100 amp breaker tripping

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mlnk

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Customer's residential 100 amp breaker trips by itself sometimes. Other times the 50 amp breaker to the garage/guest house trips along with the 100 amp breaker. No other 20 or 15 amp breakers trip. Loads are refrigerators and sometimes coffee maker when breaker trips in the daytime. This is tripping about 4 times a day!
I have replaced the 100 amp cb, now the new one is tripping. All connections are clean and tight.
Power company has checked their side and say it is OK.

This is a new 100 amp service installed three months ago.There were no problems until two days ago. The house is 1000 SF, there is a detached garage and a guest house about 500 SF each. There are no electric heaters. There is one room air conditioner 120 volt but it is not being used. There is a 120 volt sump pump which draws 10.5 amps when it comes on. The sump pump will occasionally trip its GFCI about once every two months. New GFCI was installed three months ago. The house was painted recently and the paint pump tripped the 100 amp cb, it was reset and the painting continued without incident.
This is creating stress. Any ideas out there?
 
Have you installed a recording meter to see what the load is on the house? Does it trip at the same time of day or just anytime?
 
kwired said:
Meg the feed to the garage/guest house. Nothing is tripping on the load end of the feeder?
Click to expand...

I think this is the real problem. It is possible that the wire to the garage is causing the main to trip or sometimes causes the 50 amp cir to trip.
 
jbelectric777 said:
3) AIC (very important) how far away are you from the Poco transformer ?
Click to expand...

Are you really asking about the Amps Interrupting Capacity (AIC) of the breaker?
Or, did you mean to ask about the available Short Circuit Amps (SCA) from the utility?
 
I'm assuming the 100 amp breaker isn't tripping from load??

I'd meg the wires to the guest house. If everything megs clean, I'd shut the breaker off to the guest house for a day to help narrow it down. If the 100 amp main breaker holds you definitely know the feeder to the guest house is causing the problem. Then you start investigating...
 
1. Quit resetting it till you locate the issue.
2. Megger everything.
3. Check all motors.
4. If he CB is still on, perform a FOP at as much load as you can force on.
5. Use a min max meter to capture trip current.

If this is an instantaneous trip issue (ground or phase to phase fault) The rip current is in the 600-1000 1mp range.

You are possible causing a fire hazard every time you reset the CB and damaging the CB.

Divide and conquer.
 
Thanks to everyone for great advice. I disconnected the 50 amp feeders at both ends and checked them for continuity..about 3 ohms to each other and to ground. Previous "electrician" had buried EMT conduit. I wonder how much electricity they have been wasting over the years of leakage.......
 
mlnk said:
..Previous "electrician" had buried EMT conduit. I wonder how much electricity they have been wasting over the years of leakage.......
Click to expand...

Probably not as much as you think. Soil generally is not a very good conductor.

The resistance of the leakage likely increases as the current dries out the soil.

Even though previous guy buried EMT, why should that create 3 ohms between the conductors? Conductor insulation damage can do this in any underground raceway as they are all prone to filling with condensation.
 
I've had a 15A circuit that tripped once every few weeks and I could not figure out why. The circuit powered lights and over the range hood.

I'd get home and find some lights won't turn on and breaker is tripped. Reset it and its good for weeks or sometimes months.

Well, one day, I heard a pop behind the range hood and light went out. I pulled it out and found black marks from vaporized metal. The metal bushing had sharp edges and it was intermittently shorting things out. It was vibration/thermal expansion/contraction dependent.

You've also got to watch out for things like this:
2nkjth.jpg


It will read infinity on megger unless you use a voltage high enough to cause it to arc over.

You could use a fast hold capable DMM like Fluke 87, 18x and 28x. Set the range to 60v and monitor the voltage between ground and neutral on "fast min/max mode". min/max will be approximately symmetric. If you read something a few times over the average, that's an overload, but if you a peak max of something like 15v, that's an instant trip caused by a short.

Connecting between neutral and ground measures voltage drop on neutral or ground which is proportional to current.
 
Electric-Light said:
I've had a 15A circuit that tripped once every few weeks and I could not figure out why. The circuit powered lights and over the range hood.

I'd get home and find some lights won't turn on and breaker is tripped. Reset it and its good for weeks or sometimes months.

Well, one day, I heard a pop behind the range hood and light went out. I pulled it out and found black marks from vaporized metal. The metal bushing had sharp edges and it was intermittently shorting things out. It was vibration/thermal expansion/contraction dependent.

You've also got to watch out for things like this:
2nkjth.jpg


It will read infinity on megger unless you use a voltage high enough to cause it to arc over.

You could use a fast hold capable DMM like Fluke 87, 18x and 28x. Set the range to 60v and monitor the voltage between ground and neutral on "fast min/max mode". min/max will be approximately symmetric. If you read something a few times over the average, that's an overload, but if you a peak max of something like 15v, that's an instant trip caused by a short.

Connecting between neutral and ground measures voltage drop on neutral or ground which is proportional to current.
Click to expand...

The problem in the photo will not cause breaker tripping until some conductive foreign material gets into the area causing a lowering of resistance between conductors. When that happens you will be able to read it with a megger.
 
Electric-Light said:
I've had a 15A circuit that tripped once every few weeks and I could not figure out why. The circuit powered lights and over the range hood.

I'd get home and find some lights won't turn on and breaker is tripped. Reset it and its good for weeks or sometimes months.

Well, one day, I heard a pop behind the range hood and light went out. I pulled it out and found black marks from vaporized metal. The metal bushing had sharp edges and it was intermittently shorting things out. It was vibration/thermal expansion/contraction dependent.

You've also got to watch out for things like this:
2nkjth.jpg


It will read infinity on megger unless you use a voltage high enough to cause it to arc over.
Click to expand...
Along the lines of what kwire stated. What voltage might you think that would be? To jump the cut insulation in your picture?
 
Electric-Light said:
I've had a 15A circuit that tripped once every few weeks and I could not figure out why. The circuit powered lights and over the range hood.

I'd get home and find some lights won't turn on and breaker is tripped. Reset it and its good for weeks or sometimes months.

Well, one day, I heard a pop behind the range hood and light went out. I pulled it out and found black marks from vaporized metal. The metal bushing had sharp edges and it was intermittently shorting things out. It was vibration/thermal expansion/contraction dependent.

You've also got to watch out for things like this:
2nkjth.jpg


It will read infinity on megger unless you use a voltage high enough to cause it to arc over.

You could use a fast hold capable DMM like Fluke 87, 18x and 28x. Set the range to 60v and monitor the voltage between ground and neutral on "fast min/max mode". min/max will be approximately symmetric. If you read something a few times over the average, that's an overload, but if you a peak max of something like 15v, that's an instant trip caused by a short.

Connecting between neutral and ground measures voltage drop on neutral or ground which is proportional to current.
Click to expand...

That picture looks a lot like a conductor I saw in a 15kV MCC one time. It was in the process of being wired up and I happened to take a look inside. The electrician said he would take care of it. The next day I looked at it. There was regular black tape over the slits, probably about ten layers thick. It was not my problem, and presumably they knew what they were doing more than I would, being as I rarely even see 15kV conductors, but I always wondered if tape was the right way to fix such a thing.
 
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