More Hot breakers

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lazorko

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We got another call from a different client concerned about overheating breakers . Breakers were burned, but this time there was enough bus to look at. This Siemens 125A panel had an aluminum bus. Brushing with a wire brush brought off lots of dusty oxide deposits. We are recommending replacing the panel (with copper bus).
 
Re: More Hot breakers

Hot? Overheating?? What temperature is this based upon? Can you hold your hand on it and leave it there without being uncomfortable? Or, is it just warmer that what one expects it to be.
Temperature is a relative term when using the term "hot" when something is just warmer than one expects it to be.
 
Re: More Hot breakers

It would be interesting to know HOW HOT the breakers were, There are companys that sell calibrated temperature stickers to put on electrical equipment. The other option is to do a thermo scan. With the panle cover off scan the breakers to see what is the temperature of there connection to the bus.
 
Re: More Hot breakers

This Square D document says that the UL standard for breakers permits a 55?C rise at the line and load terminals. The breakers are rated for a 40?C ambient so this would permit the line and load terminal temperature to reach 95?C before the UL standard is exceeded. 95?C = 203?F
Don
 
Re: More Hot breakers

This has been a pet peve of mine that these alu-buss panels have been allowed at all. I dont think there made with that new aluminmin alloy that the newer alu wires are, and when there installed in a enviroment thats a little high in moisture they oxidize badly and then start failing. I have had to repair or replace many of them from this. the worst places for them is laundry rooms, garages and basments and dont even think about outdoors even though they make them in a NEMA 3-R they shouldn't! for most of them wire brushing the buss and nolox on the breaker stabs works well and I nolox any breaker 30 amps and above as these are the ones with the most failure rate. and dont use them when the panel is located on a outside wall where you have cold temps as this is also a problem with them. That dont leave many places to put them does it?

You get what you pay for.
 
Re: More Hot breakers

Don and Bennie. Now you're talking some real numbers and not just "hot." As and former application engineer I always advised the client to know the real temperature and to explore the OCPD for a posible cause of elevated heating such as stabs and terminations. If the OCPD is loaded the actual temperature will be warmer but still within that which is allowed per what Don illustrated.
So, a OCPD which is running warmer than one would anticipate may be examined for a posible cause of heating. But if no failure is evident and the OCPD is within that which is allowed by UL then it should be concluded that it's a nonproblem .
 
Re: More Hot breakers

We didn't take actual temperature measurements - but when the plastic molded case has BURNED away, I would say that the breaker got too hot.

In this case, heating started at the point of contact between the bus and the breaker, and not at the conductor/screw terminal, so I would also discount the load side termination/wiring as a problem.

Hurk27, while I always use antioxidant on aluminum conductors, I have not used it on a panel bus. Sounds like wire brushing and coating would be a good treatment when adding a new branch to an old Al bus.
 
Re: More Hot breakers

While on going bus oxidation is hard to stop in the short term. Consider performing a FOP (fall of potential) measurement accross the circuit breakers as you install them (laod must be on).
Measure from the line bus to the load conductor. Millivolt readings will vary with load, as you perform more of these test (while measuring current) you will get a feel for an acceptable level of voltage drop.
 
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