CONTACT LIFE OF VACUUM CIRCUIT BREAKERS
Paul N. Stoving
Senior Advanced Design Engineer
John F. Baranowski
Staff Engineer
Kyle Distribution Switchgear
Cooper Power Systems
South Milwaukee, Wisconsin
Thomas A. Edison Technical Center
Cooper Power Systems
Franksville, Wisconsin
Introduction
Purchasing a recloser or a circuit breaker is an investment. Due to concern about this investment, many utilities
have had a growing interest in the number of times that the contacts are capable of interrupting fault current.
While many variables must be considered, one key measurement is the amount of wear that may occur to the
contacts. Certain technologies, such as vacuum circuit breakers and specifically axial-magnetic field vacuum
circuit breakers, have drastically increased the available life of switchgear. In response to this, the manufacturers
of vacuum switchgear have initiated several methods of testing this life. These methods fall into various categories:
a) mechanical life: The number of no-load operations that the mechanism and contacts can operate with no
undue wear. This is typically on the order of 10,000 operations. However, there is no current flowing
through the contacts at the time, so the only loading is purely mechanical.
b) mechanical life with ac voltage: This is similar to (a), but with some amount of ac voltage (typically either the
device’s rated voltage, about 15 to 38 kV, or a corresponding ac withstand voltage, about 50 to 70 kV), over
the contacts. This is usually done as a diagnostic–if the device ceases to withstand voltage, then it has lost
vacuum. The number of operations is still typically on the order of 10,000. However, there is usually a currentlimiting resistor in series with the test unit, so the amount of current flowing though the device is negligible.
c) switching with load current: The contacts are opened and closed with ac voltage and some current available, but the current is limited to load current levels (about 400 to 800 A). The number of operations is
smaller than before, typically about 2500 operations. Sometimes a test similar to a duty cycle (as described
below) is performed, where there are several different percentages of rated current used. However, these
percentages are based off of load current instead of fault current levels. Then the number of operations
may be higher, around 4000.
d) interruption with fault current: The contacts are opened and closed with ac voltage and fault current available. Typically, a duty cycle is followed, such as specified by ANSI Standard C37.60 [1]. In order to determine interruption life at 100% of rated fault current, the lower current interruptions are usually extrapolated into full ratings. This is discussed further in the following section.
e) extrapolation to full life: This is similar to (d), but the final contact erosion is measured. After a duty cycle,
a typical measured contact erosion is on the order of 0.1 to 0.5 mm. Based on allowable contact erosion
(about 3 mm), this is then extrapolated to a higher number of operations, typically on the order of 500 rated
fault level interruptions. This is usually valid only under certain conditions.
The difficulty lies in drawing correlations between these different methods of measurement. If one is used to
receiving life data from one method, others can be misleading. A standardized means of measuring interruption life would be desirable.
Duty Cycles and Extrapolation
A typical operating duty for a vacuum recloser as defined in ANSI Standard C37.60 consists of 116 fault current interruptions at three levels: 16 interruptions at full current, 56 interruptions at 50% of full current, and 44
interruptions at 20% of full current. This is actually defined as representing the "half life" of the contacts, where
full life would be represented by completing two such operating duties.
ANSI Standard C37.61, Appendices B and C [2] explain the derivation of the operating duty and a method for
extrapolating this duty to equivalent operations at only one fault level. While this derivation is intended for oilfilled units, it can serve as a starting point for vacuum contacts.
1
Contact Life of Vacuum Circuit Breakers
Using two duty cycles, or a full life duty on a 12.5 kA unit as an example, the total duty factor can be derived
as follows:
(12500 A)1.5
(6250 A)1.5
(2500 A)1.5
Duty Factor:
x
x
x
32
112
88
232
Operations
Operations
Operations
Operations
=
=
=
=
44.7
55.3
11.0
111.0
x
x
x
x
106
106
106
106
(1)
Then this total duty factor can be converted back to equivalent interruptions:
111.0 x 106 ÷ (12500 A)1.5 =
80 Operations at 12.5 kA
(2)
Note that for oil units, there is an exponent of 1.5 that is used for weighting the current levels. For vacuum
interrupters, if the arc between the contacts is in the diffuse mode, which is true for low current levels, such as
switching load current or small fault levels, or if the contact is of the axial-magnetic field type structure, then
an exponent of unity has traditionally been used. Therefore, the proceeding duty would be recalculated as:
(12500 A)
(6250 A)
(2500 A)
Duty Factor:
x
x
x
32
112
88
232
Operations
Operations
Operations
Operations
=
=
=
=
400
700
220
1320
x
x
x
x
103
103
103
103
(3)
As before, this total duty factor can be converted back to equivalent interruptions:
1320 x 103 ÷ (12500 A) =
106 Operations at 12.5 kA
(4)
This is summarized by Figure 1, which plots calculations (2) and (4) for different current levels. However, it should
be noted that neither of these takes other factors into account, such as arcing time, frequency, or asymmetry.
Expected Number of Operations
10000
EXP = 1.5
EXP = 1.0
1000
100
10
0
4
8
Fault Level (kA)
Figure 1.
Estimated life curves based on a duty cycle.
2
12
16
Contact Life of Vacuum Circuit Breakers
Time-Integrated Current
The authors recommend, as a common ground for comparing vacuum interrupters that arc in the diffuse mode,
the usage of time-integrated current for measuring contact life [3]. This is equal to the amount of current the
contacts have interrupted integrated during the time that arcing between the contacts occurs. The unit of this
measurement is A·s, or, usually more conveniently, kA·s. For a 12.5 kA rms excitation, if the contacts part just
after a current zero, as shown in Figure 2a, the arc conducts for a half cycle of fault current, until the next current zero. At 60 Hz, the time-integrated current would be:
.0083
I · s=∫
0
12.5 · √2 · sin(377 · t) dt
(5)
= 0.094 kA · s
If the contacts do not open until the current peak, as shown in Figure 2b, then there is only a quarter cycle of
arcing, resulting in:
.0083
I · s=∫
.0042
12.5 · √2 · sin(377 · t) dt
(6)
= 0.047 kA · s
A
B
Gap
Gap
Contacts
Part
Contacts
Part
Time
Current
Time
Current
Figure 2: a, b.
Arcing time.
Clearly this number will depend on mechanism characteristics. However, if assuming that the time that the
mechanism begins to open is random and that all arcs will clear at current zero, the average parting moment
will be at current peak. Likewise, it can be assumed that asymmetry should cancel out and average to the symmetrical waveform. If the actual waveforms were recorded during testing, then these assumptions would not
be necessary and the integration should be done for each individual trace.
However, in order to estimate time-integrated
TABLE 1
current for testing that has already been comTIME-INTEGRATED CURRENT, DIFFERENT TESTS
pleted, these assumptions may be used as a
guide. An approximation of the amount of arcApproximate
ing, in time-integrated current, for the cases
Typical
Current
Number
of
Time-Integrated
mentioned earlier is shown in Table 1.
Test Method
Level (A)
Interruptions Current (kA·s)
In these terms, switching with fault current for
a) mechanical life
0
10000
0
500 operations has the highest value. However,
b)
voltage
~0
10000
~0
recall that this is an extrapolation that is based
off of fewer operations; it has not actually
c) load current
600
2500
5.9
occurred. There is a need for this number of
d1) 1 duty cycle
12500
116
2.6
operations and the resulting time-integrated
d2) 2 duty cycles
12500
232
5.2
current to be actually tested.
e) extrapolation
12500
~500
24.4
3
Contact Life of Vacuum Circuit Breakers
Synthetic Testing
A 12.5 kA axial magnetic field vacuum interrupter was tested in Cooper Power Systems synthetic lab [4]. In a
synthetic circuit, the fault current and the recovery voltage come from tuned circuits, not a generator. The circuit used is shown in Figure 3. The first half of the circuit provides the fault current. Just before current zero,
the second half is switched in, providing the recovery voltage. The first half of the circuit is then switched out.
Due to the precise timing of synthetic testing, it is also possible to open the contacts at a specific point in the
current cycle, eliminating the randomness of mechanism timing mentioned earlier. In this test, the contacts
were always opened so that there was a nearly a half cycle of arcing, corresponding to the case in Figure 2a.
A typical unused 12.5 kA copper chromium contact, as was in the test device, is shown in Figure 4.
2.1 mH
Backup Breaker
100 Ω
17.1 uf +
60 kV
750 uh
Main
Charge
Source
400
100 Ω
Water Box
≈ 150 Ω
7.2 kV
50 kVA
9390 uf
100 or
200 Ω
100 Ω
2 kW
.3 A FWD
150 kV PIV
Discharge
Switch
50 k Ω
225 W
.02 uf
60 kV
20 k Ω
225 W
10 kVA
Test
Specimen
2 nF 25 kV +
.3 A FWD
150 kV PIV
Mica Caps
+
20 kV
120/240
01 uf
†
6.5 uf
400 Ω
†
72 Ω
20 Ω
7 A FWD
50 kV PIV
Discharge
Switch
Trigger
Gap
Switch
.5 uf
250 V
5kΩ
450 W
Adjust
for TRV
+ Mica
Voltage
Divider
Ratio
517/1
Specimen
Current
1 V/ kA
Wiel
Charge
Source
.0078 or
+ 0.1 uf
Laser
Gap
Switch
Wiel
Injection
Figure 3.
Synthetic circuit.
After testing, it was observed that the unit had performed 116 fault level interruptions. These were at a variety
of fault levels from 8 to 24 kA. Note that this is for a symmetrical current wave shape. While this interrupter is
rated at only 12.5 kA, the higher current waveforms were used to simulate asymmetrical conditions. By integrating each shot individually, and summing the results, the authors have determined that this unit saw a total
of about 16.5 kA·s. The contacts were examined and found to have an average total erosion of about 1.4 mm.
This is still short of a typical erosion limit of 3 mm. The contacts are shown in Figure 5. The slight discoloration
shown in the contacts is due to oxidation that occurred during the examination of the contacts.
Figure 4.
Surface of unused 12.5 kA contact.
4
Figure 5.
Surface of anode and cathode, 16 kA·s.
Contact Life of Vacuum Circuit Breakers
0
2
4
6
8
10
Fault Current (kA)
0
-4
-8
-12
-16
Arc Voltage (V)
Scheduled
Actual
Time-
Current Level
Number of
Number of
Integrated
kA
Shots
Shots
Current (kA·s)
4
80
80
2.84
8
80
81
4.72
12
80
85
7.54
16
80
83
9.42
20
80
85
12.47
24
80
88
15.67
12
Fault Current (kA)
Totals:
20
21
1.87
500
523
54.52
0
2
4
0
2
4
6
8
10
6
8
10
0
-5
-10
-15
-20
Arc Voltage (V)
0
-10
-20
-30
10
8
6
4
2
0
Time (ms)
-20
Figure 7: a, b, c.
Current, Arc Voltage, and Contact Gap:
later verification shot.
0
Contact Gap (mm)
TABLE 2
TESTED DUTY
Contact Gap (mm)
These contacts were tested with a constant polarity.
One was always the anode and the second was
always the cathode. Thus there was some uneven
wear. In a typical real-world situation, the polarity
would be random, resulting in an even anode/
cathode distribution for each contact. Therefore, a
second, more ambitious test program was designed,
as shown in Table 2. Using an oscilloscope, the
actual amount of arcing was recorded for each
interruption. After testing, some additional rated
current interruptions were performed to verify normal functionality of the contacts. Plots of current,
arc voltage, and contact travel for an early 12.5 kA
interruption are shown in Figure 6a-c, respectively.
The same is shown for one of the later verification
shots in Figure 7a-c. As can be seen, the contacts
were still performing the same even after duty.
In all, these contacts saw 523 operations with fault
level current, for a total of 54.5 kA·s [5]. When
translated back to equivalent operations at 12.5 kA,
the contacts saw the equivalent of 581 half cycles of
arcing at 12.5 kA, 60 Hz, or, for a typically random
opening time, an equivalent of 1162 quarter cycles
of arcing at 12.5 kA, 60 Hz.
After testing, this interrupter was examined closely. It was still capable of supporting 110 kV ac withstand voltage. The contact resistance had
increased from 42 µΩ to 70 µΩ. The average total
contact erosion was 2.5 mm. The contacts are
shown in Figure 8.
-10
-20
-30
10
8
6
4
2
0
2
4
6
8
10
Time (ms)
Figure 6: a, b, c.
Current, Arc Voltage, and Contact Gap:
early test shot.
Figure 8.
Surface of contacts, 54 kA·s.
5
Contact Life of Vacuum Circuit Breakers
Deriving Equivalent Duty
Using this data, the authors have estimated the number of operations that should yield the equivalent amount
of erosion (2.5 mm) at any current level, since the arc does not go into the constricted mode and stays diffuse.
This results in the estimated life curve based on constant time-integrated current shown in Figure 9. This plot
assumes the conservative estimate of half cycle of arcing at 60 Hz. If the application will yield only a quarter
cycle of arcing, then twice the number of operations may be possible. If the system is 50 Hz instead of 60 Hz,
the number of operations will be 50/60, or 83% of what is shown on the curve. By conserving time-integrated
current, it is easy to estimate the number of fault current operations possible.
However, what does time-integrated current really mean? It intuitively makes sense that the life of a vacuum
circuit breaker should be inversely proportional to the amount of current it interrupts, and the of amount time
that arcing occurs. In fact, the duty factors described earlier hinted at this. But what is really happening?
Recall that it was mentioned that extrapolation methods were only valid if the arc between the contacts was in
the diffuse mode. This would be true if the current being interrupted has a low value, such as switching levels
or small faults. This is also true for higher fault levels if the contact structure is of the axial magnetic field type.
In the diffuse mode, an arc in a vacuum circuit breaker has an interesting property. The arc voltage is nearly
flat near the current peak, as can be seen in Figures 6 and 7.
The level at which this plateau occurs is largely independent of the current level. With this particular contact
structure, an 8 kA interruption has a voltage plateau at about 32 V; a 20 kA interruption is at about 42 V. A
150% increase in fault level has resulted in only a 31% increase in voltage. Assuming for a moment that voltage is therefore a constant, this means that when summing A·s, this is also proportional to using V·A·s. This
is simply time-integrated power (W·s), or energy (J). In other words, the interruption life of vacuum contacts is
a function of the amount of arc energy that they are capable of dissipating during their life. Adjusting for the
slight variations in arc voltage from low to high current operations, the authors have determined that the first
pair of contacts dissipated approximately 660 kJ before examination and the second dissipated approximately
2.0 MJ. A plot of expected life using 2.0 MJ as a base is also shown in Figure 9.
As vacuum contacts vary in design between different manufacturers, it is difficult to predict the actual arc voltage in an across-the-board fashion. Thus, since most interrupters should be used within their rated fault interruption level, predicting life off of constant time-integrated current yields a simple estimate and is more practical to use than constant energy. However, if comparing against a vacuum contact design in which that arc is
not diffuse, such as a radial magnetic field contact structure (sometimes called rotating arc), it may still be necessary to use the constant energy approach.
Expected Number of Operations
10000
55 kA·s
2 MJ
1000
100
0
4
8
12
Fault Level (kA)
Figure 9.
Estimated life curves.
6
16
20
Contact Life of Vacuum Circuit Breakers
Conclusions
Over the past few years, several methods have been proposed for recording and extrapolating the life of vacuum
circuit breaker contacts, resulting in many different and usually conflicting claims. As a means of reconciling
these, and providing a common ground for vacuum contacts, the authors recommend the usage of time-integrated current. This would rate vacuum contacts based on the amount of current interrupted and the length of
time of arcing. This provides a simple means of translating from one manufacturer’s testing method to another.
Two specimens were tested in a synthetic circuit. After 16.5 kA·s, the first had an average of 1.4 mm erosion;
after 54.5 kA·s, the second saw 2.5 mm of erosion. These follow a trend that supports the relationship between
time-integrated current and contact life.
It should be noted that this is only one method of looking at the useful life on a vacuum circuit breaker. There are
many factors that go into a completed unit. The authors advise that the user still follow the individual manufacturer’s recommendations considering usage and regular maintenance: for example, measuring wear referenced
to a contact erosion groove or by using a 60 Hz high-potential test to verify the integrity of the insulation of a unit.
Acknowledgement
This work was based on a suggestion by E. Fred Bestel, Manager of Technology Development at Kyle
Distribution Switchgear, Cooper Power Systems. The authors would like to thank him for his guidance and
assistance with this work.
References
[1] "Requirements for Overhead, Pad Mounted, Dry Vault, and Submersible Automatic Circuit Reclosers for
AC Systems," American National Standard, ANSI/IEEE C37.60-1981, section 6.3.4, "Operating Duty Test,"
Table 4.
[2] "Guide for the Application, Operation, and Maintenance of Automatic Circuit Reclosers," American National
Standard, ANSI C37.61-1973/IEEE 321-1973, Appendices B and C.
[3] P.N. Stoving and J.F. Baranowski, "Interruption Life of Vacuum Circuit Breakers," Proceedings IEEE 19th
International Symposium on Discharges and Electrical Insulation in Vacuum, Xi’an China, 2000, pp.388391.
[4] "Final Test Results, VSAM 712 Serial Number 9920271," July 1999.
[5] "Final Test Results, VSAM 712 Serial Number 9920243," April 2000.
7
Contact Life of Vacuum Circuit Breakers
©2002 Cooper Industries, Inc.
Bulletin 02025 • July 2002 • New Issue
P.O. Box 1640
Waukesha, WI 53187
www.cooperpower.com
KDL
7/02