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Jason, Service Technician
Category: Electrical
Satisfied Customers: 4278
Experience:  Over 20 years of experience in all types of installations, troubleshooting, and repairs.
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I need to know how to test a main breaker to tell if it is

Resolved Question:

I need to know how to test a main breaker to tell if it is bad or not. The problem I have is that I am losing half the power in my house while half of it stays on and there are no circuit breakers tripped. The power board checked their side and found nothing wrong. I have a murray 200amp mian breaker panel.
Submitted: 7 years ago.
Category: Electrical
Expert:  Jason replied 7 years ago.
HiCustomer Do you have a multimeter that will read AC voltage? If not, are you able to get one? Basic models are available at any home improvement store for around $25. The digital models are the easiest to use.

Customer: replied 7 years ago.
Hi jason, yes I have a multi-meter.
Expert:  Jason replied 7 years ago.
Good. Set it to AC volts, and if you have the option of setting the range, set it higher than 240 volts. With the panel cover removed, very carefully touch one meter probe to one of the main lugs at the main breaker, and the other probe to the other lug. You should see a reading around 240 volts.

Next, take a reading on any double pole breaker in your panel by touching one meter probe to each lug on the double pole breaker. If your main breaker is bad (which is fairly common), you will likely see a 0 reading, or a very low reading.

You can also read between the 2 bussbars if you have them exposed anywhere in the panel. If necessary, you can remove 2 adjacent breakers to expose both bars.

Expert:  Jason replied 7 years ago.
Another test you can try is turning the main breaker off and back on to see if power is restored. If that restores power, but it goes out again a short time later, that's another indication that the main could be bad. It's not a "smoking gun", but it's good evidence.

Customer: replied 7 years ago.

so, if I test the main breaker first and see a reading of 240 volts and then test a double pole and read o or have a very low reading then my main is bad?


That will cause the power loss in half my house with no circuit breakers tripped? Also when the heat/air unit kicks on the lights dim pretty low as it has just started doing this. Is this another sign that the main is going bad?

Expert:  Jason replied 7 years ago.
Assuming your utility company checked everything on their side (this includes the transformer/connections, the wire from the transformer to your house, and all connections in the meter base), then yes, that is strong evidence that your main is bad. However, loose connections and damaged bussbars will mimic a bad main breaker.

Customer: replied 7 years ago.
One more question. The power board did check and said they found nothing. So, Is the response you gave a sure way to test the main(check the main and then a double pole) to tell if it is bad or not? Thanks
Expert:  Jason replied 7 years ago.
Yes, I would be comfortable using that test as conclusive evidence. But I would also inspect the bussbars and the jaws on the double pole breaker to make sure they were mating properly, and that there was no corrosion between the two. I would also make sure all connections are tight. But based on what you have told me, all signs point toward a bad main breaker.

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