Troubleshooting Question

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cohosilver

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Situation: Square D 100A 277/480V I-line circuit breaker that feeds a Sqaure D 277/480V NF MLO panelboard is tripping for no obvious reason. There is no load on the panelboard (A-13.3A, B-23.6A, C-0.7A). There is no load on the branch circuits at the panelboard which only has 6 20A circuit breakers. There is no physical damage to the feeders or branch circuit conductors and the feeders were just replaced because the circuit breaker had tripped so it is unlikely that it is the conductors. The feeders are a #2 awg 4/C CT Cable Tray Cable. The breaker has tripped 4 times and the 4th time it tripped the 400A Main Breaker for the I-line panel. (Note - I am not the guy repeatedly resetting the breaker, it is the customer). The breaker does not immediately trip, it can hold for hours so everything is working whenever I arrive.

I am going to replace the 100A circuit breaker at the I-line panel with the hypothesis that it is a bad 100A circuit breaker. Other than meggering the cables to test the insulation resistance (I do not have a megger available) is there any sugggestions of what the issue could be?

I am stumped.
 
Well you might look every where to see what is being used. I know it's difficult when it's working when you get thier.
But tell them to turn every thing on.
If the 400 had not tripped It would most likely be a bad 100 amp breaker. Though it still might.
 
cohosilver said:
Situation: Square D 100A 277/480V I-line circuit breaker that feeds a Sqaure D 277/480V NF MLO panelboard is tripping for no obvious reason. There is no load on the panelboard (A-13.3A, B-23.6A, C-0.7A). There is no load on the branch circuits at the panelboard which only has 6 20A circuit breakers. There is no physical damage to the feeders or branch circuit conductors and the feeders were just replaced because the circuit breaker had tripped so it is unlikely that it is the conductors. The feeders are a #2 awg 4/C CT Cable Tray Cable. The breaker has tripped 4 times and the 4th time it tripped the 400A Main Breaker for the I-line panel.(Note - I am not the guy repeatedly resetting the breaker, it is the customer). The breaker does not immediately trip, it can hold for hours so everything is working whenever I arrive.

I am going to replace the 100A circuit breaker at the I-line panel with the hypothesis that it is a bad 100A circuit breaker. Other than meggering the cables to test the insulation resistance (I do not have a megger available) is there any sugggestions of what the issue could be?

I am stumped.
Click to expand...

(Bold added by me)

I find it hard to believe that the 100 amp breaker in any way can cause the 400amp main to trip, unless the 100 amp breaker has an internal fault (which would most likely have caused a fire or explosion by now.)

480 sparky has the best answer, use a recording ammeter as I am willing to bet there is something that is fed by that 100 amp breaker that has a momentary fault.

Or just wait until something blows up then as Adam Savage would say "Well there's your problem. "

And school your customer on the fact that continually resetting a breaker especially at that voltage level is dangerous and one day he's gonna get a nasty and possibly fatal surprise.
 
There is no load on the panel board (A-13.3A, B-23.6A, C-0.7A). There is no load on the branch circuits at the panel board which only has 6 20A circuit breakers. There is no physical damage to the feeders or branch circuit conductors and the feeders were just replaced because the circuit breaker had tripped so it is unlikely that it is the conductors.
Click to expand...

Just randon thoughts:

But I'm just really thinking a none reamed conduit! even though you said cable tray... :)

No physical damages; these were tested and taken out, assumed bad and just taken out, was this their trial to
only error in trying to fix the dropping breaker? Has anyone any answers as to the nature or inspect the removed wire?
Did anyone lay their hands on it and inspect it? (Kevlar gloves)

Did you put a tool on the lugs? Did you check all the grounding? It's a pinched wire in transition from tray to panel, tray to device.

How long is the history on this? Have you completely walked the circuit?
Does the circuit run through various temperatures? Did you check the tray for a ground fault?
Is the wire all the correct insulation value and at all the same Hz in the cable tray?
Does the circuit run through various temperatures? Is there steam lines in area?

What does the bus look like? Take any temperature reading on all the breakers, have you considered maybe
isolation or moving this one breaker?

Did they move this breaker in its life cycle of the service? Are they dial up breakers in host panel?


I am going to assume the ABC is load, read at the hundred amp breaker! Did you load up the C phase and see if something might happen? What on the six circuits? What in the facility has any unusual use or release of energy?
 
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The most likely cause is a short circuit on one of the branch circuits. An overload is the least likely cause, especially because you say the 400A breaker tripped once.

You need to know how much fault current the 100A breaker has been interrupting. UL listed breakers are only rated for one full AIC operation (probably about 18kA for your breaker) before they must be replaced.
 
You need to break this down. I would start with 600v inline fuse holders wired right into each of the six 20 amp breakers. Install fast acting fuses dialed in close to the full load of each circuit.

I just had a service call for a small retail store where a main 70 amp fuse was blowing every couple weeks. The fuse was located in the landlord's electric room in a 480v mdp. The only real load in the store was a 7 1/2 ton package hvac unit on the roof with 60 amp fuses. The load on the unit was only 29 amps so I installed 40 amp fuses and called my buddy who is a hvac tech. He said to turn the thermostat up to 90 to put the unit up to full load and within three minutes there was a severe noise and a large flash from one of the fuses- problem found. One of my experienced guys could not believe that a 70 amp fuse would blow before a 60.
 
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