Tech Brief
The New High-Voltage
Circuit Breaker Interrupting Ratings
Based on K Factors of 1
by Jim Bowen
Powell Electrical Manufacturing Co.
T
he ANSI standard for high-voltage circuit breaker test procedures,
IEEE C37.09, was revised in 1999. This standard defines the shortcircuit tests required to certify a medium-voltage circuit breaker’s
interrupting rating. In the past the interrupting rating changed as a function
of the voltage at which the breaker was applied. The new standards are an
effort to recognize that modern vacuum and SF6 interrupting technologies
more closely represent a constant current interrupting device, independent
of the nominal system voltage. The standards evolved from the 1945 revision
when breakers were rated based on interrupting MVA (see figure next page).
In 1968 the standards established varying K factors to adjust interrupting rating as a function of the voltage. The K factor reflects the performance of the
Short Circuit Interrupting Current [kA]
Circuit Breaker Comparison at 5kV
60
50
40
New 1999 Rating
50 kA
Old 1968 Rating
41 kA
350 MVA
New 1999 Rating
36 kA
Old 1968 Rating
30
29 kA
250 MVA
20
0
2.30
3.85
4.00
4.16
4.76
Voltage [kV]
Fgure 1 — Circuit Breaker Comparison at 5 kV
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available at that time. This practice
ended with the 1999 change, where
circuit breaker K factors were all set
to a value of one. The K factor of 1
results in all high-voltage breakers,
tested to the 1999 version of C37.09,
having a constant interrupting rating irrespective of nominal system
voltage. The changes in interrupting
rating at the system voltage can be
seen in the graphs to the left and on
the next page.
Using the 1968 standards, the 250
MVA breaker had a 29 kA interrupting rating if applied at 4.76 kV
while the same breaker applied at
4.16 kV had a 33 kA interrupting
rating, and when applied at 3.85 kV
it had a 35.5 kA rating. Using the
1999 standards, a newly certified 36
kA breaker with a K factor of 1 will
retain the 36 kA interrupting rating
independent of the applied voltage.
Important Note: Circuit breaker
short-circuit interrupting rating is
the symmetrical rms current at the
time power contacts part. These new
rating structures do not change the
rating of circuit breakers certified
prior to the 1999 revision. They
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Old MVA Short Circuit Rating
Circuit Breaker Comparison at 15kV
70
Closing and Latching Capability
50 kA
Old 1968 Rating
Old 1968 Rating
30
20
28 kA
750 MVA
25 kA
New 1999 Rating
Old 1968 Rating
18 kA
500 MVA
10
0
11.00 11.50 12.47 13.80 15.00
Voltage [kV]
Figure 2 — Circuit Breaker Comparison at 15 kV
RMS Current
37 kA
1000 MVA
36 kA
New 1999 Rating
Rated SC Current =
Rated Max Voltage
Current X ————————
Operating Voltage
Symmetrical
Interrupting Capability
Min Interrupting
@ Max Voltage
Rated SC
Current
40
New 1999 Rating
K Rated SC Current
50
Maximum Symmetrical Interrupting Capability and
Carrying Capability for the Number of Seconds in
5.10.2.5 and 5.8
Max Interrupting
@ Min Voltage
53 kA
1.6 K Rated SC Current
Short Circuit Interrupting Current [kA]
New 1999 Rating
60
1/K Rated
Max Voltage
Operating Voltage
Nominal System Voltage
Rated Maximum Voltage
only impact breakers that are certified to the new testing standards. The new 1.0 K factor ratings will simplify
breaker application and align with the long-standing IEC
nomenclature. It is important that we do not make the
mistake of trying to apply a short-circuit rating that varies
as a function of the voltage to circuit breakers certified to
have a K factor of 1.
Figure 3 — “Old MVA Short Circuit Rating”
Jim Bowen graduated from Texas A&M University in 1976 with a
BSEE. He has Exxon in all facets of electrical engineering in the petrochemical process. He held the position of regional engineer for Exxon
Chemicals Europe for three years. In January of 1997, Jim joined Powell
Electrical Manufacturing Company as Technical Director, providing
leadership, training and mentoring to both internal and external electrical communities.
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