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.
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
#46901 - 01/05/0512:33 PMRe: Fault Current & Arc Flash Calculations
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.
#46902 - 01/05/0502:06 PMRe: Fault Current & Arc Flash Calculations
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 Jackson, Salt Lake City
#46903 - 01/05/0503:52 PMRe: Fault Current & Arc Flash Calculations
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.
#46904 - 01/06/0507:48 PMRe: Fault Current & Arc Flash Calculations