Available Fault Current

[rand]

Hello, I have a question regarding available fault current.

At a power plant where I work, an auxiliary transformer is tapped right off the iso-phase bus duct which is routed from the generator circuit breaker to the main unit xfmr bank.

My question is if a circuit breaker is to be installed on the primary side of the auxiliary transformer, would the short circuit symmetrical current rating for the new circuit breaker have to be the same as the generator circuit breaker to handle the available fault current from the generator?

 

[packersparky]

Hello, I have a question regarding available fault current.

At a power plant where I work, an auxiliary transformer is tapped right off the iso-phase bus duct which is routed from the generator circuit breaker to the main unit xfmr bank.

My question is if a circuit breaker is to be installed on the primary side of the auxiliary transformer, would the short circuit symmetrical current rating for the new circuit breaker have to be the same as the generator circuit breaker to handle the available fault current from the generator?

The circuit breaker AIC rating would have to be at or above the amount of fault current available at its terminals. The only way the generator circuit breaker would play a role in this is if it has been tested as a series rated combination with the transformer breaker. Very unlikely.

You would have to do a calculation to find the amount of fault current at the breaker terminals.