### Author Topic: Calculating Short Circuit Arc Flash energy  (Read 1676 times)

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#### HackedFridgeMagnet

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##### Calculating Short Circuit Arc Flash energy
« on: March 09, 2016, 02:17:46 am »
Does anybody know how to roughly calculate the Short Circuit Arc Flash energy given things like voltage, fault loop impedance and fuse rating?

Guidelines on this or rules of thumb? Looked at a few papers. I understand how it works but I have got to admit I'm not making much headway applying the formulas.

In particular what would happen from a single phase fault,  say 240V, 0.3 ohm fault loop impedance but protected by a 100A service fuse. Not really sure of the curve of this fuse anyway.

#### uncle_bob

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##### Re: Calculating Short Circuit Arc Flash energy
« Reply #1 on: March 09, 2016, 02:31:34 am »
Does anybody know how to roughly calculate the Short Circuit Arc Flash energy given things like voltage, fault loop impedance and fuse rating?

Guidelines on this or rules of thumb? Looked at a few papers. I understand how it works but I have got to admit I'm not making much headway applying the formulas.

In particular what would happen from a single phase fault,  say 240V, 0.3 ohm fault loop impedance but protected by a 100A service fuse. Not really sure of the curve of this fuse anyway.

Hi

Well the fuse is what is going to stop it as it heads off to 800A. With 0.3 ohms in the loop, it will not go higher than that. If the 0.3 ohms is not a pure resistance, but an impedance (L+R or even more complex) you will need to know the elements that make it up.  If it's a R and C in parallel then the fault could be pretty interesting. Even with an L+R, if the inductor saturates at 200A and you have 0.01 ohms in series with it, the result would be a bit different.

Bob

#### HackedFridgeMagnet

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##### Re: Calculating Short Circuit Arc Flash energy
« Reply #2 on: March 09, 2016, 03:07:11 am »
For simplicity it would just be taken as resistive though it would be slightly inductive.

Yes the fault current isn't hard but how would I guesstimate the energy released at the short?

#### joeqsmith

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##### Re: Calculating Short Circuit Arc Flash energy
« Reply #3 on: March 09, 2016, 04:16:08 am »
For simplicity it would just be taken as resistive though it would be slightly inductive.

Yes the fault current isn't hard but how would I guesstimate the energy released at the short?

Maybe I'm not understanding what you are asking.   Energy is just the power multiplied by time.   So 1 Watt * 1 second = 1 Joule.   What's the data sheet for the fuse show for trip time?

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#### Ian.M

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##### Re: Calculating Short Circuit Arc Flash energy
« Reply #4 on: March 09, 2016, 04:17:53 am »
Well it cant be more that I*V*t and maximum energy transferred will be if V across the arc is half the supply voltage, which will be I of half the bolted short current (i.e. equal power dissipated in the arc and the loop resistance).  t will be set by the lesser of the fuse clearing time at current I or 4 seconds (as the arc is generally reckoned to have disrupted any containment that is going to fail within that time so the energy is only integrated up to that point)

There are fomulae for calculating the effective arc flash energy and IEEE Std 1584-2002 is valid down to 700A bolted fault current.

#### uncle_bob

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##### Re: Calculating Short Circuit Arc Flash energy
« Reply #5 on: March 09, 2016, 10:52:02 pm »
For simplicity it would just be taken as resistive though it would be slightly inductive.

Yes the fault current isn't hard but how would I guesstimate the energy released at the short?

Hi

Under the right conditions (the fuse is defective) the energy can be near infinite. Yes, I've seen this happen.

Arcs are interesting beasts, they have a negative resistance. More current gives you a lower voltage drop in the arc. AC arcs go to zero 120 (or 100 depending on your AC) times a second. That makes things a bit more interesting. We all seem to have assumed this is an AC arc. If it's DC it will go pretty low voltage / high current petty fast.

The next thing you need is the data on your fuse.

Bob

#### Fank1

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##### Re: Calculating Short Circuit Arc Flash energy
« Reply #6 on: March 10, 2016, 12:10:41 am »
I will assume a house service.
With a 10KVA transformer at 2% impedance.
10KVA at 240 = 41.6 amps full load ; divided by .02 = 2083 Amps Fault current.

#### uncle_bob

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##### Re: Calculating Short Circuit Arc Flash energy
« Reply #7 on: March 10, 2016, 12:13:09 am »
Hi

Ok, well if we are headed off into even more guessing. Let's say it is a factory with 1250 KVA service and you then get 20KA....

Bob

#### HackedFridgeMagnet

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##### Re: Calculating Short Circuit Arc Flash energy
« Reply #8 on: March 10, 2016, 10:52:38 am »
sorry was off doing the job today.

It was only residential with about 50m distant to a pole top transformer.
Yes it was AC.

Luckily I made no mistakes and didn't see any arc flash first hand.

My tester said I may get a fault current of around 600A. I am not that sure how they calculate this, probably off voltage and fault loop impedance.
I don't have the fuse specs except it was almost certainly 100A.

Thanks for the input anyway.

Smf