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 Post subject: Calculation of Max Incident Energy - is a study required?
PostPosted: Wed Jul 08, 2015 10:34 am 
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Joined: Fri Apr 15, 2011 7:43 am
Posts: 177
Location: Colorado
In the paper "Impact of Available Fault Current Variation on Arc-Flash Calculations" presented in the PCIC the authors discussed the maximum incident energy (IE) may not occur at the calculated arcing current or at 85% of the arcing current. Instead the maximum IE occurs some place where the 2 second rule and overcurrent protection curve meet or somewhere else (see fig 5 of the paper).

It seems to me the maximum IE may be determined without going through an arc flash/short circuit study. This can be calculated by determining the worst case IE for a protective device. This is especially true when utility data is unknown or estimated.

What is needed is the Time-current curve, system voltage, bus gap, the enclosure type, and grounding. From this the worst-case IE can be determined.


Paper can be found here ~ ... PCT_372889

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 Post subject: Re: Calculation of Max Incident Energy - is a study required
PostPosted: Fri Jul 10, 2015 12:02 pm 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
The CONCEPT has merit. There are 3 points to be concerned with. Don't lose track of any of the 3:

1. The TCC at 2 seconds, as mentioned, or whatever the maximum simulation time is. At this point arcing time is fixed (though somewhat artificially). Due to the shape of the TCC, although heat flux decreases, the increase in arcing time increases faster causing the incident energy to increase, until it reaches the maximum simulation time (usually 2 seconds).
2. The current at the fastest possible opening time matters because no matter how high the current gets (up to the AIC), again arcing time is fixed but incident energy is not.
3. The actual estimated bolted fault/arcing fault (85% rule again) which is usually lower than either of the previous two extremes.

In practice this provides a road map for mitigation:
1. If the breaker does not trip at maximum speed during arcing conditions, decreasing settings and/or turning on instantaneous reduces incident energy. If the arcing current is already triggering an instantaneous trip, then adjusting breaker settings is fruitless.
2. Reductions in arcing current result in increased incident energy until the trip time exceeds simulation time (generally 2 seconds). This gives rise to the examples where current limiting fuses result in higher arc flash than circuit breakers.

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