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Modeling a Current Limiting Fuse
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Author:  alf [ Sun Dec 15, 2013 9:11 am ]
Post subject:  Modeling a Current Limiting Fuse
When doing the short circuit analysis with a current limiting fuse (CLF), ETAP models it as if it is not in the circuit. The resulting momentary analysis shows the downstream equipment over dutied when in fact it is not. Any ideas on how to handle this?
Author:  Jim Phillips (brainfiller) [ Sun Dec 15, 2013 9:32 am ]
Post subject: 
alf wrote:
When doing the short circuit analysis with a current limiting fuse (CLF), ETAP models it as if it is not in the circuit. The resulting momentary analysis shows the downstream equipment over dutied when in fact it is not. Any ideas on how to handle this?

The way that a series rating works (current limiting device protecting downstream equipment) is the non limited current is used - such as you are correctly seeing with ETAP.

UL provides a method for testing combinations of devices in series which can lead to a listed series rating.

As an example: Let's say you have a panel rated 22kA and an available short circuit current of 37kA. The panel is protected upstream by a specific size and class of current limiting fuse. If a UL listed combination of the fuse and panel exists, it would be labeled (NEC requirement) on the equipment. It may be something like "Series Rating 100,000 Amps" or some other rating (I made this particular one up just as an example).

A long time ago (over 30 years) a method or predicting the "let thru" current was commonly used which is what I believe you might have been expecting. The breaker industry (I was at Square D at the time) found there were problems in using this method and the UL series ratings were developed. i.e. instead of predicting what may happen and sometimes having problems, test it, rate it, be sure of how it will work.

So, in your case you need to check with the breaker manufacturer to see what series rating they have and what devices upstream need to be used to achieve the rating.
Author:  alf [ Mon Dec 16, 2013 4:36 am ]
Post subject: 
Jim,

Thanks for the reply. I was using the up, over, down method which I did not realize was invalid. We have several GE Mag-Break type TEC MCCB's rated 10k on a 20k system. GE has no series ratings for these. Hmmmm.
Author:  Jim Phillips (brainfiller) [ Mon Dec 16, 2013 7:08 am ]
Post subject: 
alf wrote:
Jim,

Thanks for the reply. I was using the up, over, down method which I did not realize was invalid. We have several GE Mag-Break type TEC MCCB's rated 10k on a 20k system. GE has no series ratings for these. Hmmmm.

I thought that is what it sounded like. The NEC began permitting "up over and down" although not calling it that, a few code cycles ago. There is language about engineered series ratings. It is pretty restrictive though, requiring all sorts of documentation etc. It was to address the problem that you may have which is older equipment that never had a tested series rating. Good Luck!
Author:  Marcelo [ Tue Jun 24, 2014 6:29 pm ]
Post subject:  Re: Modeling a Current Limiting Fuse
You may be able to replace the TECs with THED or Spectra E which have similar form factor and higher short circuit ratings.

Unluckily the TEC CBs are quite small. It may be tempting to use a Class L CL fuse above, however those CB may "pop" at very low fault currents and essentially try to protect the fuse instead of the other way around. This is why, as Jim described, series ratings came about.
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