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Arcing faults
Posted by John from Cleveland, OH, US on May 1, 2008
I would say that use of HRG will greatly reduce the risk of arcing faults on a 480 V system, since the large majority of faults begin life as a line-to-ground fault. But what the others are saying is that it does NOT reduce the hazard that can exist for a three-phase arcing fault, so the required PPE is still based on the worst-case three-phase fault. This is pretty clearly addressed in IEEE 1584. There is still an arc-flash hazard since the HRG has no effect on phase-phase or three-phase faults.
You are correct. Arc Flash calculations are based on the Arc Flash current, Time, Distance and Bolted Fault Data. There is no way that the grounding system can reduce the Arc Flash Hazard Analysis.
The only thing a High Resistance Grounding system can do is limit the fault current of a single phase to ground fault. Now that the fault current is limited to 5 A., the probability of that fault escalating to a phase to phase fault or a three phase fault is greatly reduced. In contrast a single phase to ground fault in a Solidly Grounded System has the highest probability of escalating to a three phase fault.
The Red Book is stating that most faults in the electrical industry start as Ground Faults. The probability of that fault escalating to a three phase fault in a Solidly Grounded System is High, whereas, the probability of that fault escalating to a three phase fault in a High Resistance Grounded System is Low. If the Statement 7.2.2 is interpreted this way, then the statement is true.
