44
Arc Duration (mS)
Breaker Voltage (kV)
50
100
150
200
250 300
350
400
450
500
4
8
12
16
20
24
28
32
Figure 21: Estimated Arc Time based on Nameplate Breaker
Voltage
Example:
Given:
Breaker is rated at 69kV, 30kA fault interrupt
(RMS symmetrical), and it can withstand 5
operations at fault interrupt current.
Assume:
69kV breaker has an arc duration of 24mS
(0.024s).
Calculate:
Danger Limit = (NF ) I
2
T
Danger Limit = (5) (30kA)
2
(0.024s)
Danger Limit = 108,000,000 A
2
S
Danger Limit = 1.1 x 10
8
A
2
S
Note:
The answer is expressed in amp
2
-seconds (A
2
S),
converted to scientific notation, base 10, rounded to
two significant digits.
Incomplete Nameplate Data
There are occasions when the breaker nameplate data
does not state how many full-rated faults (symmetrical)
the breaker can withstand. For those instances, to arrive
at an estimate of the full-rated fault operations capability,
use the number of rated load current operations a breaker
can withstand and employ the equation N
F
= (N
L
I
L
) /
I
F
,
where N
F
is the number of fault current operations, N
L
is
the number of rated load breaker operations,
I
L
is the rated
load breaker current, and
I
F
is the full-rated fault current.
ANSI / IEEE C37.06-1989 states that all outdoor circuit
breakers, all voltages, must be able to undergo 100 trip
operations at rated load interruption before an inspection of
the contacts is warranted.
Example:
Given:
Breaker is rated at 69kV, 20kA fault interrupt
(symmetrical), 1200A rated load break. No
information is given on the number of fault rated
interruptions.
Assume:
69kV breakers have an arc duration T of 24mS
(0.024s). 69kV breaker can withstand 100 rated
load break operations, N
L
, before an inspection
(ANSI / IEEE C37.06-1989).
Calculate
: NF = (N
L
I
L
) / IF
NF = (100) (1200A) / 20kA
NF = 6
Danger Limit = (NF ) I
2
x T
Danger Limit = (6) (20kA)
2
(0.024s)
Danger Limit = 57,600,000 A
2
s
Danger Limit = 5.76 x 10
7
A
2
s
Note
:
The answer is expressed in amp
2
-seconds (A
2
s),
converted to scientific notation, base 10.
To set the Contact Life Danger Limit, select the
CONFIGURATION tab and click “EDIT”:
Program:
Contact Life Danger Limit = “5.76e7”
If breaker is Rated in MVA, not kA, or is being Applied at
Voltage other than Nameplate:
There are occasions when the breaker is rated in MVA at
a given voltage. No kA value is available. To arrive at kA
maximum rated fault (symmetrical) current, it is necessary
to convert the MVA to kA on the voltage base. Employ the
equation
I
L
= S
3
/
√
3 (V
LL
) to derive single phase current,
where
I
L
is the individual phase current in kA, S
3
is the three
phase power in MVA, and V
LL
is the line-to-line voltage in kV.
Example:
Given:
Breaker is rated at 69kV, 2,500 MVA
interrupting duty
Breaker is to be applied at 69kV.
Calculate:
I
L
= S
3
/
√
3 (V
LL
),
I
L
= 2,500 MVA / 69kV
√
3
I
L
= 20.943kA, or approximately 21kA
Danger Limit = (NF ) I
2
x T
Danger Limit = (100) (21kA)
2
(0.024s)
Danger Limit = 10,584,000,000 A
2
s
Danger Limit = 1.06 x 10
10
A
2
s
Program:
Contact Life Danger Limit = “1.06e10”
