Arc Flash and Short Circuit

Electriman

Senior Member
Location
TX
To calculate Arc Flash Short circuit current(Bolted fault current) is used. But the short circuit current that we consider to evaluate MV equipment and LV (Low Voltage) is different based on IEEE. I was wondering if we have the same senerio when we calculate the arc flash results.

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.
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
To calculate arc flash it is the arcing current that is used in the IEEE equations. Bolted fault current is converted to arcing current using the formulas in IEEE 1584, one for <1000V and another for >1000V

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.
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
Most folks use 3 cycle info for LV because that is what the SCCR on the nameplate is based on.
IEEE Blue Book Definition for rated short time withstand current:
"The maximum rms total current that a circuit breaker can carry momentarily without electrical, thermal, or mechanical damage or permanent deformation. The current shall be the rms value, including the dc component, at the major peak of the maximum cycle as determined from the envelope of the current wave during a given test interval. (IEEE Std. C37.100-1992)"

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.
 
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