What Is A Circuit Breaker?

Circuit breakers are electrical devices whose purpose is to protect all parts of the circuit in which they are engaged from electrical conditions deemed inappropriate or unsafe. Different circuit breakers offer different attributes and features, but the most basic circuit breaker monitors current for overload and short circuit. Each of these conditions is hazardous to the circuit, the electrical system, the load, the supply and operators of the equipment. When exposed to an overload or a short circuit (or any of dozens of other specific programmed conditions), the device is designed to open automatically to interrupt the circuit quickly and completely. Fault interruption is very dangerous and produces arcs that can contribute to explosive events in air break devices.

Circuit breakers operating at optimum efficiency and accuracy can play a huge part in reducing potential arc flash hazard. In low-voltage applications, most circuit breakers have all or most of their protective behaviors integrated into the device, while in higher voltage breakers, it is common to mount the protective relaying separate from the actual breaker element. However, even that is changing now in the most modern applications.

A circuit breaker is either integrally designed or programmed to trip or open automatically at designated points on an inverse time-current curve. The higher the overload, the sooner the breaker will open. A breaker may allow 125% of rated current to flow for five or more minutes, whereas a breaker exposed to 600% or more (fault condition) of rated current should trip instantly. In addition to basic short circuit and overload protection, most 600V class breakers of the last several generations allow someone to customize the curve on which the breaker trips.

Breakers may monitor and protect for ground fault conditions, under or over voltage conditions, under or over frequency conditions and phase failure or phase sequence. They even may be paying attention to the condition of other circuits in the same system (a feature sometimes known as zone interlock).

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