Main breaker tripping

[joerivera79]

I have a 100A main breaker that is tripping, I placed a amp clamp on it and the breaker is not overloaded. One of the hots coming from the meter feels but the other is not, anybody ever run into this before?
Any thoughts?
Thanks

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[Little Bill]

I have a 100A main breaker that is tripping, I placed a amp clamp on it and the breaker is not overloaded. One of the hots coming from the meter feels but the other is not, anybody ever run into this before?
Any thoughts?
Thanks

Sent from my SAMSUNG-SGH-I337 using Tapatalk

"One of the hots coming from the meter feels"............???

I'm guessing you meant to say feels hot. If so the breaking is tripping on thermal/overheating. Most likely you either have a loose connection at the lug or it has been loose for a while and has arced leaving a high resistance connection. Could be the breaker is damaged internally and would need replacing.

It's pretty sure that if it's not tripping on overload or something shorted you have a problem at or with the breaker.

 

[joerivera79]

Yes. That was my thought, voltage was good and tightened the lugs but still felt warm. Did not trip while I was there but customer is considering a panel change with a new main breaker.

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[GoldDigger]

While considering the panel change, try pulling the breaker and looking at the bus stab contact points.
Any visual damage or signs of overheating will just get worse, and might affect any new breaker in the same location.

 

[ActionDave]

Yes. That was my thought, voltage was good and tightened the lugs but still felt warm. Did not trip while I was there but customer is considering a panel change with a new main breaker.

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I agree it sounds like a bad connection. If a panel change does not happen you need to get the wire out, cut off the burnt part and get to some fresh conductor. Maybe even replace the breaker.

 

[StarCat]

Line Connections

Line Connections

I dealt with a similar problem on a 600 AMP service, and found the burned connection on the Transformer Secondary.

 

[templdl]

I have a 100A main breaker that is tripping, I placed a amp clamp on it and the breaker is not overloaded. One of the hots coming from the meter feels but the other is not, anybody ever run into this before?
Any thoughts?
Thanks

Sent from my SAMSUNG-SGH-I337 using Tapatalk

My thoughts:
I have an infrared temp probe for my Fluke 26 which has helped me identify a source of heating. If yuu have resecured and then verified that the line/load connections are tight and still have an issue there may be a chance that one on the contacts could be compromised. If so that could be a source of heating which often results in the breaker deratings which results in nuisance tripping.
If you suspect this simple us you VM to measure the voltage from the line too load connections for each pole looking for a voltage. Ifthe contacts are OK you should not measure any voltage. Should you read a voltage that would indicate that there in a voltage drop which would result in heating. If so the breaker should be replaced. As a temporary fix exercise the breaker numerous times which cases the contacts to rub together and reduce ntact resistance.

 

[ptonsparky]

You will read voltage drop across line to load of the CB. Hopefully in the milivolt range. With a constant near equal current on both legs the VD should be nearly equal. 100 mv and 400 mv would indicate trouble on the higher.

Use A "better" meter. Your standard tester may not have enough resolution.

 

[templdl]

You will read voltage drop across line to load of the CB. Hopefully in the milivolt range. With a constant near equal current on both legs the VD should be nearly equal. 100 mv and 400 mv would indicate trouble on the higher.

Use A "better" meter. Your standard tester may not have enough resolution.

My objective is to test for a voltage not to slplit hairs with and exact measurement in milivolts which should be able to be accomplished with a basic quality digital voltmeter. In order to trip a breaker thermally enough heat musyt be generated in order to do so as a result of a noticeable voltage drop across the contacts and not looking for specific values but to compare the line to load values of both poles and to use reasoning to determine if contact resistance was a contributing factor. Exercising the breaker should reduce contact resistance reducing the voltage drop.

 

[GoldDigger]

The heat that trips the thermal portion of the breaker comes from a fixed resistance in series with the load, not voltage drop across the contacts.
It is a voltage drop from line terminal to load terminal, but that is much more than just the contact resistance.
The resistance of the mag trip coil, OTOH, is very small.

 

[ptonsparky]

The heat that trips the thermal portion of the breaker comes from a fixed resistance in series with the load, not voltage drop across the contacts.
It is a voltage drop from line terminal to load terminal, but that is much more than just the contact resistance.
The resistance of the mag trip coil, OTOH, is very small.

If the breaker contacts or other internals are bad you will have VD from top to bottom and 300mv is a flag. Been there done that too many times to ignore it. We use a camera now.

 

[kwired]

My objective is to test for a voltage not to slplit hairs with and exact measurement in milivolts which should be able to be accomplished with a basic quality digital voltmeter. In order to trip a breaker thermally enough heat musyt be generated in order to do so as a result of a noticeable voltage drop across the contacts and not looking for specific values but to compare the line to load values of both poles and to use reasoning to determine if contact resistance was a contributing factor. Exercising the breaker should reduce contact resistance reducing the voltage drop.

OP says the breaker has been tripping - seems to me that tripping and resetting would "exercise" it.

 

[templdl]

OP says the breaker has been tripping - seems to me that tripping and resetting would "exercise" it.

Interesting. After supporting molded case circuit breakers as a sales and applications engineer for 18 years you may know more than me as well as the design engineers who have many patents that I was privileged enough to get to know and to have worked with.
Exercising a breaker is opening and closing a breaker numerous times and not simply resetting and closing a breaker after a trip.
When a breaker is exercised the moving and stationary contacts asctuaslly rub against one another as well as helping the moving parts lubricate.
There is no guarantee that this will resolve a contact issue but more so than not it does help.