I hear that using high resistance grounding is suppose to eliminate or reduce the arc flash. Yet it looks like the IEEE calculations and computer programs are set up so if a system is high resistance grounded, the arc flash incident energy goes up - not down or eliminated. Any thoughts on this?

Having Hi Z grounding reduce or eliminate the arc flash incident energy depends on whether the arc flash involves L-G or three phase events. The IEEE calculations are based on a three phase model assuming the arcing event involves all three phases. With this three phase model the grounded case provides a path to ground and there is slightly less arc flash energy leaving the box. In an ungrounded or high resistance grounded system more arcing energy leaves the box - it has no where else to go. If it was a line-ground fault, a high resistance ground does reduce the current to such a low level, there would not be an arc flash. So... Hi Z grounds do eliminate the incident energy if it is line-ground but increases the incident energy if it is a three phase event.

High Impedance Grounding

I hope we will one day see a model that will include risk as a factor in deciding the PPE. This is something I expect out of the IEEE1584/NFPA70E Collaboration. If someone has provided High Resistance Grounding, they have basically eliminated the arc flash hazard for all SLG faults, and prevented them from growing into three-phase faults. Something like 90% of all faults begin as SLG faults. There is no way to take credit for this legally via the IEEE 1584, NFPA 70E or OSHA at this time. Again, we need to expect a risk factor to be considered. This will credit employers who have installed and maintained high resistance grounding, who are providing a much safer design in regards to arc flash.

High Resistance Grounding and Arc Flash - Training

There was an excellent FREE training offered on-line by EC&M this last week on this subject. If anyone wants a copy of the training presentation, email me at mnmurphy@usit.net.