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Old 03-16-2008, 09:46 AM   #1
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Main Circuit Breaker Test


I tested the voltage of the two main hot wires going into my main circuit breaker (the circuit breaker was off). The voltage read 124V on each. I then turned the circuit breaker on and re-tested the voltage. One wire tested at 0V and the other tested at 122V. Does this mean I have a bad main circuit breaker?

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Old 03-16-2008, 10:51 AM   #2
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Main Circuit Breaker Test


What made you take this reading in the first place...
Did you take the readings at the same location.

Oh, and your welcome.....

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Old 03-16-2008, 10:55 AM   #3
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Main Circuit Breaker Test


I took the readings because I lost power in certain locations of my house last night. Yes, I took the reading in the same location.
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Old 03-16-2008, 10:56 AM   #4
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Main Circuit Breaker Test


IF you are taking the readings on the main service WIRE then the breaker being on or off should not effect your readings. If you took the first readings from the wire and the second reading you took was off the bus bars then it sounds like the main is bad.

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Old 03-16-2008, 10:58 AM   #5
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Main Circuit Breaker Test


With the breaker turned off you should read around 120 volts from the conductor lug to ground on each leg on the line side. You should read around 240 across both lugs. With the breaker turned to the on position you should read the same as what you read on the line side.
Bad breakers sometimes cannot be determined by checking the voltage. A current check should be performed and both values should be very close.
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Old 03-16-2008, 11:00 AM   #6
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Main Circuit Breaker Test


You say you took these reading on the INPUT side of the main?
Then turned it back on and took the same readings?

HOW can this be a bad main then???? You are getting bad readings on the LINE side, NOT the load side.
Without a load you can get a false positive reading. As soon as a load is placed on the line the fault is exposed.

You have a problem outside the house. Call the POCO or an electrician and have them check the connections at your house, on the pole and in the meter pan.

THIS IS NOT a main breaker problem.
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Old 03-16-2008, 11:06 AM   #7
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Main Circuit Breaker Test


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Originally Posted by Speedy Petey View Post
You say you took these reading on the INPUT side of the main?
Then turned it back on and took the same readings?

HOW can this be a bad main then???? You are getting bad readings on the LINE side, NOT the load side.
Without a load you can get a false positive reading. As soon as a load is placed on the line the fault is exposed.

You have a problem outside the house. Call the POCO or an electrician and have them check the connections at your house, on the pole and in the meter pan.

THIS IS NOT a main breaker problem.

I have never experienced this before. Just curious as to what would cause the false positive by adding a load
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Old 03-16-2008, 11:11 AM   #8
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Main Circuit Breaker Test


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Originally Posted by bwinters12 View Post
I have never experienced this before. Just curious as to what would cause the false positive by adding a load
CURRENT FLOW!!!!!!!!!
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Old 03-16-2008, 11:16 AM   #9
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Main Circuit Breaker Test


Thats a very detailed answer. How would current flow explain a reading of 120 volts on a feed that has no voltage.
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Old 03-16-2008, 11:17 AM   #10
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Main Circuit Breaker Test


You have to read voltage BETWEEN the legs.

The 120 on the dead leg is being beckfed from the live leg via a 240V circuit or multi wire branch circuit.



ALWAYS test phase to phase.
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Old 03-16-2008, 11:21 AM   #11
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Main Circuit Breaker Test


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You have to read voltage BETWEEN the legs.

The 120 on the dead leg is being beckfed from the live leg via a 240V circuit or multi wire branch circuit.



ALWAYS test phase to phase.
He stated he shut the main off and read 120 on both legs so it could not be back fed through the 240 v circuits cause they are not getting any power
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Old 03-16-2008, 11:28 AM   #12
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Main Circuit Breaker Test


There is no current flow on the legs when the main is off. With the main on the current flows on the one good leg and the open leg is exposed since current is trying flow on it but cant.
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Old 03-16-2008, 11:29 AM   #13
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Main Circuit Breaker Test


Quote:
He stated he shut the main off and read 120 on both legs so it could not be back fed through the 240 v circuits cause they are not getting any power

Since he is a newbie I assumed that he didn't necessarily describe things correctly. They tend to mix up the whole in/out thing.

Anyways....ALWAYS test phase to phase.
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Old 03-16-2008, 11:40 AM   #14
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Main Circuit Breaker Test


Ok guys, here is a picture of what I am working with. I think I know what's wrong. I am only touching the breaker that says "main" in the photo. Do you see those two screws on the breaker that says "main"? That's where I am taking my voltage measurements using a multimeter.

Scenario 1: "main" breaker is set to "off" position. I put one multimeter lead on a ground and the other on the left side screw of the "main" breaker. Multimeter reads 120V. Next I put one multimeter lead on a ground and the other on the right side screw of the "main" breaker. Multimeter reads 120V.

Scenario 2: "main" breaker is set to "on" position. I put one multimeter lead on a ground and the other on the left side screw of the "main" breaker. Multimeter reads 4V. Next I put one multimeter lead on a ground and the other on the right side screw of the "main" breaker. Multimeter reads 120V.
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Old 03-16-2008, 01:48 PM   #15
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Main Circuit Breaker Test


There's no question about it, you have an open somewhere from the main breaker back into the power lines. It could be anywhere. Time to get the POCO involved, it's likely their problem. (They actually like fixing stuff on Sunday, it's overtime.)

The reason you get 120V with the breaker off is because of 'capacitive coupling' of the wire. If you're using a digital multimeter, its input impedance is very high, meaning it doesn't 'load' the circuit much. Wires next to each other that are open on both ends (like the left one is now) tend to pick up voltage from the other ones. Even if they're insulated. Usually the current is very low, so no hazard exists, but voltage induced this way in an overhead power line several miles long can easily kill a lineman. (And has!)

In your case, there's enough capacitive coupling the make the meter read, but when you turn the breaker on, the load of the building cancels this out. The result is a very low voltage reading.

Trace the wire back, and make sure anything you have acess to is tight. Don't cut the seal or pull the meter, the POCO will do that for you. If it's good all the way to the meter, then it's the POCO's problem.

Rob

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