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 Post subject: What to use for 3 phase bolted fault current?Posted: Mon Oct 02, 2017 10:18 am

Joined: Wed Aug 30, 2017 3:16 pm
Posts: 8
ETAP gives 3 options for bolted fault current calculation:
- Symm. 1/2 Cycle
- Symm 1.5 to 4 Cycle
- Fault Current Decay

I can see why one would choose 1/2 cycle (most conservative) or fault current decay (realistic), but why would someone select Symm. 1.5 to 4 cycle?

Thanks.

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 Post subject: Re: What to use for 3 phase bolted fault current?Posted: Mon Oct 09, 2017 9:13 am
 Sparks Level

Joined: Tue Oct 26, 2010 5:00 pm
Posts: 192
Location: Maple Valley, WA.
The Symm. 1.5 to 4 cycle currents are used for medium voltage circuit breakers. You have to know what the current is when the contacts are opening. For instance, a 5 cycle MV breaker has a contact parting time of 3 cycles. So you need to know what the current is at 3 cycles and compare that to the breaker interrupting rating.

_________________
Robert Fuhr, P.E.; P.Eng.
PowerStudies

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 Post subject: Re: What to use for 3 phase bolted fault current?Posted: Mon Oct 16, 2017 2:24 pm

Joined: Wed Aug 30, 2017 3:16 pm
Posts: 8
Thanks, Robert.

My question is related to arc flash. Why would we calculate the arcing current at 1.5-4 cycle and use it for IE calculation? This would result in either lower FCT (which COULD cause higher IE) or lower IE if the arcing current remains within the same relay instantaneous pick-up current region.

Will using 1.5-4 cycle for IE calculation result in more conservative or realistic IE?

Thanks,
Erin

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 Post subject: Re: What to use for 3 phase bolted fault current?Posted: Tue Oct 17, 2017 5:27 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Generally it is my understanding that the goal with any of the modeling if you are considering this "factor" is recognizing that we have a decaying exponential assymetrical fault current which is very much time varying. We could estimate incident energy more accurately with a piece-wise approach to the calculation. So we model the decaying exponential assymetrical fault current during the first couple cycles and calculate an incident energy for this, then do it again using the normal symmetrical fault current, and then add them together. The actual difference will be small but is a small increase in incident energy. All of the power system analysis software offers some kind of option of this nature.

It is also realistic and more accurate and conservative to include this in the the TCC (fault opening time) calculation because it exists and does indeed increase the amount of fault current. Whether it raises or lowers incident energy is a little more complicated to give a definite answer on but there's no reason not to do it. Actual lab testing fir arc flash doesn't have assymetrical fault currents but the real world does.

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