Current limiting fuse & breaker

[wbd]

Hello,

here is my situation: In an old power plant (utility owned), I have buswork that has bolted in Westinghouse DB-50 breakers with LS overcurrent trip devices. The trip device is an air delayed magnetic type of device. On the line side of the breaker, bolted into the busbars feeding the breaker are Shawmut A4BY1600 Class L fuses. This is an installation from the 1950's.

Data:
Available short circuit: 49 kA sym, 66 kA, asym, X/R 6.62
Breaker Ratings: 42 kA sym, 50 kA asym, test X/R 6.6
Fuse Data from let thru table for 50 kA short: Irms 30 kA, Ipeak 68 kA

Without the current limiting fuse in place, the breaker is overdutied. Taking credit for the fuse, the current is reduced to 30 kA, within the rating of the breaker. However, I am not sure about the peak let thru current.

The breaker manual has a note that states the asym interrupting rating is the average rms asym current. With regards to the fuse peak let thru of 68 kA, would that be equivalent to an rms asym current of 48 kA (68 kA/sqrt2)?

Thank you in advance for replies

 

[DanDel]

First off, disregard the let-thru tables over-across-down for calculating interrupting ratings. These were proved incorrect a long time ago.
All of the A4BY fuses I've seen are usually rated 200kA. When these are used integrally with a power C/B, the C/B interrupting rating is usually upgraded to 200kA. Some of those older DB-50s may not have that on the nameplate, it may still say 50kA rating.
Are these fuses mounted in part of the C/B frame, or are they mounted separately?

 

[wbd]

These fuses are mounted separately. I don't believe integral fuse mounting was ever an option for the DB series breakers.

I find your statement about disregarding the let thru tables as proved incorrect a long time ago. These are being published in application guides, see link:

http://us.ferrazshawmut.com/oem/resources/literature/Advisor Circuit Protection Applguide.pdf

 

[DanDel]

Actually, the IEEE recommendation for not using this technique (IEEE 242-2001 5.8.2; C/B Protection using current-limiting fuses) may not apply in your case, since your DB-50 is probably classified as an "older style nondynamic impedance C/B" in that IEEE section.

The actual language in the book is "Using present methods, engineering protection for modern circuit breakers exhibiting dynamic impedance through the use of repulsion (or blow-apart) contacts is not possible".

 

[dpc]

If you are working as a consultant, I think you must tell the owner (and his insurance company) that there is no published test data that assures that the CLF is going to protect the breaker. It's been there a long time and may be fine, but unless someone has done a test on the combination, there is no way I, as a consultant, would say it is OK.

Also, I would definitely recommend replacing the old trip units. Those things were not reliable or accurate when new, and they are almost certainly unreliable now. New solid-state trip units should be installed.

Or think about replacing the whole thing.

 

[wbd]

dpc,

No I am not a consultant but the EE at a small IOU. I have been working on getting antiquated equipment replaced but need to "prove" why.

So far, I haven't had an answer to my question in the posting "With regards to the fuse peak let thru of 68 kA, would that be equivalent to an rms asym current of 48 kA (68 kA/sqrt2)?"

 

[dpc]

Yes, for a sine wave.

 

[wbd]

Thank you, so to recap:

the fuses will limit the fault current to within the withstand rating of the breaker.

 

[Zogzog]

The DB series did have a breaker with internal fusing, fairly common actually, it is called a DBL and I have a few hundred of them both DBL-50 and DBL-25's , 3 position and single position, most with electronic trip units already installed.

 

[dpc]

No, I would definitely NOT say that the fuses will limit the fault current to the withstand rating of the breaker. I would say that that is unknown.

Maybe you should see if someone at Shawmut or Bussman will write that down and sign it for you.

 

[wbd]

Thanks, I hadn't run across the DBL bkrs, most fused of this type I had seen were GE AKU.

Well at some point somebody thought the fuses would limit the fault to allow the use of a 42kA bkr. Keep in mind this installation is 40 - 50 years old, and I don't have that much gray hair to know what the codes or design practices were back then. Plus as a utility, NEC doesn't apply.

 

[Zogzog]

Just about every breaker line has a fused version. LKD, KDON, etc...

 

[dpc]

Yes everybody used to think this would work - and probably it does. But trying to get someone to assume the liability for it not working can be a problem.

I realize that the NEC does not apply to electric utilities. But the laws of physics still do. If you do comply with the NEC, you will have a much stronger position in terms of liability, if it ever comes to that. The potential downside of a failure here is pretty substantial.

 

[wbd]

Thanks DPC

yes I realize alot but I am working on building a case to replace these breakers (open air) and open, exposed bus work and I know I will get questions like why the fuses, won't they protect the breakers from overduty, why not just replace the trip device with new with Inst trip, we're a utility, so why should we be concerned with the NEC, etc.

I should also point out that the withstand rating of the bus work is unknown as no documentation could be found other than it was built in 1931.

 

[dpc]

I hear you - the liability and safety issues are the usually the most persuasive arguments for replacement of this stuff.

Exposed 480 V bus is a continuous arc-flash hazard. Did some work for an old hydro plant that had the original (~1930) oil-filled 480 V breakers and exposed bus. They were in the process of designing some replacement gear.

 

[wbd]

dpc, that is where I am at now. My AFH shows a Cat #2 but since I know the feeder breaker to this bus work failed to open recently, I eliminated it from the AF study and, of course it goes to Extreme Danger. I also made the argument that even though the operator is 3 feet away closing the breakers to sync generators on, the breakers are all open and exposed the risk is greater.

The whole issue on the safety and condition should be answered in a couple of weeks, as upper management decided to hire an outside consultant to review my findings, inspect the physical layout/condition of the open bus/breaker work, and make their own recommendations.