New contact material for reduction of arc duration for dc application

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New Contact Material for Reduction of Arc Duration for DC
New Contact Material for Reduction of Arc Duration for DC New Contact Material for Reduction of Arc Duration for DC
New Contact Material for Reduction of Arc Duration for DC
Application
ApplicationApplication
Application
L. Doublet1, a, N. Ben Jemaa2, S. Rivoirard3, C. Bourda1, E. Carvou2, D. Sallais2, D. Givord4, P. Ramoni5

1 Metalor Technologies (France) SAS, Electrotechnics Division, BP 29, 28190 Courville-sur-Eure, France
2 University of Rennes 1, IPR , 35042 Rennes, France
3 CRETA, CNRS, 25 avenue des Martyrs, 38042 Grenoble, France
4 Institut Néel/CNRS-UJF, 25 avenue des Martyrs, 38042 Grenoble, France
5 Metalor Technologies SA, Technology Products Department, 2009 Neuchâtel, Switzerland


a Laurent.doublet@metalor.com
Abstract : The phenomenon of arcing is the major cause of electrical contact degradation in electrical
switches. Degradation involves contact erosion and/or welding. The use of special contact material and
that of specific material processing may permit contact erosion to be reduced, in particular by
shortening the arc duration. A short review of these approaches is presented in the first part of this
paper. In the second part, the development of a new self-blowing contact material is described. This
material has been tested under DC voltages from 14 V to 42 V. A reduction of the arc duration by a
factor of 4 approximately was obtained as was a concomitant reduction of the extinction gap to less
than 2 mm. This material will contribute to achieving better reliability in high current-high voltages
breaking devices, and will aid in their miniaturization, e.g. in relays.

1. Introduction
The technical and economic reasons to increase the
power levels of electrical circuits have been discussed in
numerous papers. The associated voltage and/or current
increase supported by the circuit results, in turn, in an
increase in the arc duration, and subsequent extinction
gap, during the breaking operation. Breaking devices
have been developed having original designs or
conception, which permit reduction in the arc duration,
with consequent reduction in its deteriorating effect. In
this paper, we show that it may also be possible to
introduce an innovative material which generates an
intrinsic magnetic field in the extinction gap, thus
promoting self-blowing of the arc and a consequent
reduction in its duration. After a brief review of what is
an arc and well-known solutions (mechanical, electronic,
material, …) to reduce it, we present a self blowing
material as a possible answer.
1.1 Why break higher electrical power levels
with usual break devices?
The manufacturers of breaking devices (CB, relays,
etc…) have cost reduction policies and try to reduce the
range or size of their apparatus. One consequence is that
each device must have a wide breaking capacity.
Therefore, it is subjected to longer electrical arcing,
which are more damageable for contacts materials.
Moreover, these devices are submitted to increasingly
severe constraints, such as reduced sizes and weight, in
order to limit fuel consumption in automotive or
aeronautic applications. The automotive sector also
seeks to raise the voltage of the circuit on board to
increase the capacity of electrical power and to supply a
growing number of electrical devices. All these changes
submit breaking devices, and especially the electrical
contacts materials, to more severe electrical conditions.
1.2 What are the consequences of an increase
in current and / or voltage on breaking devices
When the current is switched off by a breaking
device, it generates an electrical arc which is broadly
neutral plasma consisting of ionized species coming
from the contacts material and from the surrounding
environment (atmosphere, plastic parts…). It is a
dynamic and unstable
characteristics of this arc (duration and gap extinction,
arc voltage and arc current) depend on the parameters of
the electrical circuit, the parameters of the opening
system (speed, design) and the nature of the elements
close to the arc plasma which are able to come into the
plasma and modify its composition (contact material,
nitrogen or oxygen from atmosphere or materials from
the case).
Figure 1 shows the arc duration curves for silver
contacts versus wide current range (0.1 to 130A) at 14,
42, 112 and 360VDC in resistive load. These results
were obtained with the experimental electrical testing
phenomenon. The main

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device described in previous papers [4] and briefly
reminded §2.1.2. Each curve has two parts: a first
segment obeys to a power law with a high slope I4 and is
followed by a flatten section. Let us consider the curve
for 42VDC. Up to 2ms, the arc duration versus current
characterizes an arc in metallic phase (regime 1 or
anodic arc) whereas at longer time it characterizes an arc
in gaseous phase (regime 2 or cathodic arc) [5,6]. These
effects were extensively discussed
publications. It is interesting to note that except at
14VDC, the slopes of the first regime are parallel the
ones to the others. It can be concluded that the same
phenomena determine the arc behaviour in the whole
voltage range from 42VDC to 360VDC.

in previous
Current (A)
0.1110100
Arc Duration (ms)
1e-5
1e-4
1e-3
1e-2
1e-1
1e+0
1e+1
1e+2
1e+3
14V
42V
112V
360V
Fig. 1: Arc duration versus current in a range from 0.1 to up to 100A at
four circuit voltage (14, 42, 112 and 360VDC)
These results show that the arc durations can reach
important values needing a wider extinction. If the
contact gap, imposed by the design of the device, is
smaller than the natural extinction gap of the arc, it may
stagnate almost indefinitely until the contact materials
totally melt. That can lead to the destruction of the
device and cause fires for its direct environment. Beside
this, when the natural extinction gap of the arc increases
(because the arc is getting longer), the phenomena of
transfer between contacts pass through the above
mentioned anodic and cathodic regimes and thus,
material loss is becoming increasingly large [7-11].
Due to these various reasons, if the next generations
of breaking devices have to withstand harsher electrical
arcs, they should be equipped with arc reducing (or
suppression) systems.
1.3 What
suppression systems?
are the most common arc
By modifying certain parameters of the breaking
device, it would be possible to control the arc duration
and gap extinction. The electrical parameters of the
circuit being imposed we can only act on the physico-
chemical parameters of the breaking device. The
following (but non-exhaustive) list enumerates such
actions and their principles that help to minimize the
consequences of arcing.
1.3.1 Atmosphere
The atmosphere of the breaking can be ambient air,
vacuum or under a protective gas, such as SF6.
Alternatively, the contacts environment may be filled
with oil. In the earlier moments of the arc, the plasma
consists of species that come from the ionization of the
contact material. The pressure of the plasma is
tremendous (several tens of bars). As the contacts
separate, the pressure decreases and the gas from the
atmosphere enters into the plasma and becomes ionized.
This supplementary source of electrons maintains the
electrical conductivity of the plasma and slows down the
decrease of the current. Depending on the nature of the
atmosphere, its components are more or less ionizable:
specific atmospheres other than ambient air are
sometimes used because they promote the extinction of
the arc by providing a lack of electrons (ex: SF6). Under
vacuum, the contact material only feeds the plasma.
Therefore, its intensity decreases as the two contacts are
moving away. In this case, the gap extinction and arc
duration are reduced.
1.3.2 Kinematics
It is well admitted that for a resistive AC or DC
circuit, increasing the speed of contacts separation (e.g.
for certain appliances, by changing the stiffness of the
springs), reduces the arc duration and associated
damages as well. Various physico-chemical processes
compete during the opening of the contacts. Some of
them act in favor of the stabilization of the current in the
plasma and other work against this state of equilibrium.
Beyond these phenomenon, opening the contacts
contribute to the decrease of the current by increasing
the plasma volume. The faster the separation, the
stronger the current decrease. However, care should be
taken in inductive load where the time constant of the
inductance plays an opposite role and may increase arc
duration at high speed [4,12]
1.3.3 Horn arc contacts or sacrificial contacts
These techniques reduce erosion on the main
contacts. They limit their degradation and ensure good
conduction (at the closed state) by limiting their
temperature rise. Arc duration is not reduced but arc is
deported in areas designed to withstand it.
1.3.4 Arc chamber
As it is the case in the circuit breaker, the
developing arc moves to an arc chamber consisting of
splitting plates where it is divided into multiple serial
arcs. This promotes the arc voltage to increase and leads
to rapid extinction.
1.3.5 Double break
In the design of the breaking devices, it is not
uncommon to connect two pairs of contacts in series that
will open simultaneously and will form two electric arcs.
The energy of a single arc is almost divided by two and

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the damages are significantly reduced on the electrical
contacts.
1.3.6 Electronics systems
It is possible to implement electronic systems,
which detect arc ignition and provoke earlier arc
extinction in the breaking devices. Some solid or
electromechanical relays have been suggested in the
literature [13].
All these specific features introduced in the design
of devices involve the addition of arc suppression
systems and/or a sealed box to contain atmospheres.
They have a significant impact in terms of cost, size and
weight.
1.3.7 Material
The choice of specific contact materials is
complementary to the development of arc suppression
systems. The influence of the contact material, though
limited, reduces the extinction gap or the arc erosion
regarding to the circuit parameters (AC/DC, voltage or
current level), also the application of the circuit
(protection or control). Because the influence of contact
material occurs at the beginning of the arc, it is easy to
compare different materials according to electrical
circuit conditions.
In figure 2, six materials were tested on the
laboratory bench described in paragraph 3.1.2. The
experimental conditions were the following: 42VDC,
20cm/s, and resistive load. For our comparison, 10%
weight MeO were added to silver. We see that AgSnO2
has yielded the longest arcs, with a strong shift from
25A, while the AgZrO2 arc durations are lower. Besides
AgSnO2, other materials in this current range, such as
AgNi and AgZnO, show a linear increase of the arc
duration versus current. The additions of Gd2O3 and
ZrO2 are good alternatives to the today widely used
AgSnO2 [14].
Current (A)
010203040
Arc Duration (ms)
0
2
4
6
8
10
12
14
16
18
20
22
24
Extinction gap (mm)
0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
4.4
4.8
AgSnO2
Ag
AgNi
AgZnO
AgZrO2
AgGd2O3
AgZrO2
AgSnO2
Fig. 2: 42VDC resistive, t=f (I), Ag, AgNi, AgSnO2, AgZnO,
AgGd2O3, AgZrO2
1.3.8 Magnetic Blowing
On some breaking devices, which should switch
strong currents, magnetic blowing systems may be used.
Most often, it is realized by introducing a serial coil on
the circuit (close to the area of arc). Current flow in the
coil produces a magnetic field oriented perpendicular to
it and ensures arc evacuation from the contact area by
producing a force orthogonal to both other directions,
according to Lorentz law. Such magnetic blowing
systems have been developed both for DC and AC
applications depending on the targeted breaking
capacity.
Magnetic Field (mT)
01020 3040
Arc Duration (ms)
0
2
4
6
8
10
12
Extinction Gap (mm)
0.0
0.4
0.8
1.2
1.6
2.0
2.4

Fig. 3: Arc duration and extinction gap versus external magnetic filed
for Ag at 42VDC, 37.5A at 20cm/s
Figure 3 shows the variation of the arc duration and
extinction gap versus a uniform external magnetic field
perpendicular to the current flow. These results were
obtained on the electrical test device described in
paragraph 3.1.2, on silver contacts at 20cm/s under
37.5A. Two magnets produced the magnetic field and it
varied by changing the gap between the two magnets. Its
value was measured using a Hall probe (sensitivity
around +/- 0,2 mT) positioned in the magnet gap. In a
second step, the Hall-effect probe was replaced by
electrical contacts to obtain the data shown in fig. 3.
Figure 3 shows that the arc duration at 0mT is in
agreement with data in fig 2. The arc duration decreases
drastically as the magnetic field is increased. At 10mT it
is already reduced by a factor of more than 2. At longer
time, it tends towards a limit value of about 1 ms.
2. Self-blowing material
The purpose of this part is to present the study on
the characteristics of a break arc, exploiting the self-
blowing properties of a magnetic phase, introduced into
the contact material. This new material permits higher
breaking capacity to be reached than traditional materials
without any change in the device design.

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2.1 Equipment and procedure
2.1.1 Contact
magnetic characterization
Cylindrical samples with a diameter of 5 mm and a
height of 5mm were prepared by conventional powder
metallurgy by mixing silver powder with magnetic
material (for confidentiality reasons, we will not indicate
here the physico-chemical the properties of the material
used: nature, shape and mass content).
The samples were magnetized at room temperature
in a super-conducting magnet with a maximum applied
field of µ0H=7T (Oxford Company). The filed was
applied along in the direction perpendicular to the
cylinder axis. Figure 4 shows the characteristic
hysteresis cycle of the phase magnetic used in our
electric contact material.
Material preparation and
-1.5
-1
-0.5
0
0.5
1
1.5
2
-4-202468 10
µoH (T)
M (T)

Fig. 4 : M=f(µ0H) for magnetic phase in electrical contact material
After magnetization, the surface induction of each
sample, Bs was measured with a Hall probe (Lakeshore
Company) placed at 1mm above the surface sample. The
samples were magnetized at three different levels
corresponding to surface inductions of 30, 55 and 100
mT respectively, as determined by the Hall probe. The
samples were called AgMM1, AgMM2 and AgMM3,
respectively.
2.1.2 Electrical characterization by break arc
duration
The PALMS Laboratory of Rennes is equipped
with a fully automated device (fig. 5), adapted to
perform contact breaking under 14 VDC up to 360 VDC.
This device has been described in details in previous
papers [4,5]. In this paper, most experiments were
performed under 42VDC and with a resistive load, at
opening speed of 20cm/s, i.e. under conditions similar to
those of automotive relays. Voltage, current, loads and
opening speed were controlled, and a digital oscilloscope
stored voltage and current characteristics of the arc
during the break. The arc parameters, namely arc energy,
arc duration and extinction gap were thus deduced.

a r c
O s c i l l o s c o p e T D S
4 2 V D C
Current probe
Arc Current
Arc Voltage
Resistance Inductance

Fig. 5: Electrical test apparatus for break arc
For all the experiments, the magnetic material
AgMM was placed at the cathode and pure silver at the
anode: figure 6 shows the position of the cathode and the
anode in the electrical tests. The respective directions of
the current, I, the magnetic induction of the sample, B,
and the resulting Lorentz force, F are indicated in the
fig.6.

Fig. 6: Sample geometry used during the electrical tests
2.2 Results and discussion
Figure 7 shows the evolution of arc voltage versus
time under 37,5A, 42VDC and a resistive load for the
following contacts materials: AgSnO2, pure Ag and
AgMM3. The arc voltage increases slowly with time for
AgSnO2 and a little faster for silver. The total arc
duration are 24ms and 8ms respectively. By contrast,
with AgMM the arc duration is reduced down to 3ms
only. The reduction with respect to the case of silver
contact illustrate the blowing effect of the magnetic
field.
Tim e (m s)
-4 -2 02468 10 12 14 16 18 20 22 24 26 28 30
Arc Voltage (V)
0
4
8
12
16
20
24
28
32
36
40
44
48
Ag-M M
Ag
AgSnO 2
Arc D uration

Fig. 7: Arc voltage versus time for AgSnO2, Ag and AgMM3 at
42VDC, 37.5A resistive
Cathode Anode

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Figure 8 shows the evolution of the extinction gap,
as a function of the current, for pure silver contacts on
the one hand and silver magnetic contacts on the other
hand. The three magnetic materials differ only by the
magnetic field they produce. The stronger the magnetic
field, the lower the arc duration. Optimum properties are
obtained with the arc length of AgMM3 which is the
contact producing the highest magnetic field. Under
37,5A, the arc duration is reduced down to 2,4 ms,
72,5% less than with pure silver. For AgMM1, which
produces the weakest magnetic field, the blowing effect
starts around 10A only. In the case of AgMM2 and
AgMM3, the blowing effect begins earlier. The reason is
that for higher magnetic field, the curved radius of the
trajectory of an electron is smaller.
Current (A)
0 10 203040
Extinction gap (mm)
0.0
0.4
0.8
1.2
1.6
2.0
Arc Duration (ms)
0
2
4
6
8
10
Ag
Ag-MM3
Ag-MM2
Ag-MM1
Ag
AgMM3
AgMM2
AgMM1
Fig. 8: Arc duration versus Current for Ag, AgMM1, AgM2 and
AgMM3 at 20cm/s 42VDC resistive
Figure 9 shows the evolution of the arc duration as
a function of the magnetic field. The value at 0mT does
correspond to the arc duration for pure silver.
Qualitatively, the curve is similar to the one shown on
fig 3. The magnetic field scales do not correspond one to
the other. This is simply due to the fact that the field
measured on the contact surface with the Hall probe (fig
9) is not the field measured in the gap (fig. 3).
It remains that with AgMM3 a spectacular
reduction in the arc duration is obtained at 2,4 ms which
corresponds to ~480 µm of extinction gap (v=20cm/s).
All together, the use of magnetic contact permits
arc blowing through the Lorenz force, as it is the case in
the classical systems using permanent magnets or coils.

M agnetic Field (m T)
0 20 406080100
Arc Duration (ms)
0
2
4
6
8
10
Extinction Gap (mm)
0.0
0.4
0.8
1.2
1.6
2.0
Ag
AgM M 1
AgM M2
AgMM 3

Fig. 9: Arc duration versus Magnetic field for Ag, AgMM1, AgMM2
and AgMM3 at 20cm/s 42VDC, 37.5A at 20cm/s
Figure 10, shows arc durations versus number of
operations at 42VDC & 37.5A for a resistive load.
Results are compared for pure silver and the three types
of magnetic contacts. In all cases, the average arc
duration shows a slow decrease during the first thousand
operations. Nevertheless, the efficiency of magnetic
blowing was preserved up to final testing. This is very
encouraging despite the fact that the maximum 6000
operations considered here is much less than the millions
of operations, which a relay must usually undergo. It is
also noticeable that the dispersion in arc duration is
larger for pure silver than for all magnetic contact
material including AgMM1, which produces the weakest
magnetic field.
Operation (X1000)
0123456
Arc Duration(ms)
0
2
4
6
8
10
12
Ag
AgM M 1
AgM M 2
AgM M 3

Fig. 10: Arc duration versus number of break operations for Ag,
AgMM1, AgMM2 and AgMM3 at 20cm/s 42VDC, 37.5A at 20cm/s

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3. Conclusions and perspectives
A new contact material has been developed which
generates an intrinsic magnetic field in the extinction gap
and leads to much faster current breaking. With fully
magnetized materials (generating the largest magnetic
field in our conditions), the average arc duration was
reduced by a factor of 4 compared to that obtained with
silver contacts. The reduction in the arc duration was
already very significant with non fully-magnetized
materials. The decrease in the extinction gap under field
tends towards a non-zero limit value [extrapolated to
around 200µm (and 1ms) from Fig. 3]. This implies that
below a certain minimum distance the magnetic field
blowing effect disappears (in our conditions). The
dispersion in the blowing efficiency was characterized
over at least 6000 operations, in the case of fully-
magnetized materials. It is already planned to
characterize further the material limits by increasing the
severity of all electrical parameters during testing.
It is hoped, on the one hand, that the reduction in
the extinction gap obtained with this new material will
permit breaking device miniaturization. On the other
hand, the reduction in the arc duration should lead to a
strong reduction in contact erosion. Thus, an increase in
the device lifetime and a reduction in the volume of
contact material may be expected, leading to significant
cost reduction.
Further studies have started, addressing the
mechanism of arc breaking and the process of material
erosion, as well as arc initial formation, contact heating
and welding. The contact itself is being further
optimized with respect to its shape and nature.
Finally, as the blowing efficiency is also dependent
on the environment of the contact material, testing of this
new material in real breaking devices is programmed.

Acknowledgments : We are grateful to Justine Bernard
for the samples preparations as well as the "magnetic"
measurements.

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