Why is impedance important?

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.


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