Main Breaker Overheating

[rapaint66]

Withing the last 2 months, my main 150 amp service breaker has started tripping when the Dryer and AC run at the same time. Non of the branch circuits are tripping. I borrowed the thermal imager from the plant and noticed that the left side is running hot at about 160 degf but the right side runs at about 115 degf. My initial thought was improper load balancing had ruined the main over the last 15 years but the guy at the local electrical supply shop said that main breaker rarely go bad. It does not appear to be a load balancing issue. Any ideas? It is a Cutler Hammer 150 volt panel.

 

[Twoskinsoneman]

My initial thought was improper load balancing had ruined the main

????

I assume the first thing you did was measure the current draw. Could you post them?

 

[hillbilly1]

Is this a "BR" series, or a "CH" series loadcenter? (The "CH" series has tan handles on the breakers) If memory serves me correctly, the "CH" main bolts to the buss, where the "BR" plugs onto stabs. If it is the latter, it is probably a burnt stab.

 

[hillbilly1]

When looking at it with the thermal imager, where was the hot spot? Line? Load? Center of breaker?

 

[rapaint66]

When looking at it with the thermal imager, where was the hot spot? Line? Load? Center of breaker?

The left line was about 160 degf and the center of the breaker was about 138. All 4 lugs on the main were cold around 110

 

[rapaint66]

????

I assume the first thing you did was measure the current draw. Could you post them?

When the air and the dryer are on together the left leg is 35 amps and the right leg is 28 amps. With just the air I am at Left 22a and right 18a. It's gotta be the main ???!!!!

 

[rapaint66]

Is this a "BR" series, or a "CH" series loadcenter? (The "CH" series has tan handles on the breakers) If memory serves me correctly, the "CH" main bolts to the buss, where the "BR" plugs onto stabs. If it is the latter, it is probably a burnt stab.

It's a BR3040NC200:weeping:

 

[Dennis Alwon]

I have seen ground faults on a branch circuit blow the main. I am not sure how this happens in a residential situation.

 

[rapaint66]

It's a BR3040NC200:weeping:

I have the BR and it bolts

 

[Twoskinsoneman]

Any VD across the breaker when it is loaded?

 

[hillbilly1]

Yeah, the BR's are warmed over Westinghouse's, I'm probably thinking of a Crouse Hinds that plugs in, don't do much residential anymore. You might check the start up amperage on the a/c, the caps may be going bad, but it's a long shot on causing the main to trip though.

 

[rapaint66]

Any VD across the breaker when it is loaded?

looks like the main breaker was weakened over time by a load imbalance. I replaced the main and the buss tray this morning, re-balanced the kitchen and HVAC unit in the attic and everything looks good so far. My microwave bit the dust last night but I'm not sure if that is related.

Thanks guys. This is a great forum and I appreciate the feedback. The format matches my favorite gun forum I belong to as well.:thumbsup:

 

[jim dungar]

looks like the main breaker was weakened over time by a load imbalance.

Sorry but this is not possible. Breakers do not weaken over time.
The UL test for breakers is hundreds of on-off operations at 100% load. Each pole of a multi-pole breaker is tested individually just as if it was a single pole device.

Most likely there was a poor connection on one the incoming lugs or at the panel bussing.

 

[iaov]

If there is heat there is resistence. Even a couple of ohms will heat a conennection. I squared X R = Watts. With very large loads (hundreds of amps) even a fraction of an ohm will cause things to warm up.

 

[kbsparky]

Sorry but this is not possible. Breakers do not weaken over time.....

I disagree with this statement. I have seen many breakers weaken over time. Sometimes it is due to a catastrophic event, such as a large surge (Lightning), or a main tripping out from a short circuit condition.

Other times, the main appears to be just "worn out" and needs to be replaced, because it is internally heating up and tripping out under loads that do not approach the breaker's rating.

 

[Twoskinsoneman]

I disagree with this statement. I have seen many breakers weaken over time. Sometimes it is due to a catastrophic event, such as a large surge (Lightning), or a main tripping out from a short circuit condition.

Other times, the main appears to be just "worn out" and needs to be replaced, because it is internally heating up and tripping out under loads that do not approach the breaker's rating.

Perhaps but due to a few amps imbalance? I don't think so Tim :happyno:

 

[jim dungar]

I disagree with this statement. I have seen many breakers weaken over time. Sometimes it is due to a catastrophic event, such as a large surge (Lightning), or a main tripping out from a short circuit condition.

Other times, the main appears to be just "worn out" and needs to be replaced, because it is internally heating up and tripping out under loads that do not approach the breaker's rating.

A 'catastrophic' event is not time.
A manufacturing error is not time.

Simply carrying load current, whether imbalanced or not, does not weaken a breaker.

 

[kbsparky]

And yet, his breaker has weakened to the point of being unusable.

While I agree that an unbalanced load of a few amps won't weaken a breaker, it's really apparent that the OP's breaker is weak to the point it needs to be replaced.

It's not always a catastrophic event that weakens a breaker. Among other things, dust and dirt accumulation (along with a healthy dose of humidity) can do one in over time.

 

[jim dungar]

And yet, his breaker has weakened to the point of being unusable.

We do not know if his breaker was 'unusable. The OP replaced both the breaker and the bussing at the same time.

Environmental factors have been known to cause breakers to fail to trip and to reset correctly, however none of the ones you have listed will 'weaken' a thermal magnetic breaker causing it to trip early.

Overheating of the breaker is still the most likely cause of the OP's un-intended trip.

 

[Jraef]

A few years back we in California were subjected to a spate of summer rolling brown outs as the PoCos struggled with the deregulation fiasco. During that time my 40 year old GE 100A main tripped off five or six times in the course of a few days. I didn't have an issue with that at first because it was really obvious that we were having brown outs. But after that, it would trip on currents that were well under 60A (I had hooked up a BMI recorder I had at the time). There never was any kind of event that would have caused the breaker to trip, each time the current was steadily hovering around the 60A mark. If it stayed there for more than a few minutes, the breaker tripped.

I believe the bi-metal thermal elements can build up a "thermal memory"; they fail to fully return to their original state and thus take less current to reach the trip bar. I know it is not supposed to happen that way with circuit breakers, but I know what I experienced too. I replaced it and had no more trouble. So maybe the mechanics were not "worn out" but if the damned thing doesn't stay on. it doesn't matter to me if the switch elements would last another 100 years, it's still a PITA.