Hi everyone,

I am doing a short circuit analysis on a manufacturing plant in the U.S. and have come across some bus duct runs in the plant that are under-rated for the calculated short circuit currents (22kA rms rated withstand, vs 27kA momentary short circuit, calculated at the end of the 100' run of bus duct). I am of the opinion that a bus duct with a higher rating should replace the one that is presently installed, however would cable limiters be a possible option to reduce the short circuit current (if operating in the first half cycle of course) to ensure the present bus duct remains within its ratings? I am thinking that a swap out with higher rated bus duct may cost more on a material basis, but may be near the same price once engineering assessment and funky installation are considered; not to mention, it is a cut and dry application for a switch out.

My biggest concern is that I understand a P.E must sign off on cable limiter installations and it really doesn't sound like the NEC has provisions for this type of application of cable limiters.

The duct is fed by 2 runs of 350MCM cable per phase in conduit.

Thanks for the help

I agree with you NEC does not[ yet] approve current limiter employing.
But I don't understand why do you think 350 MCM [CU,XLPE?] could not withstand 27 kA for 1 sec.
If you will use the known formula:
Ik=5.067/10^3*Akcmil*SQRT((TCAP/tc/alpha/ro20oC*LN((Ko+Tm)/(Ko+Ta))
a single 350 MCM copper cable XLPE insulated withstand for 1 sec. 30 kA.
TCAP=3.42;alpha=0.00393;ro20oC=1.72;
Ko=234;Tm=250;Ta=40;tc=1sec.

Hi 7anoter4,
It is actually the Square D CP bus duct with the 22kA withstand I am worried about; I am not concerned about the cable. That was the issue: can cable limiters be used to protect the bus duct, or would it be better to look at a typical current limiting fuse (i.e. class J, RK, etc.)

The part of your question that concerns me is that you do not indicate the value of the the short circuit current computed at the beginning of the bus run; you only mention the end of the run which I understand to be at the point of the longest length. A fault near the beginning will be greater, so the duct withstand needs to be considered at the start of the run. A current limiting fuse will, of course, limit the current but the current will potentially rise in the first half-cycle until it reaches the point at which it melts and begins the interrupting portion of its operation. The symmetrical equivalent of that peak will need to be lower than the withstand rating of the bus duct. This will be true of either cable limiters or current limiting branch circuit fuses. You will need to consult fuse manufacturer's current limiting charts to answer your question.

eddunnpe

I apologize eddunnpe, the value at the beginning is 27kA. I erred in the first posting.

The cable limiter would only do you any good if you can find a Listed series rating combination where the bus duct was tested by an NRTL with the cable limiter and found to be suitably protected. But don't hold your breath while you look for that wild goose.

The cable limiter would only do you any good if you can find a Listed series rating combination where the bus duct was tested by an NRTL with the cable limiter and found to be suitably protected. But don't hold your breath while you look for that wild goose.

Thank you very much davidbeach. I was thinking that was the ultimate requirement for this type of installation. I am going to stick to what I know with this one!

Can you re-run the incoming feeder to the busway and add impedance from addition length?

Does a series rating apply to bus duct? I thought the series rating was for two interrupting devices in series. My understanding of the need for series testing is that a current-limiting device may interact with a downstream interrupting device such that it won't successfully interrupt even if the apparent let-through current is less than the interrupting rating. If the overdutied device is not an interrupting device, then I don't see the rational for disallowing the use of a current-limiting device.

Series rating is highly theoretical. No reason to believe it exists in practice, thus the comment about not holding ones breathe while hunting wild geese. The deeper point being that current limiting devices only help some other piece of equipment through a listed combination; that let-through energy, up-over-and-down, or what ever approach might be attempted won't be accepted unless validated through recognized third party testing.

Series rating is highly theoretical... that let-through energy, up-over-and-down, or what ever approach might be attempted won't be accepted unless validated through recognized third party testing.
That is my understanding with series ratings (and applying CLF as well). The idea of increasing the cable impedance may actually be viable. I don't have much knowledge of material and installation costs of bus duct, but what about simply replacing this duct with a higher short-circuit rating one (eg. SQD CP type, 22kA, vs SQD CPH type, 42kA)? Is it worth the cost?