by Earl King

Created on: August 15, 2008

WHY CIRCUIT BREAKERS TRIP

The reason a circuit breaker trips will vary somewhat depending on the breaker and its application. For our present purpose we will discuss the molded-case breaker found in residential settings. This breaker is used in many other light power service applications as well.

In all things pertaining to electricity, safety must be uppermost in our minds. If you are unsure about safety, get a licensed electrician. Additionally, wire sizes, breaker sizes, and the like are subject to the rules and regulations of the NEC (National Electric Code). In some locations, the law requires compliance to this code. Decisions concerning application of the NEC should also be left to a licensed electrician. If the installation is being designed, as in a new house, the architect will serve this purpose.

The breakers under consideration are manufactured by a number of companies. To be sold and used in the United States all of them must follow the guidance of the NEMA publications. This is the National Electrical Manufactures Association. Such standardization is very important in the electrical and other industries.

Just what is a circuit breaker? What is it used for? Circuits are pathways for the flow of electricity. Basically this would be an insulated wire going from an alternating current source to a load or appliance and back again to the source. One of these wires are considered hot and the other ground.

We can identify three reasons why breakers trip. These are circuit overload, short circuit, and point of separation. To accomplish the first two, a molded-case breaker contains a thermal element and a magnetic trip element.

OVERLOAD

The wire size and type for the various circuits will be determined by the guidelines in the NEC. This will fix the continuous current rating of the breaker as well as the trip rating. This is the safe allowable current carrying capacity (per NEC). If the current reaches a value which could produce a dangerous temperature in the wires or the insulation, the thermal element will operate and trip. If there is a motor on the circuit, the trip setting may be a little higher than the current rating to allow motor starting current to flow without opening the breaker. Motor starting current will drop down very quickly before any damage is done.

The thermal element is a common bi-metallic contact. The two metals making it up will have different rates of expansion so that the contact will bend one way on heating and trip.

SHORT CIRCUIT

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