Technical description
3 Supporting functions
3.5 Circuit breaker condition
monitoring
VM50.EN004
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121
Example of the logarithmic interpolation
Alarm 2 current is set to 6 kA. What is the maximum number
of operations according Table 3.5-1.
The current 6 kA lies between points 2 and 3 in the table. That
gives value for the index k. Using
k =
2
C
k
= 10000
C
k+1
= 80
I
k+1
= 31 kA
I
k
=
1.25
kA
and the Equation 3.5-2 and Equation 3.5-3, the relay calculates
5038
.
1
1250
31000
ln
80
10000
ln
n
6
5038
.
1
10
454
1250
10000
a
Using Equation 3.5-1 the relay gets the number of permitted
operations for current 6 kA.
945
6000
10
454
5038
.
1
6
C
Thus the maximum number of current breaking at 6 kA is 945.
This can be verified with the original breaker curve in Figure
3.5-1. Indeed, the figure shows that at 6 kA the operation count
is between 900 and 1000. A useful alarm level for operation-
left, could be in this case for example 50 being about five per
cent of the maximum.
Example of operation counter decrementing when the CB is
breaking a current
Alarm2 is set to 6 kA. CBFP is supervising trip relay T1 and
trip signal of an overcurrent stage detecting a two phase fault
is connected to this trip relay T1. The interrupted phase
currents are 12.5 kA, 12.5 kA and 1.5 kA. How much are
Alarm2 counters decremented?
Using Equation 3.5-1 and values n and a from the previous
example, the relay gets the number of permitted operation at
10 kA.
313
12500
10
454
5038
.
1
6
10
kA
C