Main Breaker Tripping

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blues

Member
Location
Nevada
I have an existing 120/208v, 3ph, 4 wire panel with a 150amp main circuit breaker. When the owner opens the store and truns on all the lights the main breaker will trip within 30 minutes. He will then turn the main breaker back on and will remain on without incident the renainder of the day. Amp probe readings are as follows: A=50amps B=70amps C=42amps Neut=16amps. Any suggestion on what to look for? :confused:
 

bphgravity

Senior Member
Location
Florida
Re: Main Breaker Tripping

I would check the different temperature readings of each terminal on the main breaker and check for color change or arcing at the bus connection on the load side of the main. You may also need to check the resistance of each pole of the breaker to see if one or more internal contacts are going bad. When all is said is doen, you may not find any discprepancies. The main might be simply be going bad.

On the other hand, the owner may be doing something at initial startup which he is not indicating to you or does not realize that may be causing the problem only at first startup.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: Main Breaker Tripping

I would be looking for things that might be on this circuit that might tend to run continuously when first turned on, but later on run intermittantly. Like if he had a bunch of hot plates, heaters, fridges, on these circuit. They might all turn on and stay on for a while when first turned on, but later on they only come on intermitantly.
 

charlie tuna

Senior Member
Location
Florida
Re: Main Breaker Tripping

you have two choices:
install a data recorder on the service wires to determine if the trip is caused by an actual current draw.

continue to guess at what it might be?


there could be something causing a low voltage on the service coming into the building. maybe his neighbor has a large load that he starts at a particular time. or maybe his water heater. bad breaker. poor connection. its just a guess without the right tool!
 

sameguy

Senior Member
Location
New York
Occupation
Master Elec./JW retired
Re: Main Breaker Tripping

I had this same problem and it was that out back the patio slab broke from undermining over the years, snow etc., dropped about 6" crushing the pipe below and caused a short in the am. due to condensation and was ok untill it rained or the next am.
I cut the old pipe and repiped.
 
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