remote control equipment - Indian Railways Institute of Electrical ...

INCASE OF TWO TRANSFORMERS RUNNING IN PARALLEL -
TOTAL REACTANCE = 1% + 30% = 31%
FAULT LEVEL =
FAULT CURRENT =
100 X 100
31
322.58
25
= 322.58 MVA 60%
= 12.90 KA.
It will be seen from the above that though the fault current is less but rated short circuit
braking current value assigned to the breaker is higher. This is because IEC, IS and BS
specification have standardised the rated short circuit breaking current are as given below :-
6.0 kA, 8.0 kA, 12.5 kA, 16 kA, 20 kA, 25 kA, 31.5 kA, 40 kA, 50 kA, 63 kA, 80kA,
and 100 kA.
Therefore, as a part of standardisation, the recommeded values of rated short circuit
breaking current have been adopted Recommended values of rated short circuit current and
normal rated current are shown as under :-
CO-ORDINATION TABLE OF RATED VALUES FOR CIRCUIT BREAKERS
Rated Rated Short Rated Normal Current
Voltage circuit Breaking (Amps)
(kV) Current (kA)
12.5 800 1250
16 800 1250
72.5 20 1250 1600 2000
31.5 1250 1600 2000
12.5 800 1250
20 1250 1600 2000
123 25 1250 1600 2000
40 1600 2000
12.5 800 1250
20 1250 1600 2000
145 25 1250 1600 2000
31.5 1250 1600 2000
40 1600 2000
50 2000
81
3 1%
3 3
220 KV BUS
60%
25 KV BUS