N2 Electrical Trade Theory|The Easy Way! 39 2.7.4 Three-phase circuit-breakers


Single-phase circuit-breakers have one pole, i.e. one input and one output terminal and one set of contacts. Tree-phase circuit-breakers have three poles, i.e. three input and three output terminals and three sets of contacts.


Te contacts are mechanically coupled in such a way that all three contacts open simultaneously (common trip), even if a fault occurs in one phase only.


Tis has the advantage, for example, of preventing single-phasing of motors. Single- phasing is when one of the phases are open-circuited and the motor operates on two phases which will result in overloading and/or overheating of motors. Tis cannot be accomplished by fuses on their own.


▲ 6.16.5.1.3 Te overcurrent protection device protecting a three-phase motor must prevent the motor from continuing to operate under load if single-phasing occurs.


2.7.5 Casing


Te circuit-breakers are usually encased (housed) in a high strength, fire resistant moulded casing. Bakelite is a trademark for a thermosetting plastic which is oſten used for the casing. It has a high chemical and electrical resistance. Bakelite is suitable for pouring, when molten, into moulds and is easy to machine. It has a dark brown or black colour.


2.7.6 Trip indication


When the circuit-breaker trips, the toggle switch faces downward. Tis makes it easy to see when it has tripped.


2.7.7 Trip mechanism


Te mechanism of the circuit-breaker ensures a quick and positive making (closing) action and a quick breaking (opening) action.


2.7.8 Free trip


Te toggle switch is designed to be free trip which means that it will trip even if the toggle is held in the on position or if it is closed under overload or short-circuit conditions.


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