Current limiting fuse & breaker
Current limiting fuse & breaker
(OP)
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
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






RE: Current limiting fuse & breaker
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?
RE: Current limiting fuse & breaker
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
RE: Current limiting fuse & breaker
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".
RE: Current limiting fuse & breaker
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.
RE: Current limiting fuse & breaker
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)?"
RE: Current limiting fuse & breaker
RE: Current limiting fuse & breaker
the fuses will limit the fault current to within the withstand rating of the breaker.
RE: Current limiting fuse & breaker
RE: Current limiting fuse & breaker
Maybe you should see if someone at Shawmut or Bussman will write that down and sign it for you.
RE: Current limiting fuse & breaker
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.
RE: Current limiting fuse & breaker
RE: Current limiting fuse & breaker
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.
RE: Current limiting fuse & breaker
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.
RE: Current limiting fuse & breaker
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.
RE: Current limiting fuse & breaker
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.