Hybrid Ring/Radial Final Circuits

PE NICEIC OCTober 2015:Layout 1 06/08/2015 15:13 Page 1

TECHNICAL ADVICE

taking you step by step through procedures and requirements for electrical installations

Hybrid ring/radial final circuits

ICEIC is sometimes asked whether a ring Ring circuit cable (installed rating 20 A min) final circuit has to be Nsupplied directly from Ring !nal circuit a protective device in the distribution board or consumer unit, or whether it is permitted to supply the two ends of the Junction box (or possibly double-pole switch or contactor) rated at 30/32 A min ring via a dedicated radial circuit Cable connected to 30/32 A overcurrent protective device Radial circuit cable cable, as shown in Fig. 1, in consumer unit (installed rating 30/32 A min) thereby forming a hybrid

L OUT

N ring/radial circuit. OUT

This question can arise where, for example, the contractor wishes to Consumer unit make use of a redundant radial circuit cable previously installed for another Fig 1 Typical ring final circuit supplied via a radial circuit cable, purpose (such as a cooker circuit) to forming a hybrid ring/radial circuit supply a ring final circuit, or where a Periodic Inspection of an existing hybrid circuit, the ‘single point of of the socket-outlets ( I⌬n ≤ 30 mA) in installation reveals such an supply’ is the point at which the two accordance with Regulation 411.3.3. arrangement. ends of the ring part of the circuit are Also, if any part of the circuit cable connected to the radial part (that is, at The same question may also arise is concealed in a wall or partition, it the junction box, switch or contactor as where the contractor wishes to control must be provided with additional labelled in Fig 1). a ring final circuit by means of a switch protection by an RCD ( I⌬n ≤ 30 mA) as for functional purposes, or possibly by A hybrid ring/radial final circuit must required by Regulations 522.6.202 means of a contactor, controlled by an be arranged to meet all the relevant and 522.6.203. emergency stop push button (such as requirements of BS 7671 , as amended, in a teaching situation), the supply the main ones of which are Cable sizing with regard to being brought to the switch or summarised as follows. normal load and overload contactor via a dedicated radial circuit The current-carrying capacity ( Iz) of cable. Protection against electric the cable in the radial part of the circuit shock The answer is that it is permissible must be not less than 30 A or 32 A, to to form a hybrid ring/radial circuit, even As for any other final circuit, the match the circuit overcurrent though BS 7671 defines a ring final requirements of Chapter 41 for electric protective device rating ( In) of 30 A or circuit as ‘ A final circuit arranged in the shock protection have to be met, 32 A. Where the protective device is a form of a ring and connected to a including those of Section 411 for earth semi-enclosed (rewireable) fuse, Iz single point of supply’. For such a fault loop impedance ( Zs) and, where must not be less than In divided by applicable, RCD additional protection 0.725. Regulation 433.1.202 refers. PE NICEIC OCTober 2015:Layout 1 06/08/2015 15:13 Page 2

TECHNICAL ADVICE

The ring part of the circuit must ring parts of the circuit. These voltage 10 mA, the circuit will require a high meet the same overload protection drops added together make up the integrity protective conductor requirements as for any other ring total voltage drop in the circuit. This connection in accordance with final circuit, which is usually achieved may mean, for example, that the Regulation Group 543.7. through compliance with Regulation maximum allowable length of the ring 433.1.204. part of the circuit will be less than that Electrical Testing Despite the non-conventional form for a ring final circuit connected For verification of the hybrid of a hybrid ring/ radial circuit, the total directly to the distribution board. ring/radial circuit, the circuit details and test results for each part of the load current ( Ib) to be supplied by the Protective conductor in the ring overall circuit, the radial and the ring circuit, making due allowance for any part of the circuit diversity, must still be no greater than parts, should be recorded separately the 30 A or 32 A rating of the circuit Except where the circuit protective on the appropriate schedule protective device. Thus, for example, conductor in the ring part of the circuit contained in the NICEIC Electrical the floor area served by the circuit is formed by a metal covering (such as Installation Certificate for Small should be no greater than would be a metallic cable sheath) or enclosure Installations not exceeding 100 A, as the case if an ordinary ring final circuit (such as metal conduit) containing all shown in Fig 2, or where appropriate having the same protective device the conductors of the ring, the on an EIC. protective conductor must be run in rating was installed instead of the Use of a standard circuit might hybrid circuit. the form of a ring, as required by Regulation 543.2.9. Both ends of the be easier Accessibility of connections protective conductor ring must be It will be appreciated from this As is required for all electrical connected to the earthing terminal at article that the use of a hybrid connections, the connections the origin of the ring (that is, the ring/radial circuit arrangement can between the radial and ring parts of junction box, switch or contactor in involve significant design effort. As an the circuit (in the junction box, switch Fig 1). alternative, for household and similar installations, installers should consider or contactor in Fig 1) must be arranged Earthing for equipment having to be accessible throughout the life of the use of the standard ring or radial high protective conductor socket-outlet circuit arrangements the installation for inspection, testing currents and maintenance (Regulation 132.12 shown in Appendix 15 of BS 7671 , for refers). If the circuit is intended to supply which tables of maximum cable length one or more items of equipment are given in the NICEIC Site Guide for Voltage drop where it is known or reasonably to be Electrical Installations . Account must be taken of the expected that the total protective voltage drops in both the radial and conductor current is likely to exceed )

This certificate is not valid k r if the serial number has DCN8/ o w

been defaced or altered e h t

g n i r

ELECTRICAL INSTALLATION CERTIFICATE FOR SMALL INSTALLATIONS NOT EXCEEDING 100 A e d r o

CIRCUIT DETAILS TEST RESULTS n o s

1 n r d Circuit impedances Circuit designation 7 RCD Circuit o o

i Overcurrent protective devices 6 r

Insulation resistance e

t Maximum 4 7 h RCD y

e t c g

conductors: csa t operating p x S i

e ( ) e b i d

n r s W i

B Test n measured d e t d

i r m t m Z ) e n a times n e

i ) y t

n l u f i

* To be completed only where this consumer unit is remote from 1 o u v e 1 b

e All circuits

D button e h

r 7

Ring final circuits only o c o m w g c earth fault loop 7 m n

I d c

t r

BS (EN) d p

cpc e r Live s y i r u , n 6 f P i i

t t t o n e 7 p t s Line/ Neutral Line/Earth Neutral/Earth e

i Line/Line i the origin of the installation. operation t e c 7 (measured end to end) (At least one column

d o n t g m at 5 I e c - a

i i o c impedance, Z at I A b u s S

. t r

r n e e

t n S s x r i e r a D x m

c to be completed) e T e e B e

t D r e p m n r Record details of the circuit supplying this consumer unit in the a f o r a i B p p ( p

i (if applicable) e

a u y m e u y e r r e

h r f i o a y 1 n 2 s c O s M b t M p

T C R ( T p ( o R N S bold box. 2 2 c

l (mm ) (mm ) (s) (mA) (Line) (Neutral) (cpc) (R 1 + R 2) R2 (M ) (M ) (M ) (M ) ( ) ( ) (ms) (ms) ( ) (A) (kA) (W) W W W W W a

* n i g i

1 r

Hybrid circuit: Radial supplying ring A B NA NA NA NA NA O Ring A B NA NA Lighting circuit A B NA NA NA NA NA

) e

t a Shower circuit A B NA NA NA NA NA t s

Recording the results in this way will permit R 1+R 2 values to be recorded for the radial part of the circuit and the loop

values to be recorded for the ring part.

Note: The recorded earth fault loop impedance (Z s) value for the ring includes the radial part of the circuit.

Fig 2 An example of the Circuit Details and Test Results recorded on the Electrical Installation Certificate for small installations not exceeding 100 A

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