ECN Forum Forums General Topics of the Trade General Discussion Area Fault Current & Arc Flash Calculations

How do you determine the Fault Current (kA) value that is needed for Arc-Flash calculations?

I have some literature and slide-rule type calculators for determining Arc-Flash but you have to first know what the Fault Current (kA) is and I haven’t been able to figure out how to come up with this number.

Typically you would use the infinite bus primary method, which is determined by the KVA and impedance (Z) of the transformer.

KVA/(I*1.732/Z%)=Isc.

For example, a 300 KVA transformer, 3 phase 208V at 3% impedance:
300,000VA/ (208*1.732/.03)=8317 Isc.

Ryan,
You have to be careful using the "infinite bus" method. The results of a higher fault current will show a shorter clearing time for some OCPDs which in turn may result in the selection of a level of PPE that will not provide the required protection. Part of the PPE selection is the calories of heat energy that your skin will be exposed to. This is a function of both the level of current and the time of exposure. The quicker clearing based on the higher fault current may limit the heat energy to a value less than what will be there with a lower fault current.
Don

I've found the answer to my own question while poking around on the Internet. Here is a link to some software that will figure the fault current. It isn't free, but you can download a trial version. It has a good Help section. Go to the Electrical Design Reference website at http://www.edreference.com/default.asp

In order to find the fault current you also need to know the length & # of conductors and the raceway type.

Hi Don. I may be getting in over my head here, becasue I have never done (or seen) any PPE calculations.

Is the fault always assumed to be inside the equipment you are working in? I mean, it seems to me that trying to determine the incident energy would always sort of be a crap shoot, since you don't know at what point on the wiring system the fault might occur. For example, if you have a 100' lenght of conductor, the incident energy would be higher if the fault occured closer to the breaker, but lower if it occured 90' away from the breaker. I think this would also play into the clearing time of the fault and therefore the level of PPE. Am I off base here?

Ryan,
You calculate the fault to be where ever you want. When you pick a location (usually equipment, but it could be a j-box, etc), you determine the amount of fault current from all sources (utility, generators and motors) that can feed into the fault, and then consider all impedances that will effect those sources.
You surely can calculate 50' into a 1000' cable run, if you choose. It may make sense to do that if you are working on an energized splice 50' into the feeder.
A complete short circuit study, includes calculating every piece of equipment, and sometimes splice points. This results in hundreds and sometimes thousands of results, each one unique for where the fault was assumed to occur. If you are installing a new panelboard, then you would assume the fault occurred at that piece of equipment, so that you can choose the proper AIC or withstand rating, as needed.

Bill:
Sorry, I got here late on this one...
Bussman has available few brochures on AIC/Fault Current Calcs.

Specifically # EPR-1

Bussman website may have some info, but this booklet is great; I started using it in one of my VoTech courses.

I recently mailed a spare copy to Harold Endean, I haven't received the additional copies I requested yet.

You may find a link or e-mail for your local Bussman rep, and I'm quite sure if you request a copy you'll get it.

Also, they have a Safety package; Safety Basics with a LOT of good info, including the Fault Calcs

They use a point-to-point method, which I adapted to an Excel spreadsheet format.
WWW.Bussman.com is the site.
John
Edit to delete info for rep in NYC area as this is not Bill Addis

[This message has been edited by HotLine1 (edited 01-06-2005).]