Low Fault Current Results in High Arc Flash
Low Fault Current Results in High Arc Flash
(OP)
I need some help in understanding and possibly mitigating an issue that I have seen in several locations. It seems that every time I have low fault current available from the utility, I always end up with a category 4 or worse on the secondary side / line side of the breaker downstream. I'm not really sure what causes this or how to mitigate the issue. Any help is appreciated.






RE: Low Fault Current Results in High Arc Flash
The energy developed in a fault is related to I^2 t, where I is the fault current and t is the clearing time. You can play with this and see that there will be cases where the low fault currents result in high arc flash duties.
RE: Low Fault Current Results in High Arc Flash
RE: Low Fault Current Results in High Arc Flash
That's one way, although you will have to do a protection coordination study to ensure proper operation of the downstream breakers/fuses.
You can add arc flash detection in the locations with arc flash issues. It might be possible to reduce the phase to ground arc power by the use of high resistance grounding (if the code allows it in your installation). Some people have recommended incorporating a fast instantaneous trip, setting aside the issues of protection coordination and switching it in by means of a 'maintenance mode' switch when work is to be done in the area in question. You might want to check with the AHJ and research the issues involved with switching, tagging and locking procedures for work done under such conditions.
RE: Low Fault Current Results in High Arc Flash
to reduce the impedence to get higher fault current
RE: Low Fault Current Results in High Arc Flash
I have worked on Arc Mitigation System for one of the leading Oil company. We have installed VAMP Protection System for their LV Switchgear. VAMP relays sends the trip signal within 7ms to the breaker. So the total clearing time is 7ms plus breaker operating time. This application drastically reduce the incident energy category. Before using any such relays, the breaker opening time shall be tested. Breakers having long opening time, reduce the advantage of using such relays and mitigation system.
RE: Low Fault Current Results in High Arc Flash
overcurrent protection?
Comments please.
RE: Low Fault Current Results in High Arc Flash
RE: Low Fault Current Results in High Arc Flash
Yes but what about the cost of Bus Differential on LV Switchgear? Its way too high as compared to Arc Mitigation System
RE: Low Fault Current Results in High Arc Flash
RE: Low Fault Current Results in High Arc Flash
A voltage inhibited instantaneous overcurrent relay might reduce the tripping time, but it would be difficult to set the relay's voltage setting to discriminate between a local fault when it must trip and a fault at the next switchboard. The net effect would be the same as just using a low setting on the instantaneous overcurrent.
Another problem with voltage controlled or restrained instantaneous overcurrent settings is cold load pickup or transformer inrush. Since the voltage is zero prior to breaker closure, the low set instantaneous is armed and may trip on inrush.
RE: Low Fault Current Results in High Arc Flash
Instantaneous protection either with a maintenance switch, zone selective interlocking, or differential protection cannot be done with fuses.
RE: Low Fault Current Results in High Arc Flash
RE: Low Fault Current Results in High Arc Flash
RE: Low Fault Current Results in High Arc Flash
I realized that afterward. The relay can sense the arc fault, but doesn't have a breaker to trip; there's primary pole fusing and internal transformer fusing.