leakage circuit-breakers. Statement (b) could be covered by inspection, and not by occurrence of shock). The satisfactorily applied to each type of protection. If the rate at chance that any appliance is faulty at any time is then ab: i.e. which faults developed and the distribution of durations were each appliance will experience ab fault-years per year. known, an equivalent statement of the form (a) could be derived, Let the appliances be protected by fuses which blow at If amp in although the converse is not the case. The major difficulty over one minute. The relation between the proportion of all faults a statement of this type lies in estimating in a satisfactory manner that are dangerous, the value of rD, and the value of If is dis- the distribution of duration of faults. Statement (c) can be cussed in Section 14.1, and is illustrated in Fig. 2. For any applied to each type of protection, although in some cases with given value of If and rD it can be seen from Fig. "2 that a pro-

1000

01 0-2 0-5 12 5 10 20 50 100 200 500 1000 2000 5000 10' Impedance of fault,Z,ohms Fig. 2.—Direct earthing: proportion of all faults that are dangerous for given values of If, rp and rj-.

(a) It is assumed that 10% of all faulty appliances lie within the range AB, and that (b) 90% of all faulty appliances lie within the range CD. (c) Example: Faults within the range EF are dangerous for rj> = 3, rj> = 0 at // = 60 amp. This is 12-6% of all faults on the above assumption. Faults to right of line ZR are not dangerous, because they do not develop 40 volts across rD. Faults to left of Zj, are not dangerous, because they permit a current to flow that blows the fuse within 1 min at // = 60 amp. (All curves Zjr are for lj = 60 amp except where shown otherwise.) Appliances having greater impedances to earth than 104 ohms are not faulty. 568 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON portion FD of all faults is dangerous, and each appliance will (4.6.2) Faulty Appliance with Broken Service Earth-Wire. experience abFD danger-years per year. For a given set of This is similar to the case discussed in Section 4.5.2, and is conditions at the installation, the value of FD will, of course, introduced merely to take account of the external extension of depend on the assumed distribution of impedances of faulty the earth continuity conductor to join the line earth-wire. If the appliances. incidence and duration of broken service earth-wires are c'SE, d'SE respectively, the risk is abc'SEd'sE danger-years per year. (4.5.2) Broken Earth-Continuity Conductor or Earthing Lead. We may assume that the only fault that can occur in an earth (4.6.3) Total Hazard. continuity conductor is a complete open-circuit: faults of finite The total hazard is thus resistance do not occur. Let 1 000c be the number of earth cd -f c'SEd'SE) continuity conductors developing open-circuits per 1 000 consumers per year, and 365c? the number of days for which, on the average, these faults persist. Then, at any time, cd of all (4.7) Evaluation of Hazard with Separate Earth-Wire, Multiply appliances have open-circuited earth continuity conductors. As Earthed above, each appliance experiences ab fault-years per year, and, This case differs from that of the separate earth-wire, singly on the average each appliance experiences abed fault-years per earthed, in one major respect, namely, that all faulty appliances year with an open-circuited earth continuity conductor. Under beyond a broken line earth-wire are not in the present case this condition, all faults are dangerous, and each appliance dangerous, whilst appliances on the transformer side of the experiences abed danger-years per year from this particular break may become dangerous. A general treatment is difficult, cause. This particular element of risk is common to all earthing but specific cases can be treated fairly easily, and yield instructive practices, and is added without comment in each case discussed information. below. Consider a line / miles long, as illustrated in Fig. 3. If / is

(4.5.3) Total Hazard. Phase The total hazard with direct earthing is thus ab(FD + cd) •F- Neutral danger-years per year. Earth wire

(4.6) Evaluation of Hazard with Separate Earth-Wire, Singly Earthed Fig. 3.—System of length / with earth resistance rj at transformer and three earth resistances /•/ uniformly spaced along a separate earth This is hardly a practical condition, since it is rarely possible wire. to avoid connecting frameworks of appliances to the general mass of the earth at some installations; nevertheless, a few high- not too large, rT = 4 ohms, r{ — 10 ohms, and if the earth resistance earths would not seriously alter the results. wire is of reasonable cross-section (say, 0 06-in2 copper), this Table 1 shows that three separate risks arise. There is danger complies nearly, but not completely, with the Electricity Commis- (a) of the coincidence of a faulty appliance on the line with a sioners' regulations for systems with protective multiple earthing. break in the line earth-wire nearer the supply, (b) of the coincidence of a faulty appliance and a broken earth-continuity con- (4.7.1) Faulty Appliances and Broken line Earth-Wire. ductor, and (c) of the coincidence of a faulty appliance and a This case can be evaluated in the manner described in broken service earth-wire. The risk of high voltage-drop along Section 14.2 in relation to Fig. 3, and is the intact earth wire is excluded as negligible, as are risks such as 2 that of a fault from a phase conductor to the general mass of FSE(abel fSEgSE) the earth otherwise than through metal connected to the common FSE is a factor which depends mainly on the overall resistance earth-wire. to earth of the continuous earth-wire, and on the number of points at which earth electrodes are connected to the earth wire. (4.6.1) Faulty Appliance and a Broken Earth-Wire on the Line. The factor FSE takes the values shown in Table 2 for the specific Let / miles be the length of the line. Any appliance fault cases described; these values may be taken as typical for the beyond a break is dangerous: no appliance fault before the break is dangerous. If Px is the probability that broken earth-wires Table 2 occur per unit length of line, then the probability that breaks EARTHING BY SEPARATE EARTH-WIRE occur within a length dx is Pxdx. If a break occurs between x and x + dx, there are / — x miles of line beyond x, and, if P2 AVERAGE PROPORTION FSE OF ALL APPLIANCES ON A LINE is the probability that faulty appliances occur per unit length of THAT CAUSES DANGER ON THE OCCURRENCE OF A FAULTY line, the probability that broken earth-wires occur with a faulty APPLIANCE ON A LINE WITH FAULTY EARTH-WIRE appliance beyond is PiP2(l — x)dx. With e appliances per mile of line, then each faulty appliance beyond a break at x causes Resistance, ohms danger on e(l — x) appliances: thus the probability that an appliance is dangerous because it is on the same side of a broken Overall Electrode Electrodes 2 to earth at transformer along earth wire earth-wire at x as a faulty appliance is P{P2e(l — x) dx. From this it follows that the average probability that an appliance is dangerous because it is on the same side of a broken earth-wire Any value None 0-333 2 2 4 3 x 10 0 063 as a faulty appliance is %PxP2el . If a and b are as defined in 4 8 3 x 20 0126 Section 4.5.1, 1 OQOfSE the number of broken line earth-wires for 8 16 3 x 40 0-2415 1 000 miles of line per year, 365gSE days the average duration 10 20 3 x 50 0-290 of broken earth-wires, then on average each appliance experiences 2 (abefSEgSEl ) danger-years per year. "3 x 10" indicates three electrodes along the line, each of resistance 10 ohms. LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS 569

overall resistances in question, on the assumption as to the 12-5 ohm at 250V distribution of fault impedances discussed above. §20

(4.7.2) Faulty Appliance and Broken Service Earth-Wire. ^ ohm at 250V If all the earth electrodes are on the line earth-wire, with none ,50ohmat250V at the consumer's installation, this risk is identical with that •125 ohm at 250 V discussed in Section 4.6.2 and is abc'SEd'SE. J i i If part or all of the earth electrodes are connected at the con- 01 0-2 0-3 04 0-5 sumer's installation, and the average earth-electrode resistance Duration (fraction of time) at the consumer's installation is r'SE, then the risk is abc'SEF'D, Fig. 5.—Single-phase protective multiple earthing: assumed average where Fp is derived from Fig. 2, using the appropriate value of load duration curve per consumer. Assume load — 60 W (1000 ohms) for remaining 0 • 4 of time. r'SE on the scale of rD. (4.7.3) Total Hazard. the average, 0 • 268 of all consumers are in danger if a line neutral breaks at random: for rsm — 25 ohms/consumer, 0 07 of all The total hazard is consumers, and so on, as illustrated by Fig. 6. This factor may 2 ab{cd + p + FSEel fSEgSE) be denoted by Fsfn. Fxm varies with the assumed load on the line. The maximum (4.8) Evaluation of Hazard with Single-Phase Supply with average load shown on Fig. 5 is very high, since, for intervals Protective Multiple Earthing totalling just over half an hour per day, the total load taken by (4.8.1) Broken Line Neutral. 100 consumers is 500 kW. This is excessive, but it was desired Consider the neutral to be earthed at many points, the average not to underestimate this particular risk. If now the line is assumed to be / miles long with e consumers per mile, the earth resistance per consumer being rsm ohms, with no single earth electrode very much below the average value. Suppose experience of broken line neutrals 1 000fsm per 1 000 miles per year, and the average persistence of broken neutrals 365gsm that each consumer takes a load represented by Rsw ohms at 240 volts. For broken line neutrals occurring at random, the days per year, then there are lfsmgsm broken-line-neutral-years average proportion of all consumers on the line affected by a per year per line, and each broken line neutral causes danger on rise in the neutral potential above 40 volts is as shown in Fig. 4. Fsm of le appliances. There are thus lfsmgsmFsJe danger-years per year on le appliances, and the danger-years per year per It is only when rsm/Rsw becomes less than 0-25 that there is any marked decrease in the number of consumers affected. appliance are Fsmlfsmgsm. Let us consider a line with 20 consumers, each taking 1 kW. In this case / is not greater than 0 • 5 mile, since single-phase Since 1 kW represents 60 ohms at 240 volts, then, if each earth distribution is used only for small systems. The number of electrode has a resistance of 30 ohms (i.e. the parallel resistance consumers on the line (i.e. the total load on the line) is taken of all earth electrodes is 1 • 5 ohms), on the average 0 • 475 of all into account in determining Fsm. consumers are affected, whilst, if each earth electrode has a resistance of 150 ohms or more, 0-64-0-7 of all consumers are (4.8.2) Broken Service Neutral. affected. In areas of high soil-resistivity, where protective Two separate cases must be considered here: multiple earthing would be attractive, it is quite uneconomical to (a) That in which all earth electrodes are connected to the achieve total resistances as low as 1-5 ohms: thus, for the case line neutral. cited, it is clear that, for any practical condition, on an average (b) That in which there is an earth electrode of resistance r'sm 0 • 5-0 • 7 of all consumers on a line will be exposed to danger at the consumer's installation. of a voltage rise of over 40 volts if a neutral breaks at random In case (a) the installation is dangerous whenever the con- on that line at any time except one of very light load. sumer attempts to take load, and the risk is almost independent If an average load-duration curve for each consumer can be of the magnitude of the load. If, according to Fig. 5, it is assumed, the curve of Fig. 4 can be used with the consumer's assumed that each consumer is taking load at all times, then load curve to estimate the exposure to danger in danger-days per the exposure to danger is the total exposure to broken service consumer per year. Suppose that the average load curve per neutrals, i.e. if 1 000c^w and 365^'w are the frequency of occur- consumer is as Fig. 5. Then, for rsm — 50 ohms/consumer, on

—• •a 0-6 , fe^O-4 § J Load, J 0-4 ,Z / TTTT I |0*2 0-2 fsl I Distributed multiple £8 earths, r /consumer o' ' 2 ' 4 ' 6 ' 8""' To x 102 sm Average earth electrode resistance/consumer single-phase protective multiple earthing, r ,ohm sm 2 Ratio n - rsm/Rsw 6 ' 6 ' 12 ' lo^ ' 24 ""ib x 10 Average earth «lectrode resistance/consumer Fig. 4.—Single-phase protective multiple earthing: average fraction of three-phas^ protective multiple earthing,rpm,ohm all consumers in danger, for broken neutrals occurring at random Fig. 6.—Protective multiple earthing: average fraction Fsm or Fpm of along the line, as a function of ratio of earth resistance per all consumers on a line in danger, if neutral breaks. consumer to load resistance per consumer. --effect of removing the load of 60 W (\ amp) for 0-4 of time from Fip. 5. 570 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON rence and persistence of broken service neutrals, then the hazard per consumer due to this cause is c'smd'sm danger-years per year, and may be expressed as Tc'srrtd'sm, where T is the proportion of time during which an average consumer takes any load. Like Fsm, T is affected by the assumed load-duration curve of the average consumer. In case (b) the exposure to danger is reduced by the amount of time during which the load resistance is so high that a dangerous voltage is not developed across the earth electrode Fig. 8.—Protective multiple earthing with three-phase supply: un- resistance. For a consumer with a given load-duration curve balance due to extra load on one phase only.

(Fig. 5), the effect of varying r'sm is shown in Fig. 7. Resistances Ep = Voltage to neutral. RB = Resistance/phase equivalent to balanced load beyond break in neutral. Ru = Parallel resistance on one phase equivalent to unbalanced load. /•j = Overall resistance to earth on transformer side of broken neutral. r2 = Overall resistance to earth on disconnected portion of neutral. x = Break in neutral. •".(Hi the remaining balanced load per phase. Thus, the danger with 50-4 - a broken neutral is confined to those times at which serious unbalance exists. If, however, there are times when the unbalanced load on one phase is a very large proportion of the total (i.e. RB > 3RU), then, to produce any appreciable reduction in danger, r (= r, + r2) < Ru. With a load of 10 kVA on one phase and neg- 0 5 10 15 20 25 30 ligible load on the other two, Ru = 6 ohms, and, for rt, r2 to Farth electrode resistance r^ or fpm at consumers reduce the voltage rise below 40 volts, they must individually be installation . ohms below 1 • 5 ohms. Fig. 7.—Effect of r'sm or r'pn} on average fraction T' of duration of If the risk that the neutral and one or more line conductors break in service neutral, in which consumer is exposed to danger may break is of the same order as the risk that the neutral alone of shock. may break, the former condition represents much the more Single- or three-phase protective multiply-earthed consumer, with load duration- curve of Fig. 5. serious danger, and the risk approaches that with single- phase protective multiple earthing. Considerations noted in of the order of 25 ohms or less per consumer are necessary if Section 14.4 suggest this to be the case. In consequence it is any substantial benefit is to be gained. The risk due to broken assumed that, on a three-phase line with protective multiple service neutral may thus be taken as T'c' d' danger-years per earthing, the major risk is that the neutral and one or two phase sm sm conductors become open-circuited, the supply to the other phases year, where T' is a factor depending on r'sm and on the consumer's load-duration curve, and likely to be between 0 • 1 and remaining intact. This occurs 1 000/£m times per 1 000 line- 1 unless r' is less than, say, 10 ohms. mile-years, and such faults persist 365gpm days. If consumers sm have load duration curves as in Fig. 5, then, when the supply to (4.8.3) Total Hazard. two phases is interrupted, the average load curve per consumer connected to the neutral is that given by dividing each ordinate The total hazard is thus of Fig. 5 by 3, since the consumers on the disconnected phases

abed + Tc'snJd'sm + Fsmlfsmgsm take no load. This load is all unbalanced. When the supply to one phase only is disconnected, the average load curve per T being replaced by T' if there are earth electrodes at each consumer connected beyond the broken neutral will be twice as consumer's installation. great, but the unbalanced load will be less than this. It is thus reasonable to suppose that the effective load per consumer is in each case that given by Fig. 5, with each ordinate divided by 3 (4.9) Evaluation of Hazard with Three-Phase Supply with (i.e. the resistances marked are to be multiplied by 3). The Protective Multiple Earthing average proportion Fpm of all consumers whose installations are (4.9.1) Broken Line Neutral. raised to a dangerous voltage by a broken neutral can be read This case differs from that of a two-wire, single-phase system from Fig. 6, each division on the scale of abscissae being taken in that the risk of a voltage rise across a broken line neutral to correspond to a value of rpm three times as great as that depends on the unbalanced and not on the total load current. marked for rsm. It does not appear practicable to assess the probable unbalanced current duration for various points along a line, but certain (4.9.2) Broken Service Neutral. conclusions may be drawn. This risk is the same as with single-phase protective multiple If it be assumed that the unbalance beyond any point on the earthing (see Section 4.8.2), and is T'c'pmd'pm or Tc'pmdpm according line is due to excess load on one phase only, the circuit deter- to whether there are or are not earth electrodes at each con- mining the voltage rises on the two portions of a broken neutral sumer's installation. conductor at that point is as shown in Fig. 8. From this, however high the values of r1 and r2 may be, the voltage across the broken neutral cannot exceed 40 volts on a 240/415-volt system (4.9.3) Total Hazard. The total hazard is thus unless RB > %RU, i.e. unless there is on one phase an unbalanced load of about half as much as the remaining balanced load per phase. If, for example, the neutral had equal resistances to FpJf'pmSpm + TCpmd'pn, + <^^ earth at the transformer and terminal ends, there could be no T being replaced by T if there are earth electrodes at each danger unless the unbalanced load on one phase were 1 • 3 times consumer's installation. LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS 571 (4.10) Evaluation of Hazard with Earth-Leakage Circuit- Costs of applying earth protection to 100 consumers on a line Breaker 2 miles long, by direct earthing, separate earth-wire, protective Here, hazard arises only from the possibility of coincidence multiple earthing and earth-leakage circuit-breaker, to typical of faulty appliance and faulty protective systems. standards of earth electrode resistance in soils of different resis- The chance that a fault exists on an appliance at any time is tivity, have been worked out, and are shown in Figs. 9-14. The ab as before. If 1 000m is the number of earth-leakage circuit- breaker systems that develop a fault each year per 1 000 systems connected, and 365« is the number of days such a fault persists before it is remedied, the chance that an earth-leakage circuit- breaker is faulty is mn. Since danger arises only from coincidence of faults, and each appliance is associated with one earth- earthed; 10 Q at transformer only leakage circuit-breaker system, the danger-years per year per 20 40 60 80 lOOxlO3 appliance are given by abmn. Soil resistivity, ohm-cm However, not all earth-leakage circuit-breaker faults are Fig. 10.—Cost of providing separate earth-wire, singly- and multiply- equally dangerous. With trip coil open-circuited, tripping earthed, on a line 2 miles long with 100 consumers. Costs included: mechanism inoperative and trip coil short-circuited, the metal- Separate line earth-wire at £71 -5 per mile. work is earthed through resistances that are respectively infinity, Service earth-wire at £2 • 625 per service. about 700 ohms and about 500 ohms. The proportion of Earth electrode at £6p/l 000 per mho. appliance faults that is dangerous under each condition can be deduced from Fig. 2, and in general, to take account of these it 800 *?* . is necessary to use a factor Fe which may vary between, say, 0 • 3 and 1 depending on the assumed incidence of different types 600 of earth-leakage circuit-breaker faults and the impedances of J m - . .. - - _ _— — faulty appliances. On the assumptions in the present paper, Fe - — — 8n is about 0-96. The danger-years per year per appliance are /inn 0 20 40 60 80 100 xlO3 then given by Feabmn. This is independent of cost through its Soil resistivity, ohm-cm effect on the resistance through which the metalwork is earthed, Fig. 11.—Cost of supplying separate earth-wire, multiply-earthed, at but depends on cost in so far as by increased expenditure m or n consumers' installations, on line 2 miles long with 100 consumers. can be reduced. The total hazard is thus ab(cd + Femn). Costs included: Line earth-wire at £71 -5 per mile. Service earth-wire at £2 • 625 per consumer. Earth electrodes at £7-2p/l 000 per mho + £0-5 per consumer. (5) COSTS OF DIFFERENT SYSTEMS OF EARTHING Horizontal line shows minimum expenditure if no consumer has less than one The costs of circuit-breakers, line materials, and of specially 4-ft rod electrode at £1 -5. installed earth electrodes can be fairly closely estimated, and the 300 major difficulty in assessing total costs depends on assessing the use to be made of existing earth electrodes. In what follows, c 200 costs will be discussed on the assumption that all earth electrodes rAf\ are specially provided, but it is to be remembered that, where

~—• consumers have any form of piped water supply, including 100 .—- individual small water supplies, pumps, etc., and metal service ,—-* — -•• pipes from asbestos-cement mains, it is in many cases safe to assume that 30% of all consumers are able to provide earth 7 0 20 40 60 50 100x10° electrode resistances of less than, say, 10 ohms without expense, Soil resistivity, ohm-cm i.e. an average of, say, 30 ohms per consumer, in soils of resis- Fig. 12.—Cost of providing protective multiple earthing (all earths on tivity up to, say, 20 000 ohm-cm. Although there are reasons line) on a line 2 miles long with 100 consumers. Costs included : why these cannot be used in all cases, they might be used for Distributed line earths at £6p/l 000 per mho. example in a system with a separate earth-wire, multiply earthed.

8000 400 / 0 > d 6000 7 300 \ / \ yA \ / •554000 200 A *^ o 10/ ^- v / ---- - """" 20-, ^— 2000 100 _ — A ,—— .— • . - "40 . —— — V -— — —• 16Q 11 / — — "50 3 0 10 30 50 70 90xl0 20 40 60 80 100 xlO3 Soil resistivity,ohm-cm Soil resistivity, ohm-cm Fig. 9.—Cost of providing direct earthing on line 2 miles long with Fig. 13.—Cost of providing protective multiple earthing (all earths at 100 consumers. consumers' installations) on a line 2 miles long with 100 con- The figures on the curves indicate earth electrode resistance per consumer. sumers. Costs included: Costs included: Consumers' electrodes at £7-2p/l 000 per mho 4- £0-5 per consumer. Earth electrodes at consumers' installations at £7-2p/1000 per mho +£0-5 per Transformer electrode, 5 £2, at £6p/l 000 per mho. consumer. To provide 0- 5 Q at transformer add £12p/l 000. Horizontal line shows minimum expenditure if no consumer has less than one 4-tt Hatching shows region in which reasonable safety is obtained. rod electrode. Table 3

INDIVIDUAL RISKS ASSOCIATED WITH DIFFERENT METHODS OF EARTH PROTECTION

External risk due to Total external risk Nature of earth protection Due to broken earth Broken service neutral Broken line earth wire Broken line neutral Faulty c.l.cb. system Broken service earth On past experience (1 000 c1 <= I) com inuity conductor, etc. 10 c — 20 c*= 60

R, 2 5 10 20 30 60 • 2-46 7-70 15-4 30-9 30-9 30-9

(a) Separate Singly , earth earthed •0-470 wire ; (*) Multiply «2 2 4 8 10 earthed 0089 0177 0-340 0-403 j (e) Multiply FsEabefst earthed at «2 2 4 8 10 1 0089 0-177 0-340 0-403 •Q-031 0-031 0032 0032

FtnMimgs.1 (o) 10 20 40 I 2 4 10 20 40 80 P.M.E. Earthed 1-54 1-84 1-94 214 2-37 2-49 2-52 Single- chase

(*) Earthed at R, 1-8 8-oo 10 20 40 consumers' r o-6 i o installations • 0-82 I 37

F^lfpmg^ = 2-73 X I0-y>» R3 0-25 0-5 1 2 5 10 20 50 P.M.E. •1-37 Fpm 0 004 016 0-303 0-36 0-53 0-61 0-66 1-phase • 0 0109 0-44 0-83 0-98 1-44 1-66 1-8

FpnlfimXem -• 2-73 X 10-6F^, (A) Earthed at j 0 25-2 l-rr. Rl 0-25 0-5 I 2 J 10 20 50 Ri 0-25 0-5 2 5 10 20 50 consumers' T 0-6 10 Fpm 0 004 016 0-305 0-36 0-53 0-61 0-66 • 0-82 0-9J :^\2 35 2-81 J-03 3-17 istallations 0-82 I 37 * 0 0109 0-44 0-83 0-93 1-44 1-66 1-8

• Risk due to particular cause or total risk, x 10—6. P.M.E. = Protective multiple earthing. R\ Resistance to earth per consumer, ohms. (Note that the lower values quoted are quite uneconomic in soils average resistivity.) R2 = Overall resistance to earth of earth wire, ohms. /?3 = Overall resistance to earth of neutral, ohms. Symbolic expressions of risk are derived as explained in the text. Numerical values are calculated being values given in Section 1. The values are to be read as "danger-years" per year per consumer. LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS 573

750 costs quoted in each case are those of applying the scheme in question to a new system in which provision is already made for 650 phase and neutral line-conductors and phase and neutral service- g'550 conductors, and to installations in which there already exist 3 450 earth continuity conductors with a common terminal at the incoming supply. The cost of appropriate fittings and labour 350 20 40 60 80 lOOxlO3 is included in each case. Soil resistivity, ohm-cm Costs of individual items, estimated at 1947-48 prices, are:

Fig. 14.—Cost of providing earth-leakage circuit-breaker protection Line earth-electrode (buried copper strip): £6p/l 000 per mho. on a line 2 miles long with 100 consumers. Consumer's earth-electrodes (driven rods): £7-2p/l 000 per mho + £0-5 per consumer. Costs included: Line earth-wire (0-05 in2 hard-drawn copper earth wire on a ne*v Transformer earth resistance, 10 £2, at £6p/l 000 per mho. overhead line system): £71 lOs./mile. Earth-leakage circuit-breakers at £2-5 per consumer, including cost of lightning Service earth-wire (additional cost for new consumer, external span protection and insulation of earth continuity conductor. 30 yd, 8-yd lead-in): £2 12s. 6d. per consumer. One 4-ft rod electrode per consumer up to 70 000 ohm-cm: two 4-ft rods at higher Earth-leakage circuit-breaker (30 amp, double pole): £2. resistivities.

Table 4

ESTIMATES OF INDIVIDUAL RISKS

Undertaking Data Figure u clop ted 1 2 3 4 5

(a) Number of appliances developing an earth 2-5 <1 > 25 per 10-802 1-25 25 (1 000a) i fault each year per 1 000 appliances consumer 9 (b) Average time for which faults under (a) 1/12 — — From Indeterminate 0-5(3656) may be expected to persist, if no pro- minutes tective gear operates to days (c) Number of earth-leakage circuit-breakers 50 — — — 112 50 (1 000m) developing a fault each year per 1 000 connected (d) Percentage of faults under (c) due to 67 — — — 75 9 70 mechanical defect (e) Percentage of faults due to open-circuited 26 — — — 25 9 20 trip coil (/) Percentage of faults due to short-circuited 7 — — — 09 10 trip coil (g) Average time for which faults of earth- 42 42 42 (365n) leakage circuit-breakers persist (days) (h) Number of times per year a break on a 30 5 8-2 0 10 3 1 n 10 13 single conductor occurs per 1 000 miles of l.v. overhead line (0 Average time a break under (h) may be expected to persist: (i) phase or neutral (i) 1 (i)i — (i) 1/6 (i) Few hours (i) ±Q65gsm or gim) (ii) earth wire (ii) 30 (ii) Indefinite (ii) - (ii) - (ii) 30(365

Notes (8) But possibly a preponderance in very low and very high impedance classes. (1) On basis of one appliance per consumer; see Section 14.5. (9) Broad interpretation of replies received. (2) Depending on type of appliance. No details; see Section 14.5. (10) 1 ohm for large transformers, 4 ohms for small, when consumers' fuses are small (3) In the particular experience cited all breaks were in phase conductor; none in enough. neutral conductors. (11) Data refer to neutrals only. (4) Type of service is composite twin cable. (12) This is the figure adopted for broken earth continuity conductor or earthing (5) Includes discontinuity due, for example, to overhead line taps. lead in installations, and there seems little reason to adopt a different figure (6) Includes discontinuities other than actual breaks. here. (7) Estimated. 13) See Section 14.4. 574 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON To the cost of the earth-leakage circuit-breaker must be added siderations discussed in Section 14.3 suggest, in existing installa- the cost in labour of insulating the earth continuity conductor, tions, a rate of, say, 60 per 1 000 installations per year. If this say 5s. Od. per unit, and of supplying lightning protection, say figure is adopted, this risk becomes in most cases a greatly pre- 5s. Od. per unit. There is the further cost of an earth electrode; dominating element in the total risk. Such a figurema y be very this may be taken as £1 10s. irrespective of the soil resistivity, pessimistic for new installations, since, for example, greatly for soils with resistivities up to 70 000 ohm-cm, and somewhat improved forms of earthing clamp have now been standardized. greater at higher soil-resistivities. Other elements of cost are In order that any uncertainty as to this risk (which may be involved here, namely inspection, testing and maintenance. The regarded as an "internal" risk common to all systems) may not relatively short persistence assumed for earth-leakage circuit- obscure the other factors (which may be regarded as making up breaker faults can be justified only if the circuit-breakers are the "external" risk), the "internal" risk has been evaluated tested, say, once a quarter, and the assumed incidence of faults separately from the others, and alternative values considered. implies that 5 % must be repaired each year. If each test takes The value c = 20 is suggested as a possible average figure for an extra five minutes of a meter reader's time per quarter, future installations. assessed at 4s. Od. per hour, including establishment charge, and Further, on past experience, breakages of single service con- repairs to earth-leakage circuit-breakers cost £1 per fault, this ductors has been estimated at 1 per 1 000 consumers per year, requires an annual outlay of £11 10s. per year per 100 consumers, but a large proportion of past cases have been due to the use which can be capitalized at, say, £150. This cost could probably of unsatisfactory forms of line tap, and it may fairly be estimated be reduced by increased expenditure on the new circuit-breaker, that, in future installations, the incidence of such faults may be leading to reduced maintenance and the possibility of less reduced to 0 • 1, a figurewhic h can be justified from the experience frequent inspection. cited in Table 5. It may be noted that abed is a risk common to all systems of (6) EVALUATION OF RISKS earthing, and that a reduction of any one of these quantities The estimates of risk discussed above have been summarized might contribute as much to safety as the use of a particular in Table 3, both in symbols and numerically. To enable method of earthing. It must be kept in mind that, while con- numerical values to be assigned to the incidence of individual siderable care has been exercised in selecting values for the failures, an inquiry was addressed to five undertakings (prior to various factors, the figures should be considered in relation to 1st April, 1948) and the replies received have been summarized individual circumstances, and be taken as a guide rather than as in Table 4, together with values, for each quantity, which express absolute values. the sense of the answers. (Some individual items of other data available are discussed in Sections 14.3 and 14.4.) From these, (7) COST VERSUS RISK numerical values have been entered, where appropriate, for each (7.1) "Purchase of Safety" of the individual risks covered by the list of symbols. Figs. 15-17 show, for each method of earthing, the risk asso- For the incidence of open-circuited earth continuity conductor, ciated with a given expenditure in a soil of given resistivity. there has been difficulty in arriving at a single figure. Con- Each of these includes a small allowance (c = 1) for the incidence

Table 5

ANALYSIS OF THE CAUSES OF BROKEN NEUTRALS ON DISTRIBUTORS AND SERVICES (OVERHEAD AND UNDERGROUND) FOR A PERIOD OF 3| YEARS (1940-1943) IN WESSEX AREA

Cause Conductor 2 Fuses Class size, in Breaks Total blown or S.W.G. Trans- Un- Trees Bombs Balloons Ice Aircraft Gale port Mech. known Misc.

Distributor neutral only 0-075 1 broken 0-06 3 2 1 — — — — — 3 _ — 01 2 }' Distributor neutral and one 01 4 or more phase conductors 0-075 1 broken 006 71 j>82 34 18 20 — 2 4 2 — 1 1 76 004/0-02 6 J Service neutral only broken 004 1 0-0225 6 No. 8 13 I22 8 1 2 2 — — 2 3 2 2 — No. 10 2 Service neutral and one or 0-06 1 more phase conductors 004 1 broken 0-0225 42 No. 4 1 [•64 18 2 10 3 3 1 4 6 17 — 55 No. 8 18 No. 10 1 Gross totals 174 62 22 32 5 5 5 8 12 20 3 131

Note:—Total length of distributors involved = 1 950 miles. Total number of services involved = 150 000. LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS 575 of open-circuited earth continuity conductors, whilst the break- illustrated in this way since, on the basis adopted, the risk is ages of single service conductors has been taken at the figure constant (0-22 x 10 -ty and the cost can be obtained from (1 per 1 000 consumers per year) based on past experience (see Fig. 14. Section 6). The case of the earth-leakage circuit-breaker is not Alternatively, Fig. 18 shows, for soils of low and high resis- 400 5320*10 4 -Risk due to earth continuity 16345x10 'I \ \ xlO at zero \ conductor (internal risk) cost y N S 240;, N (no electrode^ \ \ \ \ 160 X 1 VOJ 1 300 1 IU A'

2 0 6 8 10 14 16xlO3 Cost.i Fig. 15.—Cost of providing direct earthing on a line 2 miles long with 100 consumers. .•§200 c p in ohm-cm.

^Maximum risk. Earth u 5x10 wire earthed at transformer only 100

ul 10 100 1000 10000 100000 1000000 Expenditure on earthing system for 100 consumers on a line -8 •2 miles long.f Fig. 18.—Economics of earthing in soils of resistivities 5 000 and 100 000 ohm-cm. A'A = Protective multiple earthing: earths on line. B = Protective multiple earthing: earths at consumers' installations. rqken.risk. due to earth continuity ,conductor(c°}) C = Direct earthing. 0 400 600 p = 5 000 ohm-cm. Cost.f p =100 000 ohm-cm. ® = Separate earth wire and earth-leakage circuit-breaker systems in this Fig. 16.—Cost of earthing with separate earth-wire on a line 2 miles area (see Fig. 19). All data are based on expected experience on new lines, except A' which is based long with 100 consumers. on past experience, including faulty constructions. p in ohm-cm. EF signifies the maximum amount of safety purchasable. Electrodes on line only. Electrodes at consumers' installations. 350 AB signifies the minimum expenditure with electrodes at consumers' installations: Separate earth-wire earthed on line one 4-ft electrode per consumer (giving maximum risk for each value of p). \ CD signifies the minimum risk with electrodes on line. A EF signifies the minimum risk with electrodes at consumers' installations. Risk due to earth continuity 1 conductor(c=20)(internal risk) .30x10 \\ A ! 1 i i\ \ H I 325 26 Separate \ earth-wire lv earthed at >»» —^ ^— consumers 22 Lv 's s JS^QOOO i i IJ On 300 %^ 'Go* 0 — \S 12 5 10 20 50 100 D 200 500 1000 14 Expenditure on earthing system for 100 consumers on line 120 240 360 480 600 . 720 840 2 miles long.f Cost.i Fig. 19.—Economics of earthing at acceptable safety levels in soils of Fig. 17.—Cost and risk of earthing by forms of protective multiple resistivities 5 000 and 100 000 ohm-cm. earthing on a line 2 miles long with 100 consumers. P.M.E. = Protective multiple earthing. E.L.C.B. = Earth-leakage circuit-breaker. p in ohm-cm. p =5 000 ohm-cm. Electrodes on line only. p =100 000 ohm-cm. Electrodes at consumers' installations. AB signifies the maximum amount of safety purchasable. AB signifies the minimum expenditure with electrodes at consumers' installations: CD signifies the minimum cost of protective multiple earthing earthed at consumers one 4-ft rod electrode per consumer (giving maximum risk for each value of p). installations. 576 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON tivities, how much "safety" is purchased by expenditure on the is, however, advantage in connecting the earth wire to such various systems. The amount of "safety" is calculated in the electrodes as may be available without expense. following way. The risk due to faulty appliances on systems with no earthing at all is first calculated. This is the maximum (8.2) Earth-Leakage Circuit-Breaker amount of "safety" that can be purchased. The greater part The operating level of earth-leakage circuit-breakers is fixed, can be purchased by expenditure on the earthing system as so far as earth electrodes are concerned, for minor variations in such: the remainder can be purchased only by improvement of the value of resistance specified are unlikely to affect the cost. the reliability of the earth continuity conductor. Then, if the The question of the economies to be achieved by increased maximum risk with no earthing is x', and the total calculated expenditure on the construction of the breaker cannot be dis- risk is y\ the amount of "safety" that has been obtained is cussed here. (*' ~ /)• Fig- 19 shows, for soils of p — 5 000 ohm-cm and p = - 100 000 ohm-cm, these values of (x' — y') plotted against (8.3) Protective Multiple Earthing on Three-Phase Systems expenditure on various systems. This figure is based on the Fig. 18 shows that, with this system, the cost increases greatly expected future experience on the "external" risks, i.e. the for a fairly small increase in safety. If we refer to Fig. 17, we breakage of service neutrals, c'pm, etc., has been taken at 0-1 per see that there are two levels of risk at which increase in expendi- 1 000 consumers per year, and for the risk due to an open- ture fails to give much return in reduction of risk. The first of circuited earth continuity conductor the incidence has been these is at the level of 26-24 units; the second, with electrodes at taken at 20 per 1 000 consumers per year (i.e. the mean estimate). the consumer's installation, is below the level of 17 units. The For purposes of illustration, however, the case of protective latter figure corresponds to about 2 ohms overall on a line of multiple earthing with earths on the line has also been plotted 100 consumers: this is precisely that recommended by the E.R.A. 2 on the basis of past experience (i.e. c'pm = 1-0). The inset in 1940 as giving something very close to complete safety, and portion of Fig. 18 has been plotted to an enlarged scale in Fig. 19. subsequently specified in the Electricity Commissioners' Draft Regulations governing the use of protective multiple earthing. (7.2) Selection of a System This level has generally been found economically impracticable. The figures of risk and safety can be regarded only as relative. The former level (26-24 units in Fig. 17) corresponds to 6 to For example, no account has been taken of the fact that faulty 7-5 ohms overall resistance on a line of 100 consumers, and is appliances in earth-free situations do not give rise to risk. This quite concordant with the 10 ohms specified by Australian and type of factor will probably affect all systems to the same extent, New Zealand regulations, especially when account is taken of however, and thus does not affect relative values. There is, the fact that the paper considers a single line of unusually great however, no absolute standard of quality on the basis of which length, and what is admittedly an excessive estimate of the load a system of earthing can be selected. duration curve of the average consumer. Both these factors tend to give a pessimistic view of performance at any resistance It may be suggested that a system of earthing is acceptable if level. it gives a total risk that is of the order of twice the "internal" risk (which is not affected by expenditure on the earthing sys- The discussions in the paper show that any regulations as to tem). On this view, it will be seen from Figs. 18 and 19 that all the use of protective multiple earthing should specify an average the systems discussed can be considered acceptable, although resistance per consumer, and not a figure for a line irrespective direct earthing is economically impracticable over the range of of the number of consumers. An average resistance of 600 ohms resistivities considered. Separate earth-wire schemes and the per (domestic) consumer should give satisfactory results. This earth-leakage circuit-breaker give the same high order of safety, gives an amount of safety purchased (Fig. 19) of 328 units: at which is not greatly altered by expenditure on electrodes. Pro- this level it is not an advantage to use electrodes at each con- tective multiple earthing schemes can be made to give as good a sumer's installation, except in soils of the highest resistivity. performance as these at much less cost in low-resistivity soils, In the case of a failure between high-and low-voltage windings and at comparable cost in soils of higher resistivity. If a of the transformer, the high-voltage fuse must blow. This im- slightly higher risk is accepted, protective multiple earthing poses an upper limit of, say, 50 ohms on the overall resistance schemes show great economic advantage. to earth of the neutral of any low-voltage system, however small, supplied from an 11-kV system, and a lower value if supplied (8) CHOICE OF RESISTANCE LEVEL from a lower-voltage system. Direct earthing being disregarded under the conditions here (8.4) Protective Multiple Earthing on Single-Phase Systems considered, in none of the other systems discussed is the risk Although the case of protective multiple earthing for single- very greatly dependent on the variable portion of expenditure phase systems has been treated generally in Section 4.8, it has (i.e. expenditure on earth electrodes). Once it has been decided been excluded from the comparison of systems because single- whether or not one of these systems can be adopted, therefore, phase, two-wire systems are essentially limited. Consideration choice of resistance level can to some extent be guided by cost. of a special case, namely when small groups of consumers are fed by individual radial feeders from one single-phase trans- (8.1) Separate Earth-Wire former, has suggested that, for small single-phase multiply- When a separate earth-wire is used, there appears to be little earthed neutral systems, an overall resistance to earth of the order advantage in providing earth electrodes along the line, and none of 200 ohms per consumer should be acceptable, with the over- in providing electrodes at each consumer's premises rather than riding consideration that the overall resistance should not exceed along the line, since the values of resistance economically obtain- 50 ohms when the high-voltage supply is at 11 kV, or less for able are not sufficiently low to reduce very significantly the risk lower-voltage supplies. that a faulty appliance, in conjunction with a broken earth-wire, will cause danger. It is probable that investment on regular (say (9) GUARD WIRES yearly) testing of the earth continuity conductor, particularly in In present British practice, reliance is placed on the earthed soils of high resistivity, would yield a better return than invest- neutral as a guard wire. The protection that this wire affords ment on earth electrodes at the consumer's installation. There against passage of a phase conductor into the guarded zone is LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS 577 not dependent on the manner in which it is earthed at the wire and by earth-leakage circuit-breakers: in soils of the highest neutral. It is necessary, however, to consider what is the risk resistivity (50 000-100 000 ohm-cm) these are economically that this wire may itself break and cause danger in the guarded competitive with protective multiple earthing. zone. In present practice, with direct earthing, there is no risk (c) If multiple earthing of a separate earth-wire is adopted, if the guard wire breaks and the side nearer the transformer low overall resistances are necessary to obtain an appreciable enters the guarded zone, but there is risk if the side remote from increase in safety. Little advantage is to be gained by incurring the transformer enters the guarded zone, the risk being asso- expense in the distribution of earth electrodes at each con- ciated with the risk of unbalanced load beyond the break, and sumer's installation: regular inspection and testing of earth- being independent of the value of the neutral earthing resistance. continuity conductors would probably give a better return on Fuse blowing will not normally occur, since phase currents in expenditure. Advantage is gained, however, if connection is the fault condition envisaged will still be of the order of load made, without expense, to such earth electrodes as exist. currents. Thus, a neutral with the high-resistance earth accept- (d) Protective multiple earthing, i.e. multiply-earthed neutral able with earth-leakage circuit-breaker and separate earth-wire with connection of appliance frames to neutral, is only slightly systems of protection should still be acceptable as a guard wire. less safe than other more expensive methods. The degree of The adoption of a system with multiply-earthed neutral can "safety" obtained is not highly dependent on expenditure on operate only to reduce the risk noted above: thus the adoption earth electrodes. of any of the systems here discussed should not require any (e) Although the overall resistance to earth of a protective alteration of present practice with respect to the use of the multiple earthing system should logically be specified in terms of a neutral as a guard wire. resistance per consumer, for practical purposes an upper limit The possibility of using the multiply-earthed neutral as a pro- of 10 ohms, irrespective of the number of consumers connected, tection against leakage on overhead lines deserves consideration, is recommended. since the risk involved, considered as expectation of danger at (/) On three-phase systems using protective multiple earthing, any one site, does not appear to be great. there are two levels at which a small increase in expenditure on electrodes fails to give much return in increase of safety. One (10) MISCELLANEOUS level is at about 200 ohms per consumer, the other at about Two advantages of the earth-leakage circuit-breaker system 600 ohms per consumer. The former, for a line of 100 con- have not been brought out in the above treatment. All other sumers, corresponds to an overall resistance to earth of 2 ohms, systems operate by permitting the use of a proportion of faulty which has been found economically impracticable in most cases. appliances, without danger from shock, until the fault is brought The latter is concordant with practice in Australia and New to notice, either by its becoming a fault of low impedance and Zealand, where an overall resistance to earth of 10 ohms is causing shock or the operation of over-current protection, or by specified for lines irrespective of loading and length. heating, or by the failure or destruction of the appliance. The (g) Economic arguments in favour of accepting the higher earth-leakage circuit-breaker, however, is capable of detecting degree of risk noted in (/) are supported by the rather more faults as they arise, and thus can be supposed to discourage the practical one that there must now have been accumulated tens use of defective appliances. Also, the incidence of a broken of thousands of consumer-years' experience on directly earthed earth continuity conductor has been assigned the same magnitude systems operated at a resistance level that gives a far greater on all systems. Many faults in earth continuity conductors calculated risk, and while these systems have given cause for take the form of high resistances. A resistance of a few ohms concern, there have on the whole been remarkably few serious can affect the operation of those systems of protection that accidents attributable to this cause. depend on a low-resistance path for the return of fault current, (h) With three-phase protective multiple earthing, except when whilst some tens of ohms are necessary to affect seriously the very low overall resistances are used, it is more advantageous to operation of an earth-leakage circuit-breaker. The general use invest in earth electrodes on the line than at consumers' installa- of earth-leakage circuit-breakers would thus to some extent tions. This advantage decreases as the soil resistivity increases. relieve concern regarding the maintenance of continuity of the (/) General considerations suggest that with protective multiple earth continuity conductor. earthing, on a single-phase system, a resistance to earth of, say, On the other hand, the risk associated with direct earthing 200 ohms per consumer should give an acceptable level of and the separate earth-wire have been slightly exaggerated by safety. Advantage here may rest in the use of earth electrodes neglect of the fact that some appliances are protected by 5-amp at the consumer's installation. or 15-amp fuses. The effect is illustrated by Fig. 2, which shows 0") There is one overriding consideration, namely that the how (ZH ZL) changes when If is altered from 60 amp to neutral of any system must not have an overall resistance to 10 amp, over a range of values of rD and rT. It is difficult to earth of more than, say, 50 ohms on a system supplied from an take precise account of this circumstance, but the effect should 11-kV network. If supply is from a 6-6- or 3-3-kV network, a be borne in mind. lower resistance is necessary. This is necessary in order to ensure that the fuses on the high-voltage side blow in the event (11) CONCLUSIONS of a failure between windings of the transformers. The following conclusions may be drawn from the work (k) The use of direct earthing, i.e. independent of the neutral, described, but their general validity depends on the extent to on any installation connected to a system using protective which the numerous assumptions made are representative of multiple earthing must be prohibited. practical conditions. In general, conclusions as to the effect of (/) The specification set out in Section 14.6 appears to ensure expenditure on performance of individual methods of protection a reasonable standard of safety with protective multiple earthing. will be better based than comparisons of different methods. (m) Part at least of the good showing of the earth-leakage (a) In regions of soil resistivity above, say, 5 000 ohm-cm, circuit-breaker in the paper is due to the fact that such equip- where no networks of earthed conductors exist, direct earthing ment is customarily given a regular test, with maintenance when is more costly than any other system for a comparable standard necessary. The continuing nature of expenditure on this work of risk and may be literally impracticable. may account for the poor opinion of the device held in some (b) The highest standard of safety is given by a separate earth- quarters. VOL. 97, PART II. 38 578 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON («) Attention is drawn to the fact that, where a high standard E •of safety is desired, there are many cases, particularly in high- rT < - 0) resistivity areas, where capitalized expenditure on routine testing V and maintenance, or on improvement in erection practice, may Some faults are not dangerous because they are of such high give a better return in safety than capital expenditure on earth impedance that a dangerous voltage (e.g. 40 volts) cannot be electrodes. developed across the resistance of the earth electrode. Let V be the voltage developed across the resistance rD; then, with (12) ACKNOWLEDGMENTS notation as before, The paper represents an attempt to correlate the large amount of experience on earthing available in the supply industry, and V = E- the author gratefully acknowledges the advice and assistance of Z + rD + r-p the members of E.R.A. Panel V/Ab (Economics of Earthing) No danger exists if V < 40, or if which, under the chairmanship of Mr. E. Fawssett, furnished necessary information and kept the project continuously under review. He is also indebted to his colleagues Messrs. E. E. Hutchings and W. J. Darby, and Mrs. G. M. C. Morris, for much help, and to the Director of the British Electrical and These limits are indicated by lines ZL and ZH on Fig. 2, which Allied Industries Research Association for permission to publish is drawn for If= 60 amp: for any values of rD and rT those the paper. faults only are dangerous that have an impedance within the range of the intercept between ZL and ZH. If the length of this (13) REFERENCES intercept is Zx ohms, and faults of any impedance from 4 A (1) TAYLOR, H. G.: "The Use of Protective Multiple Earthing 0-10 ohms are equally likely, Zx/\0 of all faults are dangerous and Earth-Leakage Circuit Breakers in Rural Areas," under the given conditions. Journal I.E.E., 1937, 81, p. 761. If, however, as in the body of the paper, it is assumed that (2) TAYLOR, H. G.: "The Use of Protective Multiple Earthing 90% of all faulty appliances have fault impedances between 0 and 100 ohms, the remaining 10% having fault impedances and Earth-Leakage Circuit-Breakers in Rural Areas— 4 Second Report," ibid., 1941, 88, Part II, p. 415. between 0 and 10 ohms, if impedances within each group are (3) AUSTEN, A. E. W., and TAYLOR, H. G.: "Protection of uniformly distributed within the range, and if, as in most practical cases, the range does not include any impedance above Animals from Voltage Gradients round Earth Elec- 4 trodes," E.R.A. Report Ref. F/T104, 1936. 100 ohms, ZJ0 -9/100 + 01/10 ) of all faults are dangerous. (4) FAWSSETT, E., GRIMMI-IT, H. W., SHOTTER, G. F., and This expression may be suitably modified when Zx includes im- TAYLOR, H. G.: "Practical Aspects of Earthing," Journal pedances above 100 ohms. I.E.E., 1940, 87, p. 357. (5) "Voltage-Operated Earth-Leakage Circuit-Breakers for use (14.2) Risk due to Faulty Appliance and Broken Line Earth- on Consumers' Premises," B.S. 842 : 1939. Wire, with Separate Earth-Wire, Multiply Earthed (6) "Shocks from Unearthed Apparatus, with Particular (14.2.1) Broken Earth-Wire in Section of Line Remote from Trans- Reference to Radio Suppression Devices," E.R.A. former. Report Ref. M/T51, 1937. The case considered is that of a break in the earth wire between (7) GOSLAND, L.: "The Cost and Efficiency of Protective B and C (see Fig. 3). Earthing of Low and Medium-Voltage Systems by If / is the length of the line, the length of section BC is $1. Various Methods," E.R.A. Report Ref. V/T106. Consider a break at x between B and C. Then the probability that an appliance is on the right-hand side of a broken earth- (8) "A Large-Scale Sampling Survey of Domestic Consumers," 2 E.R.A. Report Ref. K/T125a, 1948. wire at the same time as a faulty appliance is PlP2e(l — x) dx, where Px, P2, are respectively the probabilities that an earth wire is broken and an appliance is faulty, and e is the number of (14) APPENDICES appliances, each per unit length. Integrating from $1 to /, and (14.1) Proportion of all Faults giving rise to Danger of Shock on dividing by /, we find that, over the whole line, the average a System using Direct Earthing probability that an appliance is on the same side of a broken l 2 An appliance is defined as faulty when it has an impedance earth-wire between $1 and / as a faulty appliance is g 1-P1P2e/ . to earth of 0-104 ohms, and faults of any impedance may Similarly, the average probability over the whole line that an occur. With direct earthing, the proportion of all such faults appliance is on the transformer side of a break in the earth wire giving rise to danger of shock depends on the resistance of the between B and C, with a faulty appliance beyond the break, is earth electrode, and may be determined by considering the limits within which faults cause danger. Faults may be con- The kind of risk associated with being on the same side of a sidered not dangerous if they are of such low impedance that broken earth-wire as a faulty appliance is that a proportion a the fuses in the supply to energized equipment will blow in a of all appliance faults causes danger. The kind of risk asso- very short time (say one minute). ciated with being on the transformer side of a break in the earth Let E be the supply voltage, Z the fault impedance (assumed wire, when there is a faulty appliance beyond the break, is that purely resistive), rD the resistance of the earth electrode and a proportion )8 of all faulty appliances causes danger. earth continuity conductor, rT the resistance of the earth elec- Thus, broken earth-wires occurring at random within the trode at the supply transformer neutral, If the current that section BC cause an average probability of danger of blows the fuse within one minute, and / the fault current to earth. Then ^rP1P2e/2(a + 70) per appliance over all appliances on the line. Z The factors a and ]8 may be evaluated from the type of No real danger exists if / > If, i.e. if consideration used in the body of the paper in relation to direct LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS 579 earthing, and if r{, r2 are the resistances from the sections of broken earth-wire and appliance faults respectively, we see that earth-wire to the general mass of the earth, after and before the the average risk over all the appliances on the line due to breaks break respectively, the range of fault impedances which is in the earth wire anywhere along the line is dangerous to \el consumers is ThpbePfs&s&x + 14jB + 14y + 138 + 38e + 7ij)

2 This may be written abel fSEgSEFSE, and the factor FSE may be considered as the average proportion of all appliances that terms in parentheses that are negative being treated as zero. cause danger on the coincidence of a fault on any one appliance a can be evaluated from this expression, account being taken with a broken line earth-wire. If the resistances to earth along of the distribution of fault impedances. j8 is evaluated in the the line are infinite, a, y, e become unity, j3, 8,77 become zero, and F becomes $, as in the case of the separate earth-wire, same way from the same expression, with r2 and r1 interchanged. SE singly earthed. (14.2.2) Broken Earth-Wire in other Sections of Line. Similarly, it can be shown that the average probability, over (14.3) Incidence of Defective Earthing Leads and Earth all appliances on the line, that an appliance may cause danger Continuity Conductors owing to the occurrence of a broken earth-wire in the centre Data on this subject may be derived from Table 6, which section is gives information concerning the origin of complaints of shock ThPiP*P(UY + 138) received in a single year by an undertaking giving supply in two and in the section near the transformer separate areas, an urban and a rural area. Direct earthing is used in each, but, whilst in the urban area there are low- resistance earths to water mains, etc., in the rural area earths where y, e, and 8, rj are factors similar to a, jS respectively. are of relatively high resistance. In the urban area there were 7-1 complaints per 1 000 consumers per year, whilst in the rural (14.2.3) Total Hazard. area there were 28 • 5 complaints per 1 000 consumers per year. Expressing PXP2 in terms of the incidence and duration of A complaint can arise only from the combination of a faulty

Table 6

RECORD OF SHOCKS RECEIVED BY CONSUMERS, JANUARY-DECEMBER 1947

Rural area Urban area Type of Index No. Defective equipment installation Percentage Section Percentage Section No. of faults of faults percentage No. of faults of faults percentage in each section of total faults in each section of total faults

1 Fixed wiring Power 5 5 1 2 Lighting 70 70 31 63 Heating 14 14 10 21 Cooking 11 11 7 14 Total .. 100 8 49 8 2 Accessories Power 5 1 3 1 Lighting 8 1-5 33 12 Heating 41 8 38 14 Cooking 468 89-5 200 73 Total .. 522 42 274 46 3 Earth continuity conductor .. Power 2 1 Lighting 3 12 7 3-5 Heating 2 8 12 7-5 Cooking 21 80 161 88 Total .. 26 2 182 30 4 Earthing lead 12 88 Total .. 12 1 88 15 5 Earth electrode 591 4 Total .. 591 47 4 1 Final total faults* 1251 597

* Defective appliance and faulty earthing system counted separately. Rural area Urban area No. of consumers .. ..21957 44 785 No. of complaints received .. 625 318 580 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON appliance and a defective earthing system. In the urban area 6 cases in which the distributor neutral and one or more phase 85% of all complaints were due to a faulty earth continuity conductors were broken, and the supply fuses were not blown: conductor, whilst in the rural area only 6% of all complaints thus it appears reasonable to take 1 000/£m as 1. The risk in were due to this cause. It may be supposed that the occurrence this case is treated as discussed in Section 4.9. of faulty appliances imposes a random sampling test on the earthing arrangement of consumers. Clearly, in the rural area (14.5) Incidence of Appliance Faults more than 3 % of consumers were tested in this way (allowing for An estimate of the incidence of appliance faults is necessary the fact that some faulty appliances would not cause any com- in order to obtain comparison between the risks with protective plaint of shock), and of the sample tested rather less than 6 % multiple earthing and those with other types of earthing. had faulty earth-continuity conductors. It is thus reasonable Table 4, Row 1 shows a wide variation in estimates of this to suppose that, of the whole population of consumers in the incidence. The most complete data available are those shown rural area, something less than 6 % had defective earth-continuity in Table 6, from which it is seen that there are about 24 com- conductors. plaints of shock due to faulty accessories per 1 000 consumers If the incidence of faulty appliances is the same in the urban in the rural area in which direct earthing with fairly high earth- area as in the rural, 3 % of the urban consumers were tested in electrode resistance is used. Since, even with bad earthing con- this way. On a test of 3% of the whole population, 0-6% of ditions, not all faulty appliances will cause complaint of shock, the whole population were found to have open-circuited earthing the incidence of faulty appliances must be somewhat greater arrangements, i.e. 20 % of all installations may be expected to be than this. Incidence of complaints per consumer in the urban defective in this way. In fact, the urban system appliances are area is less, but this is probably because earthing conditions in much older than the rural, and, if the incidence of defective this area are good and a smaller proportion of all faulty appli- appliances in the urban area is taken to be two or three times as ances cause complaint. It is thus reasonable to assume an great as in the rural, the estimate of the proportion of urban incidence of faulty appliances of not less than 25 per 1 000 con- installations with defective earth-continuity conductors falls to sumers over the whole undertaking. 10% or 6%, figures reasonably in agreement with the estimate The average consumer will have more than one appliance, for the rural area. If each defect persists for a year the inci- probably three,8 and the incidence may be expressed as, say, dence per year may be taken as equal to the number existing. 8 per 1 000 appliances. This figure may be compared with the most detailed estimate available (summarized under Table 4, Row 1, Undertaking 4): (14.4) Assessment of the Risk that a Line Conductor may be Broken "Whilst all cookers, washing machines, wash boilers, water In evaluating total risks, in the body of the paper, it is neces- heaters and refrigerators on the system are earthed, the vast sary to obtain values for the factors 1 000fSE, 1 0Q0fsm, and majority of radiators, kettles, vacuum cleaners, and irons are 1 000fpm. In addition, it is necessary to evaluate the incidence probably not. If the latter group were also earthed, it is f'pm of occasions on which (see below) the line neutral and one possible that many more failures would become apparent than or two phase conductors are broken, the supply to the other do now under the unearthed condition. phase or phases remaining intact. From data in Table 4 on the "Wood constitutes the flooring material of by far the number of occasions per 1 000 line-mile-years on which a single greatest number of domestic consumers' premises on the line conductor on an overhead line system is broken, a reasonable system. value for the latter would appear to be 10. The matter is, how- "The following are assumed approximations only of the ever, not quite so simple as this, since danger can arise when number of faults per annum per 1 000 appliances that might more than one conductor is broken. be expected if all the above mentioned types of appliance were With a separate earth-wire, however, it can be maintained that earthed under the conditions specified: the only important cases are those in which the earth wire alone is broken, since in such cases a fairly long fault duration may Cookers 50 Radiators 10 Washing machines.. 20 Kettles 70 be expected, whilst, when a phase conductor is broken in addition, 20 the fault will be rapidly discovered by reason of failure of supply. Wash boilers Vacuum cleaners .. 50 Water heaters 10 Irons 80 If, then, there are ten cases of one of four conductors breaking, Refrigerators 10 there will be 2 • 5 cases in which the broken wire is one particular conductor, e.g. the earth wire, and the factor 1 OOO/^ has been In the paper the figure of 25 faulty appliances per 1 000 taken as 2-5. appliances per year has been adopted, and each consumer With single-phase protective multiple earthing, the danger credited with one appliance. This corresponds to an estimate arises only when the line neutral alone is broken, the phase con- of 8 faulty appliances per 1 000 appliances per year, each ductor, including the fuses at the supply, remaining intact. It consumer being credited with three appliances. appears reasonable to assume the same value for fsm as for fSE, although perhaps a higher figure may be appropriate, since (14.6) Recommendations the protective effect of the other conductors is absent. The following specification appears to ensure an economically With three-phase protective multiple earthing the situation is satisfactory standard of safety with protective multiple earthing, somewhat different. Danger arises when the neutral only is and it is recommended that the regulations governing this form broken, but this is small compared with that when the neutral of protection be revised accordingly. and one or two phase conductors are broken, the supply re- (a) (i) The neutral point of the transformer from which the specified maining intact to the remaining one or two phase conductors, system is supplied with energy (hereinafter referred to as since the system in this condition is inherently unbalanced. "the supply transformer") shall be connected to an earth Supply in these cases is, however, only to two-thirds or one-third electrode, and the said point shall be bonded to the metal of the consumers respectively. The data of Table 4 are not sheathing and metallic armouring (if any) of the electric lines concerned. helpful in this connection, but it was the experience of a par- (ii) Provided that a connection from the said neutral point to an ticular undertaking that, in 6 850 line-mile-years, there were earth electrode may be omitted where the specified system LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS 581 includes two or more distributing mains whose respective point of the supply transformer and any metalwork asso- neutral conductors are connected with earth in accordance ciated therewith which is electrically separate from any with the provisions of Para. (b). metalwork associated with the windings at the higher (b) The neutral conductor of the specified system shall be connected voltage. [Subject to (a)(ii) above.] to earth electrodes located (ii) The other earth electrode shall be connected to the metal- (i) At the end remote from the supply transformer of each dis- work (including the metal case and connected metalwork tributing main and each branch of a distributing main of the supply transformer) associated with the windings forming part of the specified system; at the higher voltage. (ii) At such additional points, as equally spaced as possible, as (iii) The earth electrode connected to the neutral point of the are necessary to ensure compliance with the provisions of supply transformer shall be situated outside the resistance Para. (switch. and has a cross-sectional area not less than that of any phase (h) As from the date when protective multiple earthing has been conductor. effected in relation to the specified system, a supply of energy (d) Every earth electrode to which connection is made in com- shall not be given to any new consumer from the specified pliance with these recommendations shall, if specially installed, system, or shall not be continued to be given to any existing consist of one or more rods or tubes driven into the general consumer from the specified system, unless the following mass of earth so as to be as nearly vertical as possible, or of requirements in relation to the consumer's installation are a metal strip or wire laid horizontally in the general mass of complied with: earth. All earth electrodes shall be so installed as to prevent (i) Metalwork enclosing, supporting or near to any part of the danger from voltage gradients at earth level, and connection consumer's installation and not designed to serve as a con- with them shall ductor shall, where necessary to prevent danger, be con- be made above the surface of the ground in nected by means of an earth continuity conductor with the a position where the said connection can be inspected when neutral conductor of the specified system at the supply necessary. terminals. (e) The connection of the neutral conductor to earth shall be as (ii) The electrical resistance of the earth continuity conductor, follows: between the point where it is connected to any metalwork (i) To an earth electrode connected to the neutral at or near as aforesaid and its connection with the said neutral con- the transformer, of such a resistance that the fuses pro- ductor at the supply terminals, shall preferably not exceed tecting the high-voltage side of the transformer will 0-5 ohm, but under no circumstances must it exceed 1 ohm. operate should breakdown occur between the windings. (iii) Any conductor on the consumer's side of the supply ter- [Subject to (a)(ii) above.] minals which is connected to the neutral conductor of the (ii) To such additional earth electrodes as will give an overall specified system shall not include any fuse, automatic resistance to the general body of earth of not more than circuit-breaker, removable link or single-pole switch in 10 ohms. any part thereof. (f) It is recommended that where, at any supply transformer, the (iv) It is recommended that, before the earth continuity conductor neutral point and any metalwork associated with the windings is connected with the neutral conductor of the specified at the higher voltage are both connected to earth electrodes, system at the supply terminals, the electrical resistance of separate earth electrodes shall be used and the following the earth continuity conductor shall be verified by the requirements shall be complied with: application of a testing current of not less than 15 amp (i) One of the earth electrodes shall be connected to the neutral flowing for a period of not less than 1 min.

DISCUSSION BEFORE THE SUPPLY AND UTILIZATION SECTIONS, 22ND FEBRUARY, 1950 Mr. E. Fawssett:* I have long been certain that it was essential and saw thousands of feet of Everite pipe of several sizes. Some to get down to the economics of protection, and I was pleased villages which now rely on the local water mains for earthing to find that, when the E.R.A. Sub-Committee was able to may perhaps experience the shock of their lives—in more senses provide some representative data, the author could set out this than one. complicated problem on a comparative basis. I am in favour of the authority providing the protection and I should like to stress the fact that a considerable change in accepting the responsibility, for no one is better equipped tech- any one of the assumptions made has very little effect on the nically to do what is right and to maintain it so. The legal relative worth of the method, and I hope that the discussion will people will object, but I am certain it is the engineering solution. not be confined to destructive criticism of these assumptions, Dr. H. G. Taylor: The author refers to the fact that some years but will add to our practical knowledge of conditions throughout ago I made an attempt to bring economics into this question of the country. For instance, earth-leakage circuit-breakers are earthing, but does not go on to say that the attempt was a failure. probably the best theoretical protection devised, but experience I suspect that this was because it was based on guesses and esti- in Britain has been so diverse, ranging from unqualified approval mates which probably emanated from my own head, whereas to violent condemnation, that one must look deeper than Mr. Gosland, in tackling the matter in a more complete manner statistics of failure to get a true picture. The present types than I was able to do, has based his results on information available are subject to climatic conditions, and those areas in obtained from industry. which they have proved satisfactory combine good atmosphere The value of the deductions he has reached depends on the with a programme of unit-changing at regular intervals. If validity of the large number of assumptions made. In one case, a really well-sealed breaker were available at little greater cost, the figures obtained vary over a range of 3 000 : 1, and the there would be much to be said for its extended use, although author must have had a very difficult task to select a figure which, a sound solid connection seems to be preferable to any delicate presumably, was regarded as some sort of an average in that mechanism. range. Many village networks are partly underground and partly We have always assumed that if the fuse blows it is sufficient overhead, and for these I favour a continuous earth-wire on the protection, but we allow one minute for this to happen, and in overhead line connected to the cable sheath and to the substation that period a fatality can occur. That is a point which must not neutral. I went into the works of a village plumber recently be overlooked. The author says that it is assumed that most appliance faults are discovered by inspection and not by the • Mr. Fawssett was to have opened the discussion; in his unavoidable absence his contribution was read by another member. occurrence of shock. People who work in the industry will 582 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON know the position better than I, but I doubt that; I think neutral point—otherwise the sum of the normal neutral and that shock is the most common method of revealing faults in fault currents would balance the phase current and the protection appliances. would be inoperative. The distribution of fault current, in the In Fig. 17, the dotted curves make sudden descents, and I am ground, necessary for correct operation, is shown in Fig. A. puzzled to know why a curve produced from a formula should behave in that peculiar manner. There is probably some per- fectly normal explanation. I have a similar criticism of Fig. 19, where most peculiar curves are given relating the expenditure on earthing to safety units purchased. I cannot conceive what formula would produce some of those peculiar kinks and twists in the curve. In the top right-hand corner of Fig. 19, the T author shows tiny short curves for earth-leakage circuit-breakers. Are they curves or points? If they are curves, why are they not extended to the origin? If they are points, why are they shown as short curves? Fig. A.—The application of current-balance earth-leakage protection It is now almost twenty years since I joined the staff of the to meet conditions arising from an apparatus fault beyond a E.R.A., and earthing research has been going on at a varying break in the neutral conductor. rate ever since that time. I approached the subject with an open mind, and eventually came to the conclusion that there was much In Section 3.7, the author dismisses the current-balance or to be said for protective multiple earthing. I am interested to differential earth-leakage trip in its application to consumer's learn that the author, who has come to the subject of earthing in apparatus rather cursorily on the ground that it is not in general fairly recent years, has reached substantially the same conclusion. use. This device affords protection under critical conditions in He says that protective multiple-earthing schemes can be made which neither fuses nor the voltage-operated earth-leakage trip to perform as well as normal earthing methods, at much less cost, would be effective, and it is, I suggest, worthy of attention, in low-resistivity soils. That is a very important conclusion. although at present the cost is, perhaps, high for domestic use. The subject of earthing has always been confused by the question The question of cost brings me to the basis of the paper. None of cost. Recommendations were originally suggested for pro- will dispute that failures may occur in any system, but this is tective multiple earthing which were too severe and costly, but different from striking a balance sheet with safety opposite now it seems that a satisfactory measure of safety can be obtained expediency, the latter doubtless showing some immediate credit at a reasonable cost. but with doubtful long-term consequences. I wonder whether adequate attention has been paid to those I would draw attention to the requirements of the law as examples of protective multiple earthing which have been exe- regards earthing; the two statutory codes of regulations have cuted in this country and have been working for many years. these definitions: I refer to the rural area near Nelson, in Lancashire, where pro- Earthed means connected to the general mass of earth in such a tective multiple earthing has been used for at least ten, and manner as will ensure at all times an immediate discharge of probably fifteen years, and also to a number of American electrical energy without danger. hospitals constructed in this country during the war for the Connected with earth means connected with the general mass of U.S.A. Forces, where, I understand, protective multiple earthing earth in such a manner as will ensure at all times an immediate was used. I think that it would be very useful if detailed accounts and safe discharge of energy. of these two cases were published. Here the question of time arises. There is no mention of Mr. F. H. Mann: In the case of a multiple-earthed neutral, one minute or one second; the significant word is "immediate," the greatest danger occurs if the neutral is severed, and this and, while we may admit, as scientific engineers, that the word involves the assumption that we are dealing with a spur-line is difficult to define, we must not lose sight of the intention of distributor, because, in such conditions, the risk of breakage is the regulations. most severe. If the system is interconnected, the chance of any These definitions hold no sanction for contracting-out on part of the neutral becoming disconnected is small and could only safety; their requirements are absolute and admit of no com- occur as a consequence of a double break. promise. Nor do the stringent terms of the relevant regulations If a break occurs in a single line, the neutral is deprived of a contain any hint that the pass might be sold to economic number of its earth connections, with a consequent rise in considerations. potential. Beyond the break, the conditions will be worse, since, Mr. A. H. F. Linton: The author has compared various systems in addition to the loss of earth contacts, the direct connection of earthing in terms of the period of danger from shock, but with the star point will have been severed. Thus, if a fault actual fatal accidents are our chief concern. An examination of involving a breakdown of apparatus whose casing is connected the fatal accidents involving consumers' installations which were to the neutral occurs beyond the break, the likelihood of the reported during the last 14 years shows that 33i% included the resistance of the remaining electrodes being low enough to allow earthing factor, which embraces such items as high-resistance the fuses to blow is remote, with the probability that mains earth electrodes, defective earth-continuity conductors and un- voltage will be maintained in the apparatus. earthed appliances in "earth-fraught" situations. By "earth Hence, the more remote from the substation the greater the fraught" I mean the exact opposite to "earth free." risk if a break occurs, and the position of the consumer at the The majority of these accidents were due to the use of an end of the line is not to be envied. unearthed appliance in an earth-fraught situation, such as an The question of protection raises some problems, and, so far iron, kettle or portable radiator connected by twin flex and as the substation is concerned, a broken neutral can only be plugged into a convenient lampholder. However unprofessional detected by a voltage-rise earth-leakage circuit-breaker if a this type of temporary connection may be, the human element pilot wire is connected to the remote end of the neutral. A must be recognized. It is, and always will be, the logical current-balance earth-leakage trip should, however, respond, practice of electrically ignorant domestic consumers so long as provided that there is an earth electrode connected directly to the two-pin lampholders, twin flex and portable appliances remain LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS: DISCUSSION 583 available. As this is the most prevalent type of fatal accident earth-leakage circuit-breakers shall not trip at 15 mA, such it should be the chief target of the designer of protective systems. leakages need not be considered dangerous. All leakages are None of the fuse-operated earthing systems considered by the potentially dangerous, but some lower limit had to be set with author can offer protection against this type of accident. I these devices to prevent unwanted operation due to normal should like, therefore, to refer to the author's conclusions, in leakages from cooker hot-plates. In practice, it is found that which he puts earth-leakage circuit-breakers and the separate- hot-plate leakages are normally higher than this, and the operating earth-wire system on a par. I suggest that, for domestic con- currents for leakage trips are now usually 60-75 niA in order to sumers in rural areas where earthing conditions are difficult, overcome the risk of unwanted operation. an earth-leakage circuit-breaker offers the highest standard of In my opinion, until such time as a closer examination and safety, or, at any rate, will do so as the mechanism is improved comparison of current-operated and voltage-operated forms —and there is plenty of scope for improvement—whereas a of earth-leakage protection, especially in connection with the fuse-operated earthing system does not offer much scope for time factor, has been made, any consideration of the author's improvement. recommendations in connection with protective multiple earth- An earth-leakage circuit-breaker not only provides the same ing would be premature. protection as other systems, but, in addition, offers a better chance Mr. A. E. Morgan: I had an experience many years ago at of survival to the user of the unearthed appliance than the fuse- a small village on a rocky subsoil in North Wales, where it was operated system. The chief types of accident which fuse-operated considered that earth-leakage circuit-breakers were the only systems prevent are those involving earthed appliances, but the solution to the problem of lightning protection. We installed records of the last 14 years show only a small proportion of fatal a number with a 30-volt earth-leakage trip and for the first accidents due to the prevention of fuse operation by high-resis- few months had trouble due to the lightness of the operating tance earth electrodes. Voltage gradients near high-resistance mechanism, arising from mechanical disturbance. It was an system earths during the same period were responsible for only area very subject to lightning disturbance and the earth-leakage three fatal accidents, and in each case there were three contribu- coil burnt out on two or three subsequent occasions. Various tory factors; a high-resistance system earth and an appliance methods were adopted, but gradually, and for that reason alone, fault combined to set up a voltage gradient, the third factors the earth-leakage trip was dropped as a possible solution to this being, respectively, a stay wire, a steel fence, and a bare earth- trouble. lead down the pole. Mr. P. B. Frost: I find it very difficult to believe that many This shows that the danger to human life from voltage gradients people have studied this extremely complicated paper, with all in the vicinity of a system earth is negligible, provided that its curves and diagrams, really thoroughly. The author doubt- there is no third factor to increase the potential difference, such less realizes that the very large number of assumptions on which as a stay wire or a steel fence. These can be avoided by careful his estimates are based must cause a certain amount of uneasiness siting and by insulating the earth lead. This is another good about the value of the conclusions reached. We must guard reason for installing the earth electrode at a span or two away against placing too much reliance upon average values, for they from the transformer pole, where there are usually stays. are often very far from extreme values, and we ought to know Mr. T. C. Gilbert: As protective multiple earthing will be used the extent of the risk to a very small rural community on the mainly in the rural areas, where the fire menace is more serious assumption that all conditions are unfavourable. than in towns, I feel that this aspect of earth-leakage protection In Section 14.6, dealing with the conditions on which pro- should have been brought into the paper, so that all relevant tective multiple earthing might be approved, it is stated that there factors could receive consideration. It was suggested in E.R.A. must be an earth electrode at the remote end of each distributor, Report F/T122 that lire risks with protective multiple earthing and in addition that the overall resistance to earth must not might be greater than with ordinary earthing, but this important exceed 10 ohms. It should be emphasized that if a 10-ohm point is ignored in the paper. resistance to earth were obtainable by the sole use of a good Another omission is consideration of voltage-operated methods earth electrode at the remote end of each of two distributors, the of neutral voltage control at the substation, although voltage- distributed earths could not be dispensed with because pro- operated earth-leakage circuit-breakers have been dealt with in tective multiple earthing depends on there being a relatively low connection with the consumer's installation. The method has resistance to earth on either side of any break in the neutral. been mentioned in E.R.A. Report F/T41; closely linked to the Of the conclusions, (e) was added later, following a general adequate control of neutral potential is the time delay associated wish for a safer arrangement by the committee dealing with it. with fuse protection, persistence of leakage being a most serious It called for a resistance of not more than 10 ohms to earth on matter. the multiple-earthed neutral, however few the number of con- The paper assesses the annual charge covering testing and sumers. Conclusion (j) calls for a minimum resistance of 50 maintenance of earth-leakage circuit-breakers, but I cannot find ohms, so as to allow for breakdown from the 11-kV side of the any similar allocation for the alternative systems. Testing and transformer. Does not (e) make (;) unnecessary? checking of the electrodes will be a much more serious item and One point which has been mentioned, but not emphasized, is will affect the comparative costs shown. I do not consider that that in protective multiple earthing we are throwing the whole the possibility of lightning damage is serious with devices which responsibility for earthing, and the cost of it, upon the supply conform to B.S. 842, although there was some difficulty with authority, whereas in the past it has been necessary for the early types. individual consumer to provide his own earthing arrangements. In Section 4.8.2, it is stated that in case (a)—the neutral That will not be very welcome to the industry as a whole, because unearthed on the consumer's premises—"the installation is the paper has shown that safety is a very expensive matter. dangerous whenever the consumer attempts to take load." In The cost of earth-leakage circuit-breakers or direct earthing fact, it is dangerous all the time, as the phase is connected to would, of course, fall on the consumer himself. the neutral through the meter shunt coil. The factor T in the The suggestion that it would be wise to connect any small calculation would therefore appear to have no value, and the private water systems, such as a short pipe coming down from result is thus misleading. the hillside to a tank or a water pump, to the multiple-earthed I do not agree that, as B.S. 842 requires that voltage-operated neutral, would, in my view, be dangerous, because, whereas 584 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON human beings require a fairly severe shock, under unfavourable conditions, to suffer serious results, animals are a great deal more Scheme 1 Scheme 2 sensitive. Such small water systems are almost certain to feed into water troughs, and there is a risk of horses and cattle being killed by the relatively low potentials which commonly appear on a multiple-earthed neutral under normal conditions. No-volt gravity- I should like the author to explain why direct earthing must not operated trip be mixed with protective multiple earthing. I warmly endorse the practice, to which Mr. Linton referred, of placing the service earth electrode of a pole-mounted transformer a span or two away from the terminal pole. This pole is usually in a corner of a field where animals always congregate and rub their backs on the stays, the worst possible place to risk an earth potential-gradient. Finally, if I lived in a farmhouse with an old damp brick Earth-free Alternatives for As in scheme 1 floor I should prefer to have earth-leakage circuit-breaker situation non-earth-free protection; but I should also provide rubber mats in front of situation the cooker and the sink. Mr. A. E. J. Whitcher: The author refers to the system of Fig. C.—Neutralization for large systems. separate-earth-wire multiply earthing, and regards it as impossibly expensive on a rural system. I should like him to say that the undesirable even as a second line of defence, and a switch on the protection afforded by such a system is better than that afforded lines of Fig. D is being considered. by any of the alternative systems described. I am concerned The external risks of this system are very low indeed, being about this because in a system now using metallic-sheathed-and- armoured cables, properly bonded, these sheaths can provide the requirement of a separate earth-wire with multiply earthing, 4 but with the stringency in metal supplies we may, in the future, have to use non-conducting materials for cable sheaths. Should 2/iF this occur, then problems similar to those the author has described HP- may arise on urban systems, and I should like him to make it Switch contacts clear, therefore, that where it is available the separate earth-wire held together ///y with multiply earthing offers a better standard of protection. by fuse^ T

Messrs. R. B. Rowson and J. R. Willy {communicated): The i paper is of particular interest in its use of probability notation to enable many imponderables to be considered in isolation and '. Spring subsequently combined; and by its refuting of direct earthing in rural areas. Prior to the 1st April, 1948, we had worked for some Fig. D.—Proposed switch. years on this problem and had concluded that, of the alternatives mentioned in the paper, a separate earth-wire was undesirable, as a failure does not call attention to itself. Protective multiple mainly due to the risk of an open-circuit in section ab of Fig. D. earthing in conformity with the 1937 Regulations is usually The risks are: unattainable, and earth-leakage trips as a first line of defence eifsm +fpm)tesmpm X m' X It' + c'sm X d'sm X m' Xfl' are too liable to lightning and mechanical troubles. We felt that neutralization deserved more attention, in combination with 5O 3 \ 2 1 15 90 the full exploitation of earth-free situations. - (rao) 2 ' 2 x 365 TOOO'365 1 1 15 90 H.V 1 000 2 x 365 1 000 365 0-75 0-005 X Balancer 106 106 m' has been taken as, say, 30 % of 50, and 365 n' at 90, it being assumed that the switch would be checked each quarter. Fig. B.—Neutralization for small systems. The cost of this system should not exceed £1 10s. per consumer. For small systems Fig. B was evolved. Here, the hazards are: Scheme 1 (Fig. C) reduces the external risk to as near zero as possible at the expense of some consumption of current. Inter- (a) A combination of broken neutral and failure of balancer. nally, the continuity conductor could be avoided, thus reducing (b) A broken service-neutral. the greatest risk by using a no-volt trip at each control point. This latter is reduced to almost zero by using a concentric We hope that these proposals may help to solve a very difficult service cable. The external risk is thus almost zero if the problem. balancer has the same risk as the neutral, and the cost quite small. Mr. R. H. Rawll (communicated): The paper has been pre- For larger systems Fig. C was evolved. In scheme 2, C and L sented at a very opportune time. Following the nationalization form a series resonant circuit if the neutral opens. When the of the electricity supply industry the various Area Boards have consumer is taking load, the trip is supplied via the load. In already demonstrated their desire for, and taken practical steps either event, operation with earth resistance up to 3 kilohms is to provide, reliable and safe installations and appliances for obtained. Toggle switches which operate infrequently are their domestic consumers. In this connection, however, it is LOW- AND MEDIUM-VOLTAGE OVERHEAD LINE SYSTEMS: DISCUSSION 585 equally important and essential to achieve the utmost efficiency carried out effectively by the usual practice of connecting the of the different methods of protective earthing employed, so earthing lead to the water service pipe. that the risk of shock—when faults do occur—may be reduced It would seem that a strong case can be made out for a much to a minimum. It should also be remembered that, in the near wider use of the earth-leakage circuit-breaker. This particular future, there is likely to be a very considerable extension of device has the advantage not only of detecting faults on defective supplies to the rural areas of the country, where the provision of appliances as they arise, but of continuing to operate satisfactorily reliable earthing arrangements is a far more difficult matter than in the event of the resistance of the earth-continuity conductor is the case in urban localities. increasing to a figure which would seriously affect the operation It is observed that the paper refers specifically to overhead-line of those systems of protection that rely on a low-resistance path systems, but I think the author will agree that many of the data, for the return of the fault current. The satisfactory performance observations, conclusions, etc., contained therein would also of earth-leakage circuit-breakers can, nevertheless, only be apply to underground-cable systems. achieved if they are tested at regular intervals to see that they The really important point is that faulty appliances associated function correctly. The author suggests that they should be with open-circuited earthing leads constitute a risk common to inspected every three months. It is essential, however, that in all systems of earthing, and consequently it is important that addition to being inspected they should also be tested. As it is every endeavour should be made to reduce this particular risk, unlikely that the consumer would bother or even be competent since this would contribute as much to safety as the use of a to carry out such a test, this would mean that the supply authority specific method of earthing. It should also be borne in mind would have to undertake this work if it was to be done at all. that there are many instances where unearthed portable domestic Objections to this have often been raised in the past, on the appliances of metallic construction develop faults which involve grounds that it would make the supply authority responsible for their containers becoming alive, but the potential danger of this any damage arising from a leakage of current on a consumer's is detected neither by the blowing of a fuse—due to the absence installation caused by the failure to function of the circuit- of an earth wire—nor by the experience of shock on handling— breaker. I think that this is a position which must sooner or because the flooring is often of non-conducting material. That later be faced and accepted by the supply authorities, if a reason- this state of affairs does exist to a considerable extent is confirmed able degree of safety from shock under earth-leakage conditions by the report of one of the supply undertakings, quoted by the can be assured consumers in those cases where the installation of author. All this gives point to the necessity for the more an earth-leakage circuit-breaker is the only practical method of extensive use of domestic appliances with totally-insulated protection. exteriors, which modern developments in plastic materials, etc., Table 5 of the paper analyses the experiences of one supply are now making increasingly possible, so that the risk of shock undertaking with respect to the cases of broken neutrals on from such apparatus would be eliminated. The regular testing, distributors and services, which, it is stated, are both overhead inspection and maintenance of earth wires, etc., on consumers' and underground. It would be interesting to know what pro- premises, in order to ensure that open-circuits and high resistances portion of these faults referred to underground cables, as it would therein are early detected, is a matter of extreme importance. seem from the list of causes given that the majority of these Unless such earthing arrangements are maintained in an efficient would only affect overhead systems. condition, money spent by the supply authority on a definite In the author's list of recommendations for ensuring a reason- method of protective earthing is largely wasted and unjustified able standard of safety with protective multiple earthing, a in view of the degree of safety which can be achieved. Too little testing current of not less than 15 amp is to be applied for not attention has been paid to this in the past, the usual attitude less than one minute to the earth-continuity conductor, to verify being—particularly in the domestic installation—that once the that its electrical resistance is below the maximum specified. earthing lead has been installed it can be forgotten. However, it is not stated what voltage is to be utilized for this The recent introduction of water mains constructed of asbestos purpose, nor whether it is to be direct or alternating. This and other non-conducting materials, and their probable extensive information should also be given, together with an indication use in the many new housing estates to be erected in the near of how the testing current is to be obtained. Presumably this future, focuses attention on the fact that, in such conditions, would be through a double-wound transformer connected to the direct earthing of a consumer's installation can no longer be supply terminals.

THE AUTHOR'S REPLY TO THE ABOVE DISCUSSION Mr. L. Gosland (in reply): The discussion has been pleasingly Similarly, the apparent conflict of opinion between Mr. Gilbert diverse, and it is difficult to give a satisfactory answer to the and Mr. Morgan, on susceptibility to lightning, can only be numerous points raised within the space available. Initial resolved by analysis of experience. Mr. Frost makes the point reference to the importance of economic aspects comes aptly that, in certain circumstances, the consumer may wish to pay from Mr. Fawssett, whose suggestion of the formation of a panel more for extra protection; but who is to advise consumers on on this subject led to the attempt to reconcile divergent views such a point? on the value obtained for money spent in different ways. A Analysis to resolve such conflicts of opinion involves numerical review of the discussion about the earth-leakage circuit-breaker estimates or assumptions of the incidence and duration of by Messrs. Fawssett, Linton, Gilbert, Frost, Mann and Rawll individual defects. Mr. Fawssett, Dr. Taylor and Mr. Frost illustrates the point well. The advantage which Mr. Linton refer to the fact that these are based on scanty and divergent sees does exist, with circuit-breakers designed to B.S. 842, when data, if not assumed. Nevertheless, any engineer attempting the "earth" in the earth-fraught situation constitutes an extension evaluation of the relative merits of methods of earthing must of the earth-continuity conductor connected to the operating have quantities of this kind at the back of his mind, and there coil of the earth-leakage circuit-breaker, but not otherwise; if, seems to be advantage in stating explicit figures, so that the as Mr. Gilbert suggests, operating currents are normally outside foundations of opinion may be exhibited and discussed. As the limits of B.S. 842, the advantage hardly exists, since a Mr. Fawssett indicates, some check on this has been made and dangerous current can pass without tripping the breaker. the conclusions of the paper are thought to be unaffected by 586 GOSLAND: THE COST AND EFFICIENCY OF EARTHING ON OVERHEAD LINE SYSTEMS: DISCUSSION quite large changes in any one assumption. I should be happy fact, allow for the continuous load of the meter coil (see Fig. 5). to receive data or suggestions on which better estimates could Earth-leakage circuit-breakers are debited with cost of testing be based. and maintenance, because available data on performance relate Messrs. Fawssett, Frost and Rawll raised the question of to breakers so treated, and general opinion is that such treatment responsibility. I am hardly qualified to express an opinion on is necessary. On the other hand, no evidence exists that testing this point, but if any great saving in overall cost, without increase is usual with other methods, but, as stated by Mr. Rawll, there in risk, could be achieved by measures involving acceptance of is little doubt that some routine testing is desirable with all responsibility by the supply authority, national economy would methods, and that with most it would produce a useful return be aided by their adoption. The question is difficult, largely in safety. As regards deferring the choice between methods of because the magnitudes involved are so ill defined. protection, the matter is urgent and protective multiple earthing Mr. Fawssett and Mr. Frost referred to mixed systems. A has the advantage that it can be improved either directly or directly earthed installation should not be connected to an (being cheap) by the superposition of more expensive methods overhead system using protective multiple earthing, since direct at a later date. earthing properly carried out involves an earth-electrode re- Mr. Frost correctly states the relation of conclusions (e) sistance of, say, one ohm at the consumer's installation, whilst and (J). Nevertheless, from all regulations there are occasional protective multiple earthing implies a resistance of some ohms dispensations, and (e), if adopted, may sometimes be alleviated, between the system neutral and earth. The consequences of a but (/) remains an overriding consideration. The paper is more solid-earth fault at the directly earthed installation can readily cautious in the matter of use of private water supplies as electrodes be visualized. The condition cited by Mr. Fawssett is analogous, than Mr. Frost implies, suggesting their use only with the although the case against a system so mixed is not quite so clear separate earth-wire multiply-earthed system. With protective if the consumers fed by cable have earth-leakage return to the multiple earthing, individual cases require careful consideration, cable sheath. for the reasons which Mr. Frost mentions. Mr. Mann and Mr. Frost point out that, with protective It is, I think, true that the underground system described by multiple earthing, risks are not evenly spread. This aspect is Mr. Whitcher, with a separate earth-wire consisting of a cable properly taken account of in the analysis, both for protective sheath properly bonded, or an extra conductor multiply earthed, multiple earthing and the separate-earth-wire system, but is should be completely satisfactory, since such an earth wire is concealed in the process of averaging risks. Retention of this largely protected from the risk of fracture which gives rise to variable would obscure the broad comparison attempted in the the small element of danger with separate earth wires in overhead paper. Mr. Mann regrets that current-balance protection is not systems. discussed. This is because little service experience is available, The schemes proposed by Messrs. Rowson and Willy are so that numerical assessment is not possible. Analysis is not interesting. The balancer gives an approach to a line fed from difficult, since the case is not unlike that of the earth-leakage both ends. The major risk in such a case is that of the failure circuit-breaker. of the neutral and one line; this risk would be much reduced if Dr. Taylor does himself less than justice in suggesting that the neutral were earthed at the balancer as well as at the supply. his early approach to comparison of risks was a failure; he did, I am not sure that I have properly understood the schemes of in the work cited, at least find arguments to support the very Fig. C. In scheme 2, operation of the trip would appear to sound views he advanced. He refers to the fact that fuses, leave the broken neutral connected to the mid-point of the series even with low earth-path resistances, do not give absolute safety, resonant system; there might be some advantage in putting the by reason of the finite fusing time. This point was stressed in capacitor directly in series with the winding at the cost of some the paper because it is often forgotten. The curves mentioned loss in sensitivity. It is felt that the calculation of risk presented by Dr. Taylor have rather curious shapes because they were may require the inclusion of a factor for the possibility that the produced by computation from load-time curves, themselves trip coil is shunted or supplied by a fortuitous earth. This discontinuous. The plots for separate earth-wire and earth- contribution illustrates the advantage of comparing the merits leakage circuit-breaker are short because expenditure on these of protective schemes on a numerical basis. methods of earthing has a lower limit. Dr. Taylor rightly Mr. Rawll makes an important point in stressing the possible stresses the significance of the fact that two entirely different gain in safety by wider adoption of insulating casings and by approaches each led to the opinion that protective multiple attention to earth-continuity conductors. Certainly, inspection earthing had many merits. Inquiry had, in fact, been made of earth-leakage circuit-breakers should include testing; this was about the performance of the protective-multiple-earthing the intention, and a suitable modification has been made to the schemes to which he refers. So far as is known there have been paper. Whilst the arguments of the paper could well be applied no complaints, so that they may be regarded as satisfactory. to underground-cable systems, one would wish to re-weight the Mr. Linton's point on the necessity for insulating the earth various factors before coming to conclusions. In fact, if the lead and careful choice of the site of the earth connection illu- sheath of a metal-sheathed cable is made available for the strates how much is to be gained by attention to detail. It is return of leakage current, it is probable that the external risks agreed that fuse-operated protective schemes are inherently are reduced almost to zero—as in the case with the plastic- ineffective in the conditions cited. Misuse of electrical installa- sheathed cable with extra earthing conductor, mentioned by tions will always give rise to difficulty. Mr. Whitcher, provided that the separate earthing conductor Mr. Mann suggests that only the nearest possible approach is electrically and mechanically adequate. The system covered to the ideal is adequate; carried to the logical conclusion this by Table 5 comprised roughly 60% overhead and 40% under- would involve great expense, and study of the most economical ground distributors, with most of the faults on the former. A means of achieving a satisfactory standard of safety does not satisfactory answer to Mr. Rawll's question on the testing of appear a particularly reprehensible course. I agree with Mr. earth-continuity conductors cannot be given in brief. A sub- Gilbert that a complete treatment of this subject should give committee of the E.R.A. is at present actively considering the detailed consideration to the fire risk, but this has so far proved matter, and it is expected that a complete discussion of this intractable by analysis. The calculations in the paper do, in subject will soon be available.