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 Post subject: Protection selectivity in Arc Flash Study
PostPosted: Tue Feb 15, 2022 8:19 am 

Joined: Tue Feb 15, 2022 8:18 am
Posts: 1
Performing an Arc Flash Study, i have had a problem.

I have a triphasic bar of 400 V, with Ik3 Max = 24 kA. In = 2887 amp.

The protection is set so that the short time tripping time will trip at Isd = 18 kA (it is adjustable).
If a three-phase short-circuit (24 kA) occurs, the circuit breaker would trip in a short time.

However, according to the arcing study (using ETAP software), the arcing current (Ia) only reaches to 10 kA. As 10 kA is lower than the Isd, the breaker trips in slow time, so the arc flash will be dangerous.

When carrying out an arc flash study, should it be obligatory to carry out a protection selectivity study taking account the three-phase short-circuit and arc currents? The arcing study leads me to adjust the relay so that it trips in the short range. Does NFPA70E require you to do it?

Thanks


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 Post subject: Re: Protection selectivity in Arc Flash Study
PostPosted: Mon Mar 14, 2022 9:44 am 
Sparks Level

Joined: Tue Feb 17, 2009 8:38 am
Posts: 57
Location: Westminster, MD
I don't think 70E has anything, but NFPA 70, National Electrical Code has 240.87(C)
(C) Performance Testing.
The arc energy reduction protection system shall be performance tested by primary current injection testing or another approved method when first installed on site. This testing shall be conducted by a qualified person(s) in accordance with the manufacturer’s instructions.
A written record of this testing shall be made and shall be available to the authority having jurisdiction.

This dictates that selectivity is not possible with the devices you have for arcing faults, and it must be adjusted for arcing faults. It's new for the 2020 edition of the NEC.


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