Tripped Main

[session88]

Yesterday a guy on my crew was working on a 120 VAC Circuit that has a small Ice-Cube relay on it. When he De-Energized the relay it shorted out internally. It also sparked out a bit. The 120 VAC source is from a Single Pole 20A QO style breaker in a 30 Space Main Lug (no main breaker) Panel (Panel A).

The faulted relay blew throught the single pole 20A breaker without tripping it than blew through the upstream Main Lug Sub-Panel that feeds the 30 Space Panel (Panel B). Panel A is fed by Panel B with a Two Pole 100A Breaker. Neither the 20A breaker in Panel A or the Two Pole 100A breaker in Panel B tripped.

The 3 Pole 100A Breaker across the building that feeds Panel B tripped (Panel C).

Why didn't the 20A or the Two Pole 100A trip? Could we lessen the chances of this occuring again by replacing the 30 Space Main Lug Panel with a similar panel with a Main Circuit Breaker?

Panel C is fed from our Large UPS System. Critical loads. We now need to figure out how to prevent similar faults from tripping multiple UPS Fed Panels throughout our facility.

Would a TVSS connected to either of these panels help prevented the 3 Pole 100A in Panel C from tripping if the same fault occured again?

I have seen this happen on larger 480 VAC systems with much higher current loads but not on a single 120VAC circuit with nearly Zero Load.


Thanks.

 

[charlie b]

The amount of load means nothing, and a TVSS would not have saved the day. You ask why the upstream breaker tripped instead of the 20 amp breaker feeding the circuit. I answer that there is no reason to expect any other result. Clearly, the fault current flowing the relay had to come originally from the main panel. The fault current flowed through three breakers: the 100 amp on panel C, the 100 amp on panel B, and the 20 amp on panel A. All three saw the same fault current, and all three began the process of reacting to that fault current. At least one of them is going to trip, and that trip will terminate the event, so that no other breaker will see the fault current any more. The essential question is, which one will win the race?

Please understand that the fact that one breaker is rated for 20 amps and another breaker is rated for 100 amps is not enough to tell you how either breaker will react to suddenly seeing 2000 amps or more. Every model of breaker has its own trip characteristics. Yes, it is certainly possible that the upstream 100 amp breaker might trip faster than the downstream 20 amp breaker.

In order to prevent recurrence of this type of event, you have to have a ?selective coordination? study performed. Then it is likely that one or more of the breakers may need to be replaced with different models. The idea is to select an upstream breaker for which the trip characteristics allow it to remain closed during a fault for at least a short while, to give the downstream breakers an opportunity to trip first. This is not a simple analysis, and it is not cheap. Sometimes you can get an equipment vendor to do it for you, but a licensed professional engineer will have to be the approving authority for the analysis.

 

[SG-1]

If the 3P upstream breaker has ground fault protection it will beat any standard downstream breaker on a ground fault.

 

[GerryB]

The amount of load means nothing, and a TVSS would not have saved the day. You ask why the upstream breaker tripped instead of the 20 amp breaker feeding the circuit. I answer that there is no reason to expect any other result. Clearly, the fault current flowing the relay had to come originally from the main panel. The fault current flowed through three breakers: the 100 amp on panel C, the 100 amp on panel B, and the 20 amp on panel A. All three saw the same fault current, and all three began the process of reacting to that fault current. At least one of them is going to trip, and that trip will terminate the event, so that no other breaker will see the fault current any more. The essential question is, which one will win the race?

Please understand that the fact that one breaker is rated for 20 amps and another breaker is rated for 100 amps is not enough to tell you how either breaker will react to suddenly seeing 2000 amps or more. Every model of breaker has its own trip characteristics. Yes, it is certainly possible that the upstream 100 amp breaker might trip faster than the downstream 20 amp breaker.

In order to prevent recurrence of this type of event, you have to have a ?selective coordination? study performed. Then it is likely that one or more of the breakers may need to be replaced with different models. The idea is to select an upstream breaker for which the trip characteristics allow it to remain closed during a fault for at least a short while, to give the downstream breakers an opportunity to trip first. This is not a simple analysis, and it is not cheap. Sometimes you can get an equipment vendor to do it for you, but a licensed professional engineer will have to be the approving authority for the analysis.

That's very interesting. Are fuses the same? I had a call for an apartment building the elevator guys said a fuse blew downstream and they couldn't change it. It was a 3phase disco 150 amp 250volt fuse that was bad located in the electrical room. I changed it and checked the elevator room and it also had a newer disco with 150 amp fuses. The old fuse was a buss that said one time, the newer fuses and the new one I installed said time delay. The old one also said non on it, non time delay?

 

[SG-1]

That's very interesting. Are fuses the same? I had a call for an apartment building the elevator guys said a fuse blew downstream and they couldn't change it. It was a 3phase disco 150 amp 250volt fuse that was bad located in the electrical room. I changed it and checked the elevator room and it also had a newer disco with 150 amp fuses. The old fuse was a buss that said one time, the newer fuses and the new one I installed said time delay. The old one also said non on it, non time delay?

Yes, fuses are the same way and if not coordinated the upstream fuses can open first. A NON type fuse is one of the most primative (old) fuse types still in use. It is not a time delay or duel element type.

 

[GerryB]

Yes, fuses are the same way and if not coordinated the upstream fuses can open first. A NON type fuse is one of the most primative (old) fuse types still in use. It is not a time delay or duel element type.

Thanks, good to know and something I should have known but don't get many calls in that area. I can also tell the owner he should change the other two fuses which I did not do.

 

[Sparky3141]

Being new and all, I've been looking back at older threads and this is an interesting thread for me. I work at Pearl Harbor Naval Station and often encounter very old equipment. I just got a call late Friday after a customer was shutting off some lighting on a 3 phase 20 amp breaker (these were originally installed at least some 50 odd years ago without switches) and the breaker faulted and smoked up a storm. It tripped the panel 100 amp main, skipped past the 200 amp that fed it and the 350 amp that fed that one and tripped the feed in the adjacent building which is the high voltage station. (breaker not high voltage) I assumed that the most sensitive breakers tripped and as soon as I cleared the fault by removing the faulty breaker and securing the circuits that it ran, reset the breakers that it'd be fine like it has always done...and it was.

We turned the remaining lights on after resetting the breakers and all was fine and I walked outside. Then it happened; when the customer was turning the lights back off, when he opened the last 20 amp single pole breaker, the same breakers tripped again! The originally affected circuits were wired nutted off in the panel and this time there was no fault in the breaker. I left it off, reset the breakers again and told him not to mess with it again until Monday and I try to wrap my brain around this.

So I said all that to ask this: how can I get a fault current from turning the circuit off? I know there is an arc draw inside the breaker due to the load (less than 10 amps) but can that create or be construed as a fault current and how can I check to find out? Being that a selective coordination study is a beast too big to tackle and way out of my league, are there any other options? Would appreciate any additional input. (uh, should I have started a new thread? Not sure, being new and all.)

 

[GoldDigger]

If only one of the breakers in the series includes ground fault detection and something about the turn-off process produces ground current, even briefly, it could cause only that breaker to trip.


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[Sparky3141]

Yes. I didn't get a look at the MDP in the station that feeds into the building so I was thinking that that one may have ground fault protection which could account for it tripping. But the main in the service panel with the circuits on it does not and it was tripping simultaneously. That's my conundrum.

But like I said, this stuff is really ancient; might that have something to do with it? And if it does; is there a definitive method of measuring it? It's just a lighting panel in a warehouse setting so I can mess with it as much as I like tomorrow to troubleshoot.

I hate to tell them to replace the whole panel if there are any other options but I have to tell them something and that's the way I'm leaning right now. It's mostly 1/2" and 3/4" EMT's feeding from the top on the surface so it shouldn't take a guy about a day to do I 'spect.

 

[kwired]

Being new and all, I've been looking back at older threads and this is an interesting thread for me. I work at Pearl Harbor Naval Station and often encounter very old equipment. I just got a call late Friday after a customer was shutting off some lighting on a 3 phase 20 amp breaker (these were originally installed at least some 50 odd years ago without switches) and the breaker faulted and smoked up a storm. It tripped the panel 100 amp main, skipped past the 200 amp that fed it and the 350 amp that fed that one and tripped the feed in the adjacent building which is the high voltage station. (breaker not high voltage) I assumed that the most sensitive breakers tripped and as soon as I cleared the fault by removing the faulty breaker and securing the circuits that it ran, reset the breakers that it'd be fine like it has always done...and it was.

We turned the remaining lights on after resetting the breakers and all was fine and I walked outside. Then it happened; when the customer was turning the lights back off, when he opened the last 20 amp single pole breaker, the same breakers tripped again! The originally affected circuits were wired nutted off in the panel and this time there was no fault in the breaker. I left it off, reset the breakers again and told him not to mess with it again until Monday and I try to wrap my brain around this.

So I said all that to ask this: how can I get a fault current from turning the circuit off? I know there is an arc draw inside the breaker due to the load (less than 10 amps) but can that create or be construed as a fault current and how can I check to find out? Being that a selective coordination study is a beast too big to tackle and way out of my league, are there any other options? Would appreciate any additional input. (uh, should I have started a new thread? Not sure, being new and all.)

Have you confirmed that turning that switch off actually opens one or more contacts or does it by chance simultaneously close something else and that is what has the problem? Think Chicago three way or similar set up that may use double throw switches and opens one circuit but closes another when operated.

The opening of upstream overcurrent devices is all about the trip curve of involved devices and if it is undesired or unacceptable for the upstream device to trip in certain situations then more careful selection of devices is necessary, and usually means a different type of fuse or specialty breakers will be needed. The use of common miniature type breakers usually gives somewhat unexpected results because they will all have similar ratings and is kind of a crap shoot as to which one will trip first under certain conditions.

 

[Sparky3141]

I didn't get back there today and now with the holidays not sure when the shop will be opened back up. A little befuddling but I'll get in there and do my best; thanks for the inputs.