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trip curve

1) n. The relationship between the amount of current going through a circuit breaker and the time it takes the circuit breaker to trip. This relationship exists because of the mechanisms within a circuit breaker, and the curves differ depending on the type of mechanism (be it thermal magnetic, arc-fault, magnetic, or other). For the purposes of this description, the trip curve being discussed is that of the thermal magnetic type, which has both a thermal and a magnetic mechanism for tripping the circuit breaker in the event of two fundamentally different types of electrical faults.

The first mechanism is a bimetallic strip that heats up over time. The more current flowing through it, the faster it heats up. As it heats, it bends, then comes in contact with another piece of metal, and once that happens the breaker trips. This is the thermal mechanism, which shows mercy to loads that are near the current rating of the breaker. The purpose of this mechanism is to prevent an overload from lasting long enough to melt the insulation of the wires. This mechanism trips because too many things are plugged into a circuit at the same time.

The second mechanism is the magnetic mechanism. The magnetic component will trip instantly as soon as a certain threshold of current is reached. Regardless of how high above the threshold the current draw is, this mechanism will turn off the circuit in the same amount of time. It's designed to keep electronics from exploding in the event of an electrical surge, like that of a lightning strike or should two wires short across each other.

The thermal component exists for overload situations where too many appliances are on at the same time and is forgiving because in cases like 1800-watt hair driers, that need lots of current but only for a couple minutes; either a person will dry their hair fast enough and without problems or the thermal mechanism will eventually heat up and turn the circuit off before damage occurs to the wiring. Then the magnetic mechanism prevents damage/injury in the event of a short-circuit event (someone drops a toaster in the bathtub).

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Image Courtesy of EC&M.

The above image shows the reaction times of both mechanisms. The upper portion of the curve is the thermal reaction due to overload, while the bottom, horizontal portion of the curve is the magnetic reaction due to short-circuit.

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