Main breaker tripping but not overloaded

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Dansos

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PA
Hi guys I got called for a main breaker tripping. Disconnect is down in basement and panel is on 2nd floor apartment. Starting turning everything on and was get around 50-55 A per line (240V 200A) main breaker felt like it was getting hot on 1 side. It stayed on for around 120s and then tripped. They ALSO have a 200 A main breaker in their panel upstairs. That breaker is not getting hot and none of the branch circuits seem hot either. Possible bad breaker? Or possible loose connection in disconnect causing an arc? No signs of overheating on the terminal lugs
 
Probably main breaker and/or associated equipment needing replaced because of some moisture intrusion that’s caused corroded contacts and the like.
 
The one lug looks a LITTLE discolored. I am going to deenergize it and take it apart to see. Also, the one leg that is overheating also feels like the wire itself is getting warm. The SEU coming from meter base is warm to the touch, not hot but not ice cold like the temp is down in the basement. Both legs are drawing equal current through when everything is on. Any reasons for 1 leg to be overheating? I am leaning toward a loose lug or termination on the main breaker. I am ripping it apart today. I will keep you posted.
 
There is a loose connection on the main breaker. Could be either on line or load side. It may be possible to tighten the connection to solve the problem, but it's likely the main breaker is damaged and should be replaced.

If you darken the room and look at the main you will probably see where it's arcing. (It will also be the hottest point measured with a spot thermometer.)
 
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Before you tear things apart, use a volt meter and check to see if you have a voltage drop between the line side and load side of the MB. If you do, you just need to change out that breaker.

I'd also use and ampmeter to check the GEC for current and if you can get the jaw around the SEU, check that for unbalanced current.

Please let us know what you find/do.
 
Since he never said how old or what brand of disconnect it is. It may be cheaper to just replace the disconnect (main) than to find a breaker to fit.

I have found this to be true with a lot of old challenger and other older stuff.
 
If it is bus to breaker connection that has failed - replace both or put new breaker on different bus position. If you plug new breaker onto bus that has been overheated, it may be compromised and will never make a good connection again. New breaker may work initially but will fail sooner than if installed on good bus. Same for lug to conductor connection. If conductor has been overheated cut it back to unaffected portion of conductor, splice extension on if it will no longer reach. Overheated conductor may never make good connection again either and lead to early failure.
 
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