Electrical Breakdown in Atmospheric Air Between Closely Spaced (0.2 "M–40 "M) Electrical Contacts Paul G

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390 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 25, NO. 3, SEPTEMBER 2002 Electrical Breakdown in Atmospheric Air Between Closely Spaced (0.2 "m–40 "m) Electrical Contacts Paul G. Slade, Fellow, IEEE, and Erik D. Taylor, Member, IEEE

Abstract—The increasing importance of electrical contacts in close to home. A new miniature circuit breaker, called an arc air with micrometer spacing prompted recent experiments on the fault circuit interrupter, has been recently been introduced electrical breakdown behavior of these gaps. The electrical field for household use [9]. This device detects low-current, arcing between the contacts used in one of the experiments was analyzed using finite element analysis to model the electric field. The ex- faults on the line, causing the circuit breaker to trip before the perimental data on the electrical breakdown voltage could be di- fault can cause severe secondary damage, such as a fire. In the vided into three regions as a function of the gap spacing. First, at U.S.A. the usual home circuit is 110 V (rms), so the question close gap spacing ( R m) both the breakdown voltages as well has again risen about how could an arc burn for long enough to as the electrical fields at the cathode were similar to values mea- cause damage at these low voltage levels. If the breakdown in sured during the breakdown of vacuum gaps of less than 200 m. Second, at larger gaps ( T m) the breakdown voltages followed air followed Paschen’s Law, then the low line voltage would be Paschen’s curve for the Townsend electron avalanche process in air. unable to break down the gap after the arc extinguished at the Finally, in between these two regions the breakdown values were first current zero. However, in closely spaced gaps a voltage below the expected values for purely vacuum breakdown or purely of 110 V would be sufficient to cause electrical breakdown, Townsend breakdown. The breakdown phenomena have been dis- re-igniting the arc after a current zero. Once an arc has been cussed in terms of field emission of electrons from the cathode and their effect on initiating the observed breakdown regimes. initiated, breakdown of a contact gap is not the only means by which the arc can continue to operate through a number of Index Terms—Atmospheric pressure, electrical breakdown, electrical contacts, Paschen’s law, small contact gaps, vacuum current cycles. For example, carbon deposits from dissociated breakdown. insulation can permit a conducting path between the arcing members. An arc carrying a current of a few amperes has a temperature greater than 5000 K [8], which would be sufficient I. INTRODUCTION to ignite a fire. Finally, increasing interest is being paid to HIS paper discusses the electric breakdown of closely another potential arcing problem, namely in the hundreds of T spaced contacts (0.2 mto40 m) in air at atmospheric kilometers of closely bundled electrical wiring typically used pressure. This subject attracted some research effort in the in commercial and military jet aircraft [10]. 1950’s [1]–[3], but for the most part has been neglected until In all these cases, the proper design of the electrical system comparatively recently [4], [5]. There are a number of reasons requires knowledge of the breakdown behavior. Experiments on why discussion of this subject is timely. First, the miniatur- contacts in air at micrometer gaps have shown that the contact ization of electrical components is rapidly advancing. This gap crucially controls the breakdown voltage. An essential com- miniaturization includes higher conductor densities leading to ponent in eliminating these electrical breakdowns at micrometer smaller conductor spacings in connectors, switches, and micro gaps is to quantify the voltage limits and understand the break- electro-mechanical systems (MEMS). In these components, down behavior as a function of contact gap spacing. In this paper the spacing between electrical conductors is routinely dropping we evaluate recently published electrical breakdown measure- to the micrometer range. Even a low potential difference ments for closely spaced contacts in air at atmospheric pressure imposed across two such conductors can generate a very high and in vacuum between Pa and Pa. This data is then electric field, possibly leading to an electrical breakdown at analyzed in terms of the Townsend electron avalanche theory quite low voltages. Second, the automobile industry is actively and Paschen’s Law for breakdown in air [6]–[8], and in terms of changing the electrical system in cars from 14 V to 42 V. It is the vacuum breakdown process involving field emission of elec- commonly believed that if the voltage between two conductors trons from microprojection on the cathode contact [11]–[13]. in atmospheric air is below the Paschen minimum breakdown voltage in air of 325 V [6], [7], then a breakdown between the II. EXPERIMENTAL DATA conductors is not possible. However, an electrical breakdown of the gap between the conductors, leading to an arc, is possible The data for electrical breakdown in atmospheric pressure air even at 42 V when the distance between the conductors is a comes from two sources [4], [5]. The first set of data was de- few micrometers [8]. Third, this subject also can literally come veloped by Lee, Chung and Chiou [4]. These researchers made Manuscript received September 12, 2001; revised November 16, 2001. This careful measurements using the contact arrangement shown in work was recommended for publication by Associate Editor J. W. McBride upon Fig. 1, with contact gaps ranging from 0.2 mto40 m. The evaluation of the reviewers’ comments. cathode was a Fe polished needle and the anode was a silver The authors are with Eaton Corporation, Cutler-Hammer Products, Horse- heads, NY 14845 USA (e-mail: [email protected]). disc. The minimum breakdown voltage as a function of con- Digital Object Identifier 10.1109/TCAPT.2002.804615 tact gap is shown in Fig. 2. The second data source, also shown

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Fig. 1. The contact structure used in [4]. All dimensions are in millimeters.

Fig. 3. Comparison of the data in Fig. 2 with both the Paschen curve in air and (1).

Fig. 2. Electrical breakdown voltage in air as a function of contact gap. Data points @ A are from [4], and @RA are from [5]. in Fig. 2, comes from the work of Torres and Dhariwai [5], again using a very carefully developed contact system. These measurements were taken in a clean room, and three contact materials were tested: Ni, Al and brass. They observed no difference Fig. 4. Electrical breakdown voltage in vacuum as a function of contact gap. in outside their experimental errors for these materials. Fig. 2 shows that for contact gaps greater than 6 m, the break- in Fig. 4. This data is taken from many sources [14]. The data down voltage is in the range 300–400 V.For contact gaps less points shown here are the average values. The contact gap than 4 m, the breakdown voltage is a strong function of the ranges from 35 mto200 m and shows a linear dependence contact gap . In Fig. 3 we have taken the Paschen Curve in air at on contact gap atmospheric pressure [8] and superimposed it on the data given i.e. (2) in Fig. 2. For contact gaps greater than 6 m, the Paschen curve passes through the breakdown data. For contact gaps less than 4 Where V m Vm . The value of m, the voltage breakdown points are below the expected values for the vacuum breakdown data falls within the range of in predicted from the Paschen curve. The data in this region can be (1) for the air breakdown data in contact gaps less than 4 m. represented as a linear function of the gap spacing These experimental similarities between the breakdown data in air at very small gaps and breakdown in vacuum for gaps of (1) less than 200 m suggest that a similar breakdown mechanism governs both cases. Three other similarities are also present. The two lines are shown in Fig. 3; one is for V m First, the electric field at the contact surface for small gaps in and the other is for V m . This behavior will be air is comparable in magnitude to those observed in vacuum discussed in the following sections. breakdown. Second, this electric field is high enough to produce a field emission current from the cathode contact. Finally, III. VACUUM BREAKDOWN REGION the mean free path of the electrons in air at atmospheric pres- The experimental data for the breakdown voltage at small sure is about 0.5 m. Electrons from the cathode will have very gaps in air is very similar to the breakdown behavior of contacts few collisions before reaching the anode at small contact gaps in vacuum. The experimental data for the breakdown voltage ( m). The presence of air in these small contact gaps will as a function of the contact gap in high vacuum is shown thus have only a small effect on the breakdown process.

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Fig. 6. Example of a surface profile for a ground and polished mild steel contact [16].

Even with polished contacts, the microscopic contact surface is Fig. 5. Geometrical enhancement factor for the contact structure shown in anything but smooth [15], as seen in Fig. 6 [16]. The peaks in Fig. 1 as a function of contact gap. the contact surface result in another field enhancement factor, , at the surface of a microprojection’s peak. Thus, the total TABLE I electric field at the contact’s microsurface is given by GEOMETRICAL ENHANCEMENT FACTOR FOR THE TWO RANGES OF CONTACT GAP (4)

Putting then:

(5)

For polished cathodes, the total enhancement factor For vacuum breakdown, the critical parameter is the local is in the range 100–250 [17]. Calculations of the enhancement electric field at the contact surface. A common method to cal- factor generally state the total rather than separately deter- culate the electric field is to use the ratio (where is the mining the microscopic and macroscopic contribu- potential drop impressed across the contact gap ). When the tions. From Fig. 4, at breakdown for contact gaps 35 contact gap is small compared to the radius of curvature at the m to 200 m in vacuum is Vm . Therefore tip of a needle contact, or the diameter of broad area contacts, the at breakdown lies between Vm and peak macroscopic field is very close to this value. There will be Vm . These field strength values are comparable to cal- a geometrical effect that produces a higher macroscopic field for culations of at breakdown in air for a contact gap of 2 m longer contact gaps. For broad area contacts, the peak electric (between Vm and Vm ). The Fe cathode field is generally on the edge of the contact, and for needle con- used to generate these experimental data is a carefully prepared tacts it is generally at the contact tip. If the geometrical enhance- and polished needle. We would therefore expect that for this ment factor for a given gap is then the maximum macroscopic cathode to be closer to 100 than to 250. Thus, at the 2 m field, , between the contacts is given by gap in air would be closer to Vm . The breakdown field strength for vacuum gaps (35 m–200 m) and contact gaps in (3) air ( m) are thus very similar. The enhanced electric field at the cathode contact’s surface The first step in comparing the air breakdown data to the vacuum results in the field emission of electrons. The current density theory is to determine . We performed a finite element anal- generated by the electric field is give by the Fowler-Nordheim ysis for determining macroscopic electric field as a function equation [17], [18] of the contact gap for the arrangement shown in Fig. 1. The data in Fig. 5 plots versus the contact gap for this case. The ratio of the contact gap spacing to the radius of curva- (6) ture of the needle cathode determines the behavior of the electric field. When the gap is very much smaller than the needle’s ra- where is in amp/m , the electric field is in V/m, is the dius of curvature, the needle geometry only has a small effect on work function of the contact material in eV, and and are di- the electric field and can be used to approximate the electric mensionless functions of the parameter . field. The peak electric field, however, increases monotonically In practice, is one and is as the gap increases. For a contact gap of 40 m (i.e., near to the needle’s radius of curvature) the value of has the value of (7) about 1.6. Table I gives the ranges for the geometrical enhancement factor for the contact geometry used in the air breakdown This equation calculates the current density from the metal via experiment in [4]. Thus, for a contact gap of 2 m in air, where tunneling through the surface potential barrier. A further expla- is 160 V (Fig. 3) and is 1.033 (Fig. 5), nation of the terms of the equation can be found in [19]. Vm when the gap breaks down. Assuming that the emission current comes from a small In addition to knowing the macroscopic field, it is also impor- microprojection of area on the contact surface, then the cur- tant to examine the microscopic field at the contact surface. rent density is . Further assuming that the micro-

Fig. 7. Fowler–Nordheim plot of the prebreakdown field emission current for Cu contacts spaced 2 mm apart.

Fig. 9. Electrical breakdown process in vacuum for contact gaps less than 500 "m and in air for contact gaps less than 4 "m.

gives a value of 248. The electric field in this experiment is only Vm when breakdown occurs. Thus, the electric field for contacts in air spaced at less than 4 mis certainly sufficient to produce a very high current density at a cathode microprojection. The development of space charge in front of the cathode is often cited as a limiting factor on the current density that can be extracted from microprojections. Dyke and Trolan [20] observed that the field emission current diverged from the Fowler- Nordheim equation at current densities above Am for a 1 mm contact gap. This behavior can be attributed to the ef- Fig. 8. Prebreakdown field emission current as a function of the applied voltage for the data in Fig. 7. fects of space charge reducing the electric field at the emitter, and hence lowering the emission current. Nevertheless, the cur- scopic field at the tip of the microprojection is given by (3) rent density does still increase with increasing electric field and and the total eventually breakdown is reached. The space charge current limit can be calculated from the formula [21] (8) (11) then (6) equation can be written as where is the space charge limited current density, is the bias voltage and is the gap distance. Comparing the ratio of the current density limit for a typical small gap case in air ( (9) Volt, m) to that of a macroscopic gap typical of vacuum breakdown experiments ( kV, mm) gives If the voltage across the contact gap is varied and mea- V m sured, then a plot of vs. would give a straight (12) line with slope : kV mm Thus, space charge buildup will still affect the emission current, (10) however, the current limit is higher for smaller gaps because of the squared dependence on the gap distance. Since breakdown from which can be calculated. Fig. 7 shows a typical Fowler- in the larger gaps occurs via the vacuum breakdown mechanism Nordheim plot for Cu contacts with a 2 mm gap in vacuum [17], despite the effects of space charge, small gaps should be able to and Fig. 8 shows as a function of for the data in Fig. 7. follow the same breakdown process. grows exponentially as (and ) increases. In fact, at For small gaps in air, the space charge effect will only be of ( kV) for this contact structure and gap, will increase concern once is greater than Am . This current den- into the milliampere range. Thus, for a 1.45 times increase in sity is still too low to heat the cathode macro-projection to a (and ), from 34.6 kV to 50 kV, the current increases by level that would initiate a breakdown from the production of four orders of magnitude . The slope of the line in Fig. 7 field emission electrons alone. One method for overcoming the

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Fig. 10. Townsend electrical breakdown process in air for contact gaps greater than 6 "m. space charge limitation is to ionize the dense neutral gas des- will form above the cathode surface. This cathode plasma will orbed by the current heating [22]. The resulting ions move to- expand at ms [11], crossing a contact gap of 1 min ward the cathode, thereby reducing the negative space charge about s. With closely spaced contacts ( m), the build up and allowing the electric field at the contact surface to breakdown of the contact gap once the microprojection explodes increase. Also, ions falling into the cathode will encounter an is almost instantaneous. Thus, it is reasonable to assume that image electron that travels in the projection to recombine with the breakdown process for very small contact gaps in air is sim- it. The total current density inside the microprojection thus will ilar to that described for vacuum breakdown. The breakdown be voltage is a function of the electric field between the contacts, not a function of the contact gap times the ambient pressure. (13) Small contact spacing allows low inter-contact voltages to produce breakdowns. In the experiments discussed here, a break- This process is illustrated in Fig. 9 [22]. down voltage V is observed for a 0.25 m contact The background air between the contacts can have only a lim- gap. Thus, for contact gaps less than 4 m the electrical break- ited effect on the breakdown process. The mean free path of an down follows the sequence expected from experiments on the electron in atmospheric pressure air is m, so for contact breakdown of contact gaps in vacuum. In this process, the cur- gaps less than 4 m the field emission electrons will only have a rent density in the microprojection increases until it explodes, few collisions with gas particles. A voltage drop of 20 to 300 V forming a rapidly expanding plasma ball of metal vapor. When across the contact gap gives a reasonable probability of limited this plasma reaches the anode, the contact gap breaks down. ionization from these few collisions. As the ions drift toward the cathode, a positive space charge can develop in front of the IV. PASCHEN BREAKDOWN REGION cathode microprojection that will allow to increase. This ef- Fig. 3 shows that the as a function of contact gap for fect also helps to compensate for the collisions that may occur gaps greater than 6 m fits the Paschen Curve for air. The between the electrons and the background gas. no longer satisfies the vacuum breakdown curve for gaps When the in the cathode microprojection reaches values of m, where voltages above V would be required for the order of to Am , the explosion emission process the vacuum breakdown processes to occur. For contact gaps for vacuum breakdown, as discussed by Juttner [11] (based on of 6 m or more, field emission electrons from the cathode the work by Mesyats [12] and Fursey [13]), can occur for closely will have approximately twice the number of collisions that spaced contacts in air. As increases (Fig. 9), the temperature would occur for contact gaps of less than 4 m. This produces of the cathode microprojection increases. In fact, the tempera- more ionizing events, making it reasonable to assume that the ture inside the microprojection increases faster than the surface electrical breakdown for longer contact gaps occurs as a result as a result of the Nottingham effect [13], [23]. The Nottingham of Townsend electron avalanche theory [8]. This process is effect results from the difference in energy between the emitted illustrated in Fig. 10. For a 7 m contact gap with a voltage electrons and their replacements in the metal lattice from the drop of 330 V (Fig. 3), and assuming that is 100, then external electrical circuit. At a critical temperature, the microprojection will explode and a dense cloud of metal vapor plasma Vm Vm (14)

Inserting this value into the Fowler-Nordheim equation (see (6)) contacts is achieved when this plasma reaches the anode shows that this electric field can generate a field emission elec- contact. This sequence is very similar to the electrical tron current in the microampere range. These electrons will in- breakdown process between open contacts in vacuum (for teract with the background gas. The growth in current will be contact gaps up to 200 m). given by [7], [8] 4) For contact gaps greater than about 6 m, the breakdown process follows the classical Townsend electron (15) avalanche theory. For small gaps, however, field emission electrons from the cathode contact initiate the electron where is the contact gap, is the first Townsend coefficient, avalanche. giving the number of electrons produced per unit distance in 5) For contact gaps between 4 m and 6 m, the electrical the direction of the electric field and is the second Townsend breakdown is lower than would be expected from either coefficient, giving the number of electrons generated by sec- Paschen’s Law or from the vacuum breakdown data. We ondary processes per each primary avalanche. Breakdown oc- believe that in this range of contact gap, the electrical curs when . According to the Paschen curve breakdown results from an electron avalanche enhanced for air, the minimum voltage at which this can occur is about vacuum breakdown. 330 V, as is seen in Fig. 3. Thus, for contact gaps greater than 6 m in air, field emission electrons from microscopic projec- tions at the cathode can still initiate the breakdown process. In REFERENCES this case, however, the field emission electrons initiate an elec- [1] L. H. Germer, “Physical processes in contact erosion,” J. Appl. Phys., tron avalanche that leads to a gap breakdown when a voltage vol. 29, pp. 1067–1082, 1958. V is impressed across the open contact gap. [2] , “Electrical breakdown between close electrodes in air,” J. Appl. Phys., vol. 30, pp. 46–51, 1955. [3] H. N. Wagar, “Performance principles of switching contacts,” in Phys- V. T RANSITION REGION ical Design of Electronic Systems, Vol. 3, Integrated Device and Con- nection Technology. Englewood Cliffs, NJ: Prentice-Hall, 1971, ch. 9, For the contact gap in the range 4 mto6 m, the measured pp. 500–562. [4] R. T. Lee, H. H. Chung, and Y. C. Chiou, “Arc erosion behavior of silver is less than would be expected from either purely vacuum contacts in a single arc discharge across a static gap,” Proc. Inst. Elect. breakdown or from Townsend avalanche breakdown. In this re- Eng., vol. 148, no. 1, pp. 8–14, Jan. 2001. gion only a partial electron avalanche process would be initi- [5] J. M. Torres and R. S. Dhariwal, “Electric field breakdown at micrometer separations,” Nanotechnol., vol. 10, pp. 102–107, 1999. ated, because the electrons would only have a limited number [6] F. Paschen, Wied. Annu., vol. 37, p. 69, 1889. of collisions with the gas before reaching the anode. This par- [7] F. Llewellyn-Jones, Ionization and Breakdown in Gases. London, tial avalanche would, however, produce more ions, which would U.K.: Methuen, 1957, pp. 61–71. [8] P. G. Slade, “The arc and interruption,” in Electrical Contacts, Prin- drift toward the cathode microprojection. This enhanced flow ciples and Applications, P. G. Slade, Ed. 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Fursey, “Field emission and vacuum breakdown,” IEEE Trans. 1) For closely spaced contacts in air, Paschen’s Law is only Elect. Insulation, vol. EI-20, pp. 659–670, Aug. 1985. [14] R. Latham, Ed., High Voltage Insulation. San Diego, CA: Academic, valid for contact gaps greater than about 6 m. 1995. 2) For contact gaps less than about 4 m, the breakdown [15] R. S. Timsit, “Electrical contact resistance: Fundamental principles,” in voltage, , in air is a linear function of the contact gap Electrical Contacts, Principles and Applications, P. G. Slade, Ed. New York: Marcel-Dekker, 1999, pp. 1–88. and is given by: [16] J. A. Greenwood and J. B. P. Williamson, “Contact of nominally flat surfaces,” Proc. Roy. Soc. A, vol. 295, pp. 300–319, 1966. volts (16) [17] D. K. Davies and M. F. Biondi, “Vacuum breakdown between plane- parallel copper plates,” J. Appl. Phys., vol. 37, no. 8, pp. 2969–2977, July 1966. where is in m and is a constant. is similar in [18] R. H. Fowler and L. 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[23] F. M. Charbonnier, R. W. Strayer, and E. E. Martin, “Nottingham effect Erik D. Taylor (M’00) received the B.S. degree (with in field and T-F emission,” Phys. Rev. Lett., vol. 13, no. 13, pp. 397–401, honors) in applied physics from the California Insti- Sept. 1964. tute of Technology, Pasadena, in 1993 and the M.S. and Ph.D. degrees in applied physics from Columbia University, New York, NY, in 1995 and 2000, respec- tively. Paul G. Slade (M’71–SM’86–F’90) received the In 1999, he started working for Eaton B.S., Ph.D., and Diploma of mathematical physics Cutler-Hammer, Horseheads, NY, as a Senior degrees from the University of Wales, Swansea, Scientist/Engineer at their vacuum interrupter fac- U.K., and the MBA degree from the University of tory. His research interests include plasma physics, Pittsburgh, Pittsburgh. vacuum arc physics, electromagnetic field modeling, He has over 30 years experience covering a wide vacuum breakdown, magnetic fusion, and plasma-material interactions. range of problems associated with switching electric current. His research has covered electrical contact and arcing phenomena in air, vacuum, and SF .He has used this experience to develop new types of circuit breakers and switches. He has authored or co-authored over 70 research papers and has published the book Electrical Contacts, Principles and Applications. He is presently the Manager of the Vacuum Inter- rupter Technology Department, Eaton Cutler-Hammer, Horseheads, NY, where he is responsible for research and development, design, and applications engi- neering for the vacuum interrupter product. Dr. Slade is a member of the Institute of Physics (U.K.).

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