200 Amp Load Center Buss acting like 100 Amp Buss

[AES1005]

Situation:

There was a power outage in the affecting the residence. when power was restored the main breaker on a GE 200 amp panel begain tripping with electric furnace on aux heat. customer called power company and they checked their line and meter, everything good. I was called in and suspected the main breaker after checking the load which was 78 amps total. was called back the new main breaker tripped out.
here's what I have found so far. No matter where the load comes from different breaker circuits as soon as the load gets near 70-80 amps the main trips on temp? the left buss read 250 deg and the right buss 156 deg. I checked all of the lugs bolts on the buss everything is tight. I have not yet removed all of the breakers to inspect the full buss. Has anyone had a buss on a load center act like this, as long as the total load stays below 70 amps temp stay down @ 80-90 deg.

 

[ultramegabob]

sounds like you have a high resistence in a connection on the buss, it may be where the main makes contact.

 

[brian john]

With system loaded did you perform a fall of potential test? This is step one.

and in this case most likely all you have to do.

I would have completed this first trip, just randomly replacing circuit breakers (or any component) in my opinion is not doing yourself or your customer justice. One should ALWAYS know what is wrong and the cause of the issue before any repair.

 

[augie47]

I agree with Brian. I think with an FOP test you will find the main breaker is your culprit.

 

[AES1005]

I see where you are going, what I have done so far is install a new main breaker. same problem. I was only going to remove the breakers so I could physicaly look and the main buss. I do not own the equiptment to perform a fall of potiential test. But from here I will re-check connections on the buss. what you all are saying is making sense (high resistance). thanks.

 

[augie47]

Without testing, I would have placed my bet on the main breaker.
Please let us know what you find.

 

[LarryFine]

I do not own the equiptment to perform a fall of potiential test.

You do if you have a voltmeter. If you measure from the line terminal to the load terminal on one line of the main breaker, for example, you'll be measuring the voltage drop between those points. That's fall of potential.

Start with your meter on full voltage, and reduce the range one step at a time until you get a meaningful voltage. In theory, the voltage should be zero from the main line lug all the way to the branch breaker terminal.

 

[ibew501ed]

nice of you to explain that (even if it hurt a little )

 

[AES1005]

Thank's for the help, these are some of the things we (I) miss. Going back to the site on monday will let you know what I find.

 

[gar]

100117-1125 EST

AES1005:

If you have something like a Fluke 27 or 87 it will auto range on the AC volt position and if you need to resolve less than 0.001 V, then you can switch to the MV position and resolve 0.1 MV or 0.000,1 V. For your tests here you do not need the MV range.

Your first measurement should be from the actual wire going to the main lug, not the lug, to the far end of the buss. At 50 A this will not be 1 V on a good system. Probably is much more than 1 MV because you are reading across the main breaker. I will guess in the range of 25 MV.

Note 50 A at 1 V is 50 W, this is way too much to be dissipated in a 200 A breaker. Assuming a linear resistance in the breaker the dissipation at 200 A would be 800 W at 200 A.

If it is 25 MV at 50 A, then power is 1.25 W, and at 200 A it is 20 W. This might be high. I doubt that it takes 20 W to trip the breaker.

Next measure voltage from the input wire to the buss at a point on the buss between the main lug and the breaker. At 50 A this is probably less than 2 MV, wild guess.

See what the various voltage drops tell you.

.

 

[AES1005]

Sorry it took so long to get back, Here's what we found: Had an HVAC Tech check the heat pump, furnace everything good there. what I did find was wire going to furnace attached with three wire strands on the breaker at the furnace. (re-spliced and attached all of the #2 AL conductors to the breaker). found some resistance on one 60 amp breaker between the poles (DBLP). removed two other double pole breakers that were spare. system is now operating fine. with normal tempurature's.
In conclusion, I can only concieve that the problem was a combination of main,wire,and resistance on breaker all together causing the heat/tripping.

Thanks for everyone's input, it all helps.