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 Post subject: arc flash calcs...Posted: Wed Jul 09, 2008 11:08 am

Joined: Wed Jul 09, 2008 10:48 am
Posts: 1
Hi,

I have some questions that I hope someone can help me answer.

1) How do I get the bolted fault current for any particular cb? Is there an equation for this?

2) In the standard IEEE equations for incident energy calcs, is the variable t (which is the arcing time) the same as the clearing time seen on a tcc curve?

This is my first time doing this type of study and am lost in trying to figure these two questions. Any help will be greatly appreciated.

Thank you.

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 Post subject: Posted: Wed Jul 09, 2008 1:55 pm
 Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
The bolted fault current comes from a fault current study. See IEEE Std-141 Red Book, Electric Power Distribution, for examples.

For fuses, the arcing time is the time from the Total Clearing TCC. If the TCC is average, you have to add a margin. For breakers with separate relays, the breaker interrupting time has to be added to the relay TCC time.

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 Post subject: Posted: Wed Jul 09, 2008 2:36 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Bolted Fault Current

How do I get the bolted fault current for any particular cb? Is there an equation for this?

You will need to get the source impedance data from the utility and start the calcs from there. These will be furnished as sequence components in percent or per unit based on mva and voltage base values.

Please take this as a constructive response. These calcs are serious and errors could involve dangerous or deadly consequences.

Don't know what your background is, but if you don't understand how to find the available fault duty already, you are probably trying to climb a greasy pole. You should use this to talk to the boss so he can get you some help.

Anyone else care to chime in on this?
Alan

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 Post subject: Posted: Wed Jul 09, 2008 5:35 pm
 Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
acobb wrote:
Anyone else care to chime in on this?
Alan

Ding, ding!

Yep, sounds like the OP is in over his head. We will be glasd to help but you are asking what step 1 is in a hundred or maybe thousands of steps procedure.

I am guessing you have had this thrown on your lap to make this happen by your boss. Go back and tell him you need to buy some \$20,000 software and attend a 2 weeks class on the software to do this study, or you need some help, that way you get help.

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 Post subject: Posted: Fri Jul 11, 2008 7:05 am
 Sparks Level

Joined: Thu Jul 19, 2007 7:54 am
Posts: 201
Location: St. Louis, MO
I've gotta agree.
There's no way anyone should be doing these calcs without the basic knowledge of how to perform short circuit calculations or even where to get the information from. There can be some serious consequences to incorrect calculations here.

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 Post subject: Posted: Fri Jul 11, 2008 9:07 pm
 Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 510
Location: New England
DG,
There are several methods to calculate fault current. The easiest that I have found is the MVA method. I would recommend that you visit arcadvisor.com and read about it. For \$50 a year you get an account with access to their MVA calculator and reference tables. You can also google MVA Method Fault Current.

In the analysis, you have to calculate Isc (bolted short current) to the point under investigation. You need Isc as one of the inputs into the IEEE equations to find arcing current. You also need other things like gap distance, voltage, arc in a box or open air. The equation gives you Ia (arcing current). You then take this value, and 85% of this value, to the TCC to find clearing time.

Ia is always going to be smaller than Isc. Typically it ranges in the 70% to 90% range of Isc. You take the 85% value of Ia because on the TCC this will result in a longer clearing time, sometimes a very dramatic increase in time. Longer clearing times tend to result in higher incident energy even though the arcing current is less. The 85% and 100% results act as built in safety factors, one looking at highest arcing current, the other longer clearing times. Actual fault currents are often less than those calculated. These t values, along with some other variables like worker distance are then put into another series of equations. The first calculate what is known as Normalized Incident Energy, and then Corrected Incident Energy, which is what is put on the Arc Flash Label and determines PPE.

The math uses log functions, which aren't difficult if you remember high school math. If you have some electrical engineering and/or power background, and you just need to be taught the details, then there is help out there for you. If you have no idea of power distribution at all, then I would caution you about doing this on your own.

I can offer you a phone call to discuss your situation and recommend references for purchase if you Private Message me.

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