Arc Flash/Ground Fault 1

[mpparent]

Hi All,

While I'm still very much learning about this subject, as I look in the standard, there's no talk of ground fault settings for breakers. Probably an ignorant question, but why?

Mike

 

[dpc]

Mike,

In IEEE 1584, they state that three-phase faults should be used as the basis for arc-flash calculations. There are a couple of reasons for this:

Arc-energy release for three-phase faults should be the worst case.

Arcing faults that start as something tend to escalate (very quickly) to three-phase faults within equipment, due to the conductive plasma that forms.

So no matter what type of system grounding and ground fault protection you have, the PPE requirements are always based on the three-phase fault energy and phase protection (per IEEE 1584). I don't think NFPA 70E really specifies this one way or the other.

For outdoor substations, with the much greater spacing between phases, these assumptions probably don't make much as much sense. The Arc Pro software that is used by some utilties is based on line-to-ground faults, so I assume you can use the ground fault clearing times to determine the arc energy.

 

[stevenal]

The proposed OSHA 1910.269 (generation, transmission and distribution) tabulated values (see

http://mywebpages.comcast.net/dwallis2/subv.pdf)

are based on line to ground faults, so the clearing times should be based on ground fault settings. Better yet, when performing live line work sacrifice coordination for safety and set the relay (selectable setting groups required) to trip instantaneously.

 

[mpparent]

dpc,

Thanks for that.

Mike