Short circuit asymmetrical factor 1

[electrolitic]

Do I need to consider the calculated short circuit asymmetrical factor in case of arc fault in a MV system?
For bolted faults I already need to provide a zero crossing delay for safe breaker opening.
Thanks.

 

[dpc]

What is the purpose of the short circuit calculation? If it is for arc-flash, you can ignore any asymmetrical current if you are using IEEE 1584. If it is for equipment duty calculations, it will depend on the standard that the breaker was tested to.

Arcing fault have a lot of resistance (the arc), so the X/R ratio is generally low. This results in low dc offset current.

But no one can really answer your question without more information.

 

[electrolitic]

Thanks DPC. Considering the system high X/R ratio I need to provide an intentional time delay for the protection in order to
match the correct interrupting capacity of the breaker in case of bolted fault. The confirmation of lower X/R ratio (that implies low asymetrical factor) in case of arc fault by IEEE would permit a fast detection and a faster breaker actuation.
Am I correct?

 

[dpc]

In terms of dc offset current, you'd have to account for the worst case which is going to be a bolted fault. Arcing faults don't have any bearing on interrupting ratings (at least in my part of the world), since the current magnitude of an arcing fault is going to be less than a bolted fault.