ECN Forum Forums Code Related Discussion NEC & other Code issues Fault current data & arc flash calculations?

George:

FYI, transformer approx fault current ratings are within the Bussman Literature.

Keep in mind that these figures are approximations. I use the Bussman literature (with permission) for one of my classes.

The SPD book is a good investment for anyone looking for info, as is the website

Thanks Hotline- And I'm sure those numbers will be fine. I was trying to make a point of how miserable it is to work with some of these utility companies who have cut backs, the old timers are not there anymore and the contractors are stuck holding the bag.

My guess is the person who is asking for that information will not challenge any information the contractor provides.

In Michigan 70E usually never gets looked into until there is an "incident". OSHA (MiOSHA in Michigan) enforces 70E and electrical inspectors are rarely in that loop.

The other item I would like to mention is that Bussman has a CD that can be used for fault current calculations. I think it is free (it was to me) and it works on a PC or a Mac. That might be what the others are mentioning in this thread.

George:
I also use the "Safety Basics" materials from Bussmann. It's an interesting short <30 min item.

AS to your inuendo about supplying a 'lot of paper'....I used to do that! Be surprised how quick the inquisition came to a halt!

Yes, you do use the SCA to determine what the AIC of your protective device needs to be. But you use the SCA, not the AIC, when making arc flash calculations.

A similar concept is how we use load amps to size branch breakers, but, we do not use the branch breakers to size the main service.

jbd:
Not beating a dead horse here, but this is from my original reply to the OP:

"The flash boundry calcs are not something I know how to do; I think it's best left to an engineer."

AEP recently replaced the pad mount serving my house. I just checked the label for the rating and impedance, and did my own estimated calculation based on an underestimate of underground service "drop" length.

Of course that might be harder to do with pole pigs.

Another way to estimate fault current involves measuring voltage drop under 2 or more large known loads. You then extrapolate the voltage drop to zero to get the current. The larger the test loads, the more accurate the extrapolation can be.

If Trick 440 still needs help with the calculations and the Arc Flash values, I was at a seminar in Grand Rapids and talked to Dave Vallier (a highly qualified engineer from Bussmann) and he can help you out here in SE Michigan. Bussmann is very active with IAEI and I would recommend contacting Dave at this address: dave.vallier@cooperindustries.com He suggested I convey this message to you.

My biggest word of advice is to not overestimate the amount of available bolted fault current. A high value of current will cause the protective devices to open fast which reduces the arc flash incident energy. A more realistic value of bolted fault current may cause the devices to open slowly resulting in higher incident energy.

If I have a single fault current value (especially if it looks like an infinite bus calculation), I will run additional arc flash calculations at 50% and 75% of bolted fault.