Device duty for line to ground shortcircuit 1

[Antonio Perez Martinez]

Hello everyone,

I see that ETAP is just using three phase short circuit results to check device duty requirements for buses and breakers, although in some cases line to ground shortcircuit currents are higher than three phase ones. Is this correct or should it be checked manually?

Thanks.

 

[waross]

Short circuit ratings have to do with the capability of the device to interrupt a fault current.
Line to ground voltages are 58% of line to line voltages.
When in doubt do the hand calcs but remember to include the 58% voltage and review the interrupting capacity at the lower voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Breaker ratings are generally based on three-phase faults. Check breaker standards or consult manufacturers regarding line-to-ground fault ratings. Ground fault current can exceed 3-phase, at least in theory. This can happen for solidly-grounded generators where the zero sequence reactance can be less than the sub-transient, and also for delta-wye transformers where the zero-sequence reactance for core-type transformer can be less than the positive sequence reactance. However as you get away from the transformer, this difference quickly evaporates due to the cable or line impedance.

Having said all that, I've never had a situation where the ground fault current impacted the breaker selection. One possible exception might be for corner grounded systems.