It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer.
Example: Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source. Calculate as follows:
Normal Full Load Current =
Maximum Short Circuit Amps =
The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.