Medium Voltage Application Guide

13721.A
Percentage DC component
SWITCHGEAR
Exercise
What is the required short circuit breaking capacity of a circuit breaker, with a pole opening time of 45 ms and an
expected symmetrical short circuit fault level of 21 kA at the point of installation?
Total opening time of the circuit breaker: t = 10 + 45 = 55 ms
The percentage DC component at a total opening time of 55 ms is 30%:
100
90
80
70
60
50
40
1 = 45 ms
30
20
10
0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Time interval from initiation of short circuit current (ms)
The asymmetrical fault current level is 23 kA. A circuit breaker with a rated short circuit breaking capacity (I sc ) of
25 kA can be used.
710-12280-00A Medium Voltage Application Guide Page 93