#### Relay Failures Specific to Space Applications

######## Alexander Teverovsky, Ph.D.

alexander.a.teverovsky.1@gsfc.nasa.gov

UNISYS/NASA GSFC Operations, Lanham, MD

##### Abstract

Low-pressure conditions, as experienced in space applications, are
considered benign for many electronic components. However, for switching
devices the probability of failure may be significantly greater than at
normal atmospheric pressure due to arcing-at-break processes. This study
was stimulated by a relay failure in a 60-V power bus in a spacecraft
module, and it was intended to analyze failure modes and the probability
of their occurring under low-pressure conditions. The effects of gas
pressure, power bus voltage, and load current on arc duration and
probability of arc flashover have been investigated. It was shown that
arc duration mostly depends on switching power and gas pressure,
significantly increasing when power is rising and pressure is
decreasing. Failure analysis indicated two major mechanisms in
low-pressure conditions: (1) contact damage (excessive erosion and/or
microwelding) and (2) arc flashover to a grounded case and/or grounded
coil post. For a relay operating in a vacuum, the effect of leak rate on
the time to failure at low-pressure conditions is discussed.

###### Introduction

Telemetry data indicated the failure of a 60-V power bus in a spacecraft
module. The failure occurred during the first switching of a relay, and
it caused a 10-A fuse in the bus power distribution unit to blow open.

One conceivable failure scenario involved the possibility of transient
case arcing (arc flashover) in a hermetically sealed relay used in the
module. No indication of arc flashover was observed under vacuum and
normal pressure conditions during multiple testing of the module before
launch. However, it is possible that a fine leak in the relay may have
decreased internal air pressure while the module was operating in space.

Although failures due to arc flashover in relays are well known since
the 1960's \[1\], the mechanism of this process is not completely
understood. In particular, the effect of low-pressure conditions on
relay arcing and the possibility of flashover failures has not been
adequately addressed in technical literature. Obviously, this
low-pressure condition is of particular interest for space applications.

This work analyzes arc flashover in hermetically sealed latching relays
operating in vacuum and estimates effect that a hermetic-seal leak has
on this phenomenon. For this purpose, the effects of internal air
pressure (ranging from 760 torr to 0.2 torr), power bus voltage (12 V to
60 V) and load current (0.2 A to 3.5 A) on arc-at-break duration and
relay failures were investigated.

###### Experiments

The setup used to simulate conditions in the bus power distribution unit
is shown in Figure 1. A power source was created by combining five car
batteries in series, allowing voltage to vary in 12-V increments up to
60 V dc. Standard 10-A/250-V CSA/UL-F fuses were used to detect
flashover events. Load current was adjusted from 0.24 A to 3.6 A using
200-W wire-wound resistors. Inductances of these resistors are listed in
Table 1. Wires of 12 AWG type with lengths of approximately 2.1 m (7 ft)
were used to connect the circuitry. A straight wire of this length has
an inductance of approximately 1.5 μH. Inductance of the ammeter was
0.23 μH. Total inductance in the circuit depended primarily on the
resistor used, but it was relatively low (≈ 90 μH maximum). The impact
of the circuitry inductance is discussed later.

Three hermetically sealed latching relays were used in this study. The
contacts in these relays were rated for 12-A resistive load at 28 V dc.
According to the manufacturer application note, it is possible to derate
the contacts for higher dc voltages provided lower currents are used
during switching. At 60 V, the calculated current limit is 3.73 A.

Table 1. Inductances of resistors used in the experimental setup.

+-------------------+-------------------+------------------------------+
| ###### Resistance | **Inductance**    | **Type**                     |
+-------------------+-------------------+------------------------------+
| 5 Ω               | 19 μH             | wire-wound                   |
+-------------------+-------------------+------------------------------+
| 10 Ω              | 20 μH             | wire-wound                   |
+-------------------+-------------------+------------------------------+
| 10 Ω              | 0.5 μH            | carbon                       |
+-------------------+-------------------+------------------------------+
| 20 Ω              | 40 μH             | wire-wound                   |
+-------------------+-------------------+------------------------------+
| 50 Ω              | 87 μH             | wire-wound                   |
+-------------------+-------------------+------------------------------+

The movable relay contacts were connected to the positive battery
terminal. Arc duration, θ, was monitored with a current probe and an
oscilloscope. The sample was switched five times at each combination of
experimental parameters, and mean values and standard deviations of θ
were analyzed. Small holes were drilled in the relays before installing
them in the vacuum chamber to permit pressure equalization.

![](media/image1.jpeg){width="2.9972222222222222in"
height="2.501388888888889in"}

Figure 1. Block diagram of the test set-up.

###### Results

###### Atmospheric Pressure Condition

A typical transient oscillogram measured at contact break under normal
conditions in the 60-V/3-A circuit is shown in Figure 2. Duration of the
arc for all tests was in the range of 2.2 to 2.7 ms, and it did not vary
more than approximately 20% during switching.

Two relays with cases removed were examined under a low-power microscope
during operation. Bright arcing was observed for both relays at the
moment of contact break. The arcing, which was reproducible, was
localized at a certain area between the contacts. The arcing was visible
even when the load resistance was increased, causing the current to
decrease from 3 A to 1.7 A, 1.15 A, and even to 0.9A (at 70 Ω). This is
consistent with the minimum arcing current of silver contacts which is
generally considered to be about 0.4A \[2\].

![](media/image2.jpeg){width="2.9972222222222222in"
height="1.3868055555555556in"}

Figure 2. Typical transient oscillogram at contact break (60 V/3 A) at
atmospheric pressure.

###### Low-Pressure Conditions

**Preliminary Experiments.** The first 60-V/3-A experiment (first
contact break), conducted with sample SN 1 (contacts B1-B2) at an
internal gas pressure of 1 torr, resulted in a blown fuse, indicating
that a flashover event had occurred. The fuse was replaced, and the
experiment was repeated with the same result (i.e., the second 10-A fuse
was blown open). Following the fuse change, the chamber valve was
opened, and testing was conducted at atmospheric air pressure. No fuses
blew during multiple switching tests. The transient current oscillograms
at contact break and the arc duration were found to be similar to those
observed during the initial test, thus indicating that no damage to the
relay had occurred during flashover events.

The experiments were continued using the A-side contacts. No failures
occurred during multiple switching at low-pressure conditions. Arc
behavior was much less stable than at atmospheric pressure, and θ varied
from 15 ms to 1.1 s. Figure 3 shows an example of the transient at
contact break in the low-pressure condition.

![](media/image3.jpeg){width="2.9972222222222222in"
height="1.3972222222222221in"}

Figure 3. Transient at break (60 V/3 A) at air pressure of 6 torr, SN 1.

Experiments with contacts B1-B2 were continued, first at normal
conditions (atmospheric pressure) and then in the vacuum chamber (6
torr). No failures occurred in the experiments conducted at atmospheric
pressure and during the first seventeen switchings at low-pressure
conditions. But the eighteenth switching blew the third 10-A fuse. The
flashover event (see Figure 4) occurred approximately 15 ms after arc
ignition (θ ≈ 40 ms). After this event, the relay failed open (it was
not latching).

![](media/image4.jpeg){width="2.9972222222222222in"
height="1.9381944444444446in"}

Figure 4. "Blowing fuse" event in SN 1 at contact break (60 V/3 A) in
vacuum chamber at 6 torr; current was measured between case and ground.

**Effect of Air Pressure.** Two samples were tested at various pressures
in the vacuum chamber at the 60-V/3-A condition. Similar to SN 1, arc
duration at low-pressure conditions was much longer, less reproducible,
and significantly noisier with spikes up to 2 A peak. No fuse blowing
occurred during these experiments. The arc-duration-pressure (θ-p)
relationship had an extremum at approximately 10 torr (see Figure 5).
Similar extremal behavior of the θ-p dependence was expected. The arc is
known to be suppressed at high and low air pressures, and these effects
are used in practical design of vacuum and high-pressure circuit
breakers. However, the range of air pressure that would cause
significant increase in θ for the JL type relays was not known. Based on
Figure 5 and assuming a 5-times increase in θ (compared to atmospheric
pressure) as significant, the low-pressure condition range can be
defined between approximately 1 torr and 200 torr.

Sample SN 2 failed (the contacts stuck closed) during experiments at 19
torr after sustaining an arc for about a second.

![](media/image5.jpeg){width="2.9972222222222222in"
height="1.9465277777777779in"}

Figure 5. Arc duration variance with pressure for 60-V/3-A condition.

E**ffect of Current and Voltage.** To avoid possible damage to the
contacts by long arc duration at pressures between 1 torr and 200 torr,
these experiments were performed first at atmospheric pressure and then
at 0.4 torr. Sample SN 3 was switched first at minimal voltage (12 V)
while changing load resistors in the sequence 50, 20, 10, and 5 Ω. Then,
the voltage was increased in 12-V increments to 60 V. The results of
measuring mean θ as a function of the short-circuit current, I, for
various power supply voltages are shown in Figure 6. A power function
was used to describe θ(I) variance:

$q = q_{0}I^{\alpha}$ (1) (1)

where α and θ~0~ are parameters. Parameter θ~0~ increases with the power
supply voltage, whereas α is relatively independent of current and
voltage (above 12 V), but it increases from 0.6 to 0.9 at atmospheric
pressure to 3.6 to 5.2 at the internal air pressure of 0.4 torr.

![](media/image6.jpeg){width="2.9972222222222222in"
height="2.1013888888888888in"}

![](media/image7.jpeg){width="2.9972222222222222in"
height="1.9590277777777778in"}

Figure 6. Arc duration at break vs. short-circuit current.

The flashover event (10-A-fuse blowing) occurred during the second
switch at 0.4 torr and the 60-V/3-A condition. The oscillogram of this
event was similar to Figure 4. This time, the flashover occurred in 5
ms, whereas the arc duration was 23 ms in length. Blowing of the fuse
coincided with failure of the relay that stuck open.

**Effect of Circuitry Inductance.** Experiments with the carbon and
wire-wound resistors showed that inductance of the 10-Ω resistors did
not significantly affect duration of the arc (θ~carbon~ ≈ 940 μs,
θ~wire-wound~ ≈ 900 μs). This suggests that circuit inductance, L, in
the range of tens of microhenries can be considered as negligible. On
the other hand, it is known \[3\], that an increase in the load
inductance above ≈0.2 mH substantially increases the arc duration (i.e.,
approximately 10 times). The following approximate calculations
facilitate estimation of the condition at which the load can be regarded
as resistive.

Energy dissipated during the arcing period θ is:

$W_{\alpha} = \int_{0}^{q}{V_{\alpha}I_{\alpha}\text{dt}}$ (2) (2)

where V~a~ and I~a~ are the voltage and current of the arc.

Analysis of the arc oscillograms suggests that arc current and voltage
in these experiments can be roughly assumed as constants that are equal
to fractions (30%-60%) of the load current and switching voltage. This
facilitates estimating W~a~ ≈ kVIθ, where k is an empirical coefficient,
k ≈ 0.1-0.4.

Obviously, if the arc energy is much larger than the energy stored in
the circuit inductance (W~L~ = 0.5LI^2^), the inductance effect can be
neglected. This gives the following equation for a "negligible"
inductance of the circuitry:

$L\text{<<}\frac{2\text{kV}q}{I} = 2\text{kR}q$ (3) (3)

where R is the load resistance.

Typically, arc duration was in the millisecond range at the 60-V/3-A
conditions. At these conditions the above estimation gives L\<\< 4 mH.
This means that inductance of the circuitry with any of the resistors
used in the experimental setup can be considered as negligibly low. The
shortest arcs (in the microsecond range) occurred at low air pressure
with minimal load currents (load resistance = 50 Ω). At this condition
the negligible inductance should be below 10 μH. As the 50-Ω resistor
had L ≈ 90 μH, this circuit inductance should affect arc duration. This
most likely explains the higher-than-expected (according to the power
law) values of arc duration in the 12‑V/50-Ω experiments (see Figure
6b).

Analysis of the low-inductance criterion (Equation 3) shows that arc
duration depends on the circuit inductance in the microsecond range of
θ. To analyze resistive-load conditions, we will consider results with θ
\> 100 μs. Besides, this range of θ corresponds to the gaseous phase of
arc where the internal air pressure is expected to affect the arcing
process \[4\].

######### Failure Analysis

Sample SN 2, which failed without the arc flashover, had a melted
movable contact (anode) that was fused to the stationary contact.
Appearance of this damage was typical for a contact failure caused by
excessive power dissipation (excessive arc duration in our case). A
polymer cap (arc suppressor), which isolated the contact area from the
case, had evidence of severe overheating and burning, confirming
excessive power dissipation during arcing.

Sample SN 1 had a melted armature which had developed a bump in the area
above the post X2. This post and the welded coil wire were partially
melted (see Figure 7). Appearance of the damage suggests that flashover
occurred between the armature (connected to the movable contact) and the
grounded coil post. The damaged site was approximately 8 mm from the
normal location of the arc (between the silver contacts). However, the
polymer cap had dark burned spots indicating overheating at the silver
contacts, and both (A and B) movable contacts had evidence of excessive
erosion. These defects most likely were caused by excessive power
dissipation during relay switching under low-pressure conditions.

Internal examination of SN 3 revealed a melted post with a fused spring
and melted glass at the periphery of the glass seal around the
stationary contact. Figures 8 and 9 show damaged sites in this relay.
The damage appearance suggests a breakdown (arc flashover) between the
post connected to the armature and the header. The damaged area is at a
distance of approximately 5 mm from the silver contact location. The
polymer cap had a dark burned edge near the stressed contacts.

![](media/image8.jpeg){width="2.9972222222222222in"
height="2.329861111111111in"}

Figure 7. SN 1 failed at 60-V/3-A circuit break in vacuum chamber at 6
torr due to a flashover between the armature and the grounded coil post.

![](media/image9.jpeg){width="2.9972222222222222in"
height="2.313888888888889in"}

Figure 8. SN 3 failed at 60-V/3-A circuit break at an air pressure of
0.4 torr due to a flashover between the post connected to armature and
the case. The black area on the case is an arc track. A white arrow
indicates damage to the glass seal (see below).

![](media/image10.jpeg){width="2.9972222222222222in"
height="2.1527777777777777in"}

Figure 9. Close-up view of the melted glass seal shown above.

####### Discussion

It is believed that flashover is caused by the ionized gas diffusing
from the between-contacts arc column to the surrounding case or header
\[1\]. This diffusion can initiate a gas breakdown between the contact
and the case, and although it may not cause damage to the relay, a
failure of the circuit is quite possible. In this sense, a similar event
can be considered as a relay failure.

Types of gas molecules and gas pressure significantly affect the arcing
process. Data reported by Holm \[2\] indicate that the arc can be
sustained to much wider contact separations at low air pressures (10
torr -- 100 torr) and that arc durations may be correspondingly longer
than at normal atmospheric conditions. Also, arc behavior changes at low
pressures. The arc column becomes wider, its temperature decreases (at
pressures below approximately 10 torr), and the longitudinal potential
gradient of the arc in inert gases has a minimum at approximately 2 torr
\[5\].

Our experiments showed that flashover failures occurred within several
milliseconds after switching. This indicates that duration of the arc
may have a significant effect on the probability of flashover. Arc
duration is governed by a complex processes taking place in the gaseous
phase of the plasma.

At relatively low electrical switching levels (I\<3 A and V\<50 V), the
gaseous phase of arcing, which follows the metallic phase, starts after
200-300 μs, when the contact separation reaches approximately 100 μm
\[4\]. The process is controlled by electron emission from the cathode
spots which ionizes gas molecules and thus sustains a plasma. The
cathode spots are in permanent chaotic movement from site to site along
the electrode surface (probably due to the silver oxide decomposition
being caused by the high-temperature plasma \[6\]). This shifting of the
cathode spot constitutes a weakening and subsequent restoration
(reignition) of the arc \[2\]. The reignition process is attended by
plasma bursts that cause spikes and noise in the arc current. Similar
plasma bursts can probably ignite the gas breakdown causing flashover
failure provided the cathode spot is in a suitable location and the
burst is powerful enough. Acceleration of the cathode spot motion as gas
pressure decreases \[6\] may also increase the probability of finding a
suitable location to initiate this breakdown.

According to this model, an increase in arc duration increases the
probability that the plasma burst (which initiates the breakdown) will
occur. Correspondingly, the same factors that cause an increase in arc
duration are favorable for flashover failures.

It should be noted that an increase in θ increases the energy dissipated
in the arc and correspondingly enhances the probability of a "regular"
failure event (such as contact erosion and microwelding). This may
result in a "regular" failure occurrence before the flashover event.

###### Arc Duration

Arc duration is mostly considered to be a function of the short-circuit
current, I. Two types of arc-duration-versus-current laws are usually
used to represent θ(I) functions: an exponential law and a polynomial
law \[7\]. Although none of the functions has received the necessary
theoretical justification, the exponential function is mostly used to
describe processes in the metallic phase of arcing \[4, 8\], whereas a
power function better describes arcing in the gaseous phase. Our
experimental data, which are presented in Figure 6, seem to confirm this
conclusion.

An increase in power supply voltage, V, is also known to increase θ, but
θ(V) dependencies have not received much consideration. The minimal
voltage that is necessary to start acing in relays is about 10 V \[2\].
This is explained by the fact that to start ionization at contact break,
electrons must gain sufficient energy to overcome the energy barrier in
the metal electrode and to ionize the metal vapor. As the work function
for silver is ϕ = 4.3 eV (for silver oxide, it is less, ≈ 1-1.2 eV) and
ionization potential of silver atoms (vapor phase) is V~i~ = 7.5 V, the
minimum voltage necessary to start an arc is V~min~ ≈ ϕ/e + V~i~ ≈ 9-12
V. Ionization of the gas atoms and molecules requires more energy, so
it, correspondingly, occurs at higher voltages. For example, the V~i~
values for oxygen and nitrogen molecules are 12.1 V and 15.6 V,
respectively. Ionization of helium requires 24.5 V. Several tens of
electron-volts are required to strip electrons from internal levels of
atoms and molecules. Ben Jemaa and co-workers \[4, 8\] have proven that
the arc voltage in the gaseous phase goes through several stages with
voltage plateaus that are specific for various types of gases and that
are related to their ionization potentials. This mechanism explains
changes in the θ(I) relationship when the power supply voltage was
increased from 12 V to 24 V (see Figure 6). The increase in voltage
intensifies gas ionization, facilitates the arc-reignition process, and
correspondingly increases arc duration.

In the range of currents and voltages used, an increase in both external
parameters of the arc, V and I, resulted in a rise of θ, with both
parameters having a similarly appreciable effect. This implies that a
switching power, P=IV, may be used as a single parameter relating to arc
duration. Figure 10 shows the θ(P) variance at atmospheric pressure and
at 0.4 torr.

![](media/image11.jpeg){width="2.9972222222222222in" height="1.84375in"}

![](media/image12.jpeg){width="2.9972222222222222in"
height="2.120138888888889in"}

Figure 10. Arc duration variance with switching power at various
internal pressures in the relays.

It is apparent that θ is relatively independent of load resistance and,
correspondingly, of the power supply voltage. At θ \> 100 μs, the θ - P
relationship can be approximated with the power law:

$q = \text{AP}^{n}$ (4) (4)

where A and n are constants.

Calculations gave n = 2.8 for the low-pressure condition and n = 1.4 for
the atmospheric-pressure condition. At the low-pressure condition θ
increases with power much faster, and above approximately 80 W, arc
duration exceeds the corresponding values for the normal-air-pressure
condition.

###### Internal Gas Pressure in Leaking Relays

A decrease in the internal gas pressure with time in a leaking relay can
be approximated by an exponential function:

$p = p_{0}\text{exp}\left( - \frac{t}{t} \right)$ (5) (5)

where p~0~ is the initial (atmospheric) pressure, τ = p~0~V/L, V is the
relay volume (cm^3^), and L is the standard air leak rate (atm•cm^3^
/s).

The internal free volume, V, of the relays was estimated at
approximately 3.6 cm^3^. Time dependency for the internal gas pressure
in relays with air leak rates in the range from 10^-5^ to 10^-11^
atm•cm^3^/s are shown in Figure 11.

The low-pressure condition (below 200 torr) can be reached in less than
10 years if the leak rate exceeds 10^-8^ atm•cm^3^/s of air. The value
of 1x10^-8^ atm•cm^3^/s of air is used as a rejectable limit for relays
with internal volume of less than 33 cm^3^ (2 in^3^) (per MIL-R-6106).
Although this limit is much more severe (more than two orders of
magnitude) than the one that is used for microcircuits, it is justified
in the light of the described low-pressure condition failures.

The helium leak test is usually used to confirm the required level of
the relay hermeticity. With the standard air leak rate, L, the measured
leak rate of the tracer gas (He), R~1~, can be calculated using the well
known formula \[see, for example, 9\]. Calculations showed that to
provide the standard leak rate L = 1x10^-8^ atm•cm^3^/s of air, the
measured leak rate for the part, R~1~, should be less than 1x10^-11^
atm•cm^3^/s of He. This value is near the detection threshold of regular
helium leak detectors. Besides, in many instances helium desorption from
paint on the relay masks a real leak from the case, thus giving an
effective leak rate of more than 1x10^-8^ atm•cm^3^/s of He. If the real
(from the case) leak rate is approximately 1x10^-8^ atm•cm^3^/s of He,
then the corresponding standard air leak rate would be 3x10^-7^
atm•cm^3^/s, and the low-pressure condition would be reached in a few
months.

These estimates show that in real life, use of a leaking relay is quite
possible despite stringent screening of parts intended for space
applications. Such a relay may cause a circuit failure provided the time
of its operation in vacuum is within certain limits such that internal
pressure is below ≈200 torr, but above ≈0.3 torr. A longer vacuum
operation time further decreases air pressure and, thus, reduces the
possibility of the arc-related failures.

![](media/image13.jpeg){width="2.9972222222222222in"
height="1.9916666666666667in"}

Figure 11. Calculated gas pressure in a leaking relay in vacuum; Leak
rate, L, varied from 10^-5^ to 10^-11^ atm•cm^3^/s of air.

####### Conclusions

1\. A decrease in internal air pressure in relays significantly
increases the probability of failures caused by arc flashover and/or
contact damage. Under favorable conditions, a flashover can occur even
at the first relay switching (contact break), thus resulting in a
single-event catastrophic failure.

2\. For the examined relays rated for 12-A/28-V service, flashover
failures occurred while breaking a 3‑A/60‑V resistive-load circuit at
pressures between 6 torr and 0.4 torr. The gas breakdown (flashover)
occurred 5 to 15 ms after arc ignition. Damage appeared between the
armature and grounded coil post or relay header at distances 5 to 8 mm
from the contacts.

3\. Arc duration significantly increases when internal pressure
decreases, reaching a maximum at approximately 10 torr (from 2 to 3 ms
at atmospheric pressure to timespans of seconds at low pressure).
Excessive arc duration (more than 20 to 30 ms) at low-pressure
conditions (between approximately 200 torr and 1 torr) resulted in
excessive erosion and/or microwelding of electrodes and in overheating
of polymer caps (arc suppressors).

4\. The power supply voltage (in the range 24 V to 60 V) was found to
have a similarly significant effect on arc duration as did the
short-circuit current (in the range 0.5 A to 3 A). Within these ranges,
arc duration mostly depends on switching power, and at P \> 10 W, arc
duration can be approximated with the power law: θ = AP^n^ with the
power coefficient n being larger for the low-pressure condition (n = 2.8
at 0.4 torr) than for atmospheric-pressure condition (n = 1.4). Above
approximately 80W, arc duration at the low-pressure condition is longer,
correspondingly increasing the probability of the relay failure.

5\. Increased arc duration and flashover failures suggest that the
internal air pressure range from 0.3 torr to 200 torr can be considered
as a high-risk region. Estimates showed that internal pressure in relays
with leaks greater than 1x10^-8^ atm•cm^3^/s of He can reach this
critical region in less than a year when operating in vacuum.

####### Acknowledgements

The author would like to thank John Slonaker, Scott Hull, Frederick Felt
and Steve Esmacher for useful discussion and for review of this
manuscript. The reported work was performed at Goddard Space Flight
Center Parts Analysis Lab, and it was supported by the Office of Systems
Safety and Mission Assurance (OSSMA, code 562).

####### References

1\. J. Lawrence, *Application notes for preferred parts and materials*,
NASA, GSFC (1965).

2\. R. Holm, *Electric Contacts Handbook*, p. 309, Springer Verlag,
(1958).

3\. N. Ben Jemaa, L. Nedelec, S. Benhenda, *IEEE Trans. on CPMT*., part
A, v. 19, 82 -- 86 (1996).

4\. N. Ben Jemaa, J.L. Queffelec, D.Travers, *IEEE Trans. on CHMT*, v.
14, 113 -- 117 (1991).

5\. J.M.Somerville, *The electric arc*, p. 19, John Wiley & Sons Inc.,
NY (1959).

6\. I.G.Kesaev, *Cathode process in the mercury arc*, p. 49, Consultants
Bureau, NY (1964).

7\. *Handbook of vacuum arc science and technology*, edited by
R.L.Boxman, D.M.Sanders, P.J.Martin, p. 116, Noyes Publications, New
Jersey (1995).

8\. N. Ben Jemaa, J.L. Queffelec, R.Haug, *J.Phys. D: Appl.Phys*., v.20,
468-475 (1987).

9\. *Electronic materials handbook*, v. 1, p. 1063, Packaging, ASM
International (1989).