Ground Fault and Arc Flash 1

[WDeanN]

The topic came up on another thread, so I'm posting a follow up here. (

http://www.eng-tips.com/viewthread.cfm?qid=199403&page=1)

The basic question is: Should ground fault protection be taken into account when performing an arc flash study?

Most (MOST, not all) arcing faults will propagate to 3 phase to ground faults, and will probably be taken out with the ground fault detection (if it exists). So, should this be taken into account when calculating trip times?

Currently, we do not look at this at our location, as I have yet to see anything on this, but it would make sense to use the ground fault clearing time.

 

[dpc]

In enclosed equipment spaces, most faults start as line-to-ground but often escalate almost instantaneously to three-phase due to the ionized gases present. Three-phase faults also result in the highest arc-flash levels.

For these reasons, IEEE 1584 recommends that all arc-flash calculations be based on three-phase faults.

Certainly ground fault protection can lower the risks associated with arcing faults (and impedance grounding can reduce it even further). But unless you can lower the risk of three-phase faults to a very low level, the PPE protection needs to be based on the worst case and that is three-phase faults.

For outdoor equipment, with the much greater spacing between phases, three-phase faults are very uncommon and the new arc-flash requirements in the 2007 NESC are based on line-to-ground faults. But inside equipment enclosures, it's a different story.