Main Breaker Overheating

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Jraef said:
... tripped off five or six times in the course of a few days...
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Again, the breaker did not failure just because of time. It experience 'several' trips, and it may have had a marginally acceptable bi-metal when it was built. In a breaker, there is not enough physical movement of the bi-metal to cause it deform. Bi-metal overload relays have been used for motor protection for years, but I have never heard someone say their characteristics change based solely on age. Think how many operations the bi-metal thermostat in a oven or water heater experience without developing a thermal memory.
 
I've seen many a breaker fail and not due to overloading. Age takes a toll on everything it's just a matter of time!:eek:hmy:
 
jim dungar said:
Again, the breaker did not failure just because of time. ...
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Agree, and Mostly Agree.

We get catastrophic events here at our industrial site frequently enough to see aging breakers. Whatever the event it doesn't typically destroy a breaker, it just damages it a little. Over time the trip point gets reduced until it won't hold the rated loads without excessive heating. Then the decay accelerates until it won't hold the rated loads at all.

Strictly speaking Time doesn't kill the breakers. But a series of nasty events can damage the breaker ever so slightly that it's not readily apparent. Then it looks like time did it.

Events such as a job setter resetting overloads for several weeks before the EL gets called. Then we find out there's intermittent single-phasing.
 
Sierrasparky said:
I've seen many a breaker fail and not due to overloading. Age takes a toll on everything it's just a matter of time!:eek:hmy:
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Devices failing due to age is not an issue.
I started with the comment that a breaker does not 'weaken' just due to age. The most common failure of thermal magnetic breakers is excess heat, caused by poor connections or poor maintenance (i.e. dust, moisture)
I have seen many hundreds, if not thousands, of properly operating breakers that are in excess of 30 years old.
 
Whether breakers fail because of time is somewhat a matter of definition. I believe that most of us do not believe that a breaker that was manufactured some years ago and just now put into service would be any the worse for its age. On the other hand, if the breaker has been used for several years, it is more likely to have corroded, to have been tripped a number of times, etc. The history of the breaker is more important than its age. There are tools, no matter how old, that if working now are as good as a new one. They are independent of their history. A breaker does not seem to be such a device.
 
I find it interesting when people say they see breakers "fail", by "fail" they usually mean trip when they don't think they should trip and to me that is not a failure. I test thousands of breakers every year, 99% of those that "fail" don't trip fast enough or not at all, that is a failed breaker and something you can only determine by testing (I know, I know, not practical for small MCCB's).

As Jim mentioned, it is very rare for a MCCB to weaken over time, and that would be due to excessive operations weakening the springs but UL testing requires them to handle thousands (4000 times for the 150A breaker from the OP) normal operations. However, when it comes to a fault interuption at or near the AIC rating MCCB's are only rated for two of those at which point they need to be tested or replaced (Economics determine which).

In almost every case of a thermal mag MCCB tripping it is due to overheating of the thermal element, which is caused by either excessive current, or heating from a high resistance connection (Which I also agree with Jim was the likely cause here).
 
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I do find that New 15 and 20 amp breakers that are brand new will trip quicker with less sparks than that of a 20 year old version.
 
I replace a main about once every 2 years. Not a common occurrence but it happens. Any breaker can go bad over time.
 
I've just seen a 60 amp breaker melted to the bussbars, and the load was only a 3 ton A/C unit on #6 wire. It draws 15 amps across all 3 legs. Why would a 60 amp breaker melt under a 15 amps load? never tripped.

Clearly age and the loosening of the connection between the bussbars and the breaker. probably 1000's of cycles of the A/C unit, thousands of heat cycles caused the connection to loosen up....
 
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