Low Fault Current Results in High Arc Flash 1

[imisspla07]

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.

 

[magoo2]

The breaker clearing curve is usually an inverse chactacteristic, so lower fault current means longer clearing times. High fault currents result in faster clearing and you often have an inst element that kicks in at some present value of fault current.

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.

 

[imisspla07]

Thanks for the info. What is the best way to mitigate the issue? Changing fuses that have faster clearing times?

 

[PHovnanian]

What is the best way to mitigate the issue? Changing fuses that have faster clearing times?

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.

 

[unclebob]

Zone selective interlocking, Differential protection or find a way
to reduce the impedence to get higher fault current

 

[burntcoil]

The prime factor in Arc Flash Mitigation is Incident Energy. Having a look at different Arc Mitigation Options,the time required by ZSI and Differential Protection is enough to do the damage. Arc Mitigation is not only about clearing the fault. Its about clearing the fault in the minimum possible time.

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.

 

[gattie]

Is this not a case for voltage dependent or voltage controlled
overcurrent protection?
Comments please.

 

[davidbeach]

Bus diff can operate in 8-12ms, not appreciably slower than the 7ms you claim for the arc detection.

 

[burntcoil]

@davidbeach

Yes but what about the cost of Bus Differential on LV Switchgear? Its way too high as compared to Arc Mitigation System

 

[unclebob]

Remote operation is another way of doing things safely.

 

[rcw retired EE]

"Is this not a case for voltage dependent or voltage controlled overcurrent protection? Comments please."

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.

 

[jghrist]

What is the primary/secondary voltage and size of the transformer? Often MV-LV transformers will have fuses for primary protection and no protection on the secondary. A fault on the line side of the downstream breaker will result in relatively low primary fault current and high fuse clearing times. This is difficult to mitigate without adding relay operated primary protection, which is an expense to the utility.

Instantaneous protection either with a maintenance switch, zone selective interlocking, or differential protection cannot be done with fuses.

 

[imisspla07]

jghrist, it is a 12470/480V transformer, 1000kVA. I added the VAMP 221 relay as suggested above into my power tools software and remarkably it lowered it from a category 4 to a category 1.

 

[jghrist]

jghrist, it is a 12470/480V transformer, 1000kVA. I added the VAMP 221 relay as suggested above into my power tools software and remarkably it lowered it from a category 4 to a category 1.

What is the protection ahead of your main breaker? If it is the transformer primary fuse, then the VAMP cannot decrease the arc hazard on the line side of the main breaker.

 

[imisspla07]

jghrist,

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.