#### Electriman

##### Senior Member

- Location
- TX

The short circuit for LV circuit breaker evaluation is 1/2 cycle symetrical fault current.

The short circuit for MV and HV circuit breaker evaluation is 1-1/2 to 4 cycles symetrical fault current.

- Thread starter Electriman
- Start date

- Location
- TX

The short circuit for LV circuit breaker evaluation is 1/2 cycle symetrical fault current.

The short circuit for MV and HV circuit breaker evaluation is 1-1/2 to 4 cycles symetrical fault current.

- Location
- Rutland, VT, USA

- Occupation
- Electrical Engineer, PE

Evaluating equipment is different than calculating arcing current for arc flash. Typically, one will utilized the maximum available fault current, typically infinite bus, to evaluate equipment duty, which is what I think you asking about.

Contrast that with, for arc flash calculations, the actual available short circuit current from the utility is used.

- Location
- New York, 40.7514,-73.9925

Most folks use 3 cycle info for LV because that is what the SCCR on the nameplate is based on.The short circuit for LV circuit breaker evaluation is 1/2 cycle symmetrical fault current.

- Location
- Rutland, VT, USA

- Occupation
- Electrical Engineer, PE

IEEE Blue Book Definition for rated short time withstand current:Most folks use 3 cycle info for LV because that is what the SCCR on the nameplate is based on.

Since the peak occurs within the first 1/2 cycle this is the rating used.

Now switchgear will have a 3 cycle rating since the main breaker may not have an instantaneous function to enable coordination with the feeder breakers. Therefore a 3cy rating is appropriate as the bus has to hold together to give the main breaker time to operate.

- Location
- TX

Thank you all.