Short Circuit Analysis

[lionelpd]

I just received my short circuit study and found that some of my protective devices are not rated for the full available fault current. For example my available fault current for a 480v breaker may be 44.1kA and the rating of the breaker is 40kA. Another example is that a 480v breaker is rated for 50kA and the available fault current is 63.0kA. My industrial facility is obout 40 years old. Do I need to pruchase all new higher rated breakers where this is the situation?

 

[petersonra]

I just received my short circuit study and found that some of my protective devices are not rated for the full available fault current. For example my available fault current for a 480v breaker may be 44.1kA and the rating of the breaker is 40kA. Another example is that a 480v breaker is rated for 50kA and the available fault current is 63.0kA. My industrial facility is obout 40 years old. Do I need to pruchase all new higher rated breakers where this is the situation?

I would take a look at the assumptions made in making the calculations. Ten feet of extra wire can make a big difference. If it happens that the wire length was underestimated by even a relatively small amount, the numbers could change enough if you used the true values that you are OK.

It is also possible that some of the other assumptions used in the calculations are off. Unless a good field survey was made, it is entirely possible they used typical values rather than actual values for some of the calculations.

 

[etszap]

110.9 Interrupting Rating.
Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.

 

[ron]

If you trust the accuracy of the calculations, you must do something about it, not necessarily because of the NEC, because that would effect you only if doing new modifications/work to that equipment, but for liability now that you know.

 

[jim dungar]

NFPA 70E 210.5 requires protective devices to be maintained so that they can "adequately withstand or interrupt available fault current".

 

[davidr43229]

I agree with Ron here:

If you trust the accuracy of the calculations, you must do something about it, not necessarily because of the NEC, because that would effect you only if doing new modifications/work to that equipment, but for liability now that you know.

The only Assumptions are: did they calculate with a known available fault current at the primary or was the study performed with "Infinate" fault current available?
That is what I would ask. The rest should be just plugging in the numbers of conductors, length of conductors,pairs per phase and magnetic or non-magnetic conduit.
If after you have these questions answered (because you really can question the study) and found it to be true, think about series rating:
http://cooperbussmann.com/2/SeriesRatings.html
to protect your existing installation, however maybe think about adding a UL listed combination of high current limitation in front of those breakers, that by themselves are exceeding the existing available fault current, could be your cheaper fix.
Just my $.02

edited to correct my poor spelling.

 

[James@CHA]

I would put together a "study" of your own, and compare the values of the available fault current. This way you can be confident in the numbers beofre replacing the equipment.

I have a quick calculation set up in excel if you wouls like to try it. Send me a message and i'll email it to you.

You just need to know a few key items including transformer impedance and feeder length/size.