Circuit breaker design 1

[caporter]

I think I once read some where that circuit breakers are only tested to trip at the rated capacity 6 times. Haven't found anything recently to back that up. Is there something that states when a circuit breaker should be replaced based on the number of times it has tripped?

 

[Zogzog]

It depends what type of curcuit breaker you are asking about and what you mean by "the rated capacity".

Breakers have different ratings, an overload, and a fault. Most breakers are rated to interupt overloads thousands of times, but as few as 2 fault interuptions, again all depends on what type of breaker wea are discussing. Little breakers dont have the means of indicating what type of protective action occured, so you have to assume it was a fault and replace (or test) them after anywhere from 2-6 operations, again, depending on the breaker type.

Mnay times a manufacturer will recommend that a breaker is remanufactured or at least tested after so many operations or a period of time (2-5 years). Smaller breakers cost more to test than to replace so for those ones it makes more sense to replace than test at all.

You will need to look at the manufactures literature that was supplied with the breaker to know the exact #.

 

[jraef]

It also depends upon what you are referring to as a "circuit breaker". That is a somewhat ambiguous term when in the context of service and reliability. A Power Breaker for example is designed to be service and rebuilt, a Molded Case Circuit Breaker is only partially serviceable, and a Mini Circuit Breaker is a throw-away device.


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[caporter]

In this case the circuit breakers in question are molded case Square D in QOB. We have a few other brands but those are the choice these days. As for the rated capacity I meant the two main ratings list on each one, amperage over current and interrupt rating. The article I remember said that UL only tested them so many times and the units were designed with that in mind. If there is documentation showing that then I can provide justification for replacing them.

 

[dpc]

There are a series of tests at varying current levels in the UL standard. For fault currents near the limit of the breaker interrupting rating, I suspect it is less than six operations.

I'm sure any of the US breaker suppliers can provide you with the UL test procedures. GE used to have a nice booklet on application of molded case circuit breakers.




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[Zogzog]

Short answer: 2

Long answer:Molded case low voltage circuit breakers are typically tested to UL standard 489. UL uses the following test goals to determine if a breaker is considered to be safe (incompliance with their safety standard):
• The breaker must interrupt the maximum short circuit current two times.
• The breaker must protect itself and the connected conductor and the equipment it is installed in.
• After having been tested the breaker must be fully functional and pass a thermal calibration trip test at 250% of its rated ampacity; and pass a dielectric withstand test at two times its rated voltage or a minimum of 900 volts.
• The tested breaker must also operate properly and have continuity in all of its poles.
UL-489 listed circuit breakers are tested with a four-foot length of wire, as they must perform during the test as they would when installed in the real world, so wire is connected to make the test a bit more realistic. During the test the conductor’s insulation must not be damaged. The connected wires must not be pulled loose from the breaker-conductor termination lug. The breaker case must not be damaged as a result of cable whip forces (caused by the potentially huge amount of magnetic force developed under short circuit conditions). The connected wire acts to some degree as a heat sink for the breaker. That is, it helps to dissipate heat produced within the breaker. This is because the breaker’s case acts as not only an electrical but a thermal insulation also, in that it tends to retard the rate of heat transfer. This is one reason why breakers have wire size ranges marked on them. Too small a wire attached to the breaker cannot adequately aid in cooling the breaker.
The temperature at the circuit breaker’s terminals must not rise more than 50 degrees C. above the ambient air temperature surrounding the breaker. The UL-489 test standard has been used to test many, many circuit breakers over the years and has proven to be a pretty good standard by which the safety of circuit breakers can be determined.