Basic Circuit Breaker Design

The simplest way to protect an electrical circuit is to install a fuse. A fuse is not much more than a thin strand of wire that is enclosed in a casing, usually glass, that is located in the circuit. When the circuit is closed, in other words, when current is flowing, the fuse and anything else in the circuit all are subjected to the same current. In the event of increased current, the wire in the fuse melts and the circuit opens, eliminating any further current flow and acting as a safe guard against fire or damage to the wiring. Eaton circuit breakers perform the same function but in a different fashion.

The fuse works perfectly; the only problem is; it just works once and then needs replacing. A circuit breaker performs the identical function of a fuse, opening the circuit in the event of an overload; however, it can be re-set time after time and does not need replacing each time the circuit opens.

How does a circuit breaker work?

Think of Eaton circuit breakers as no more than a simple switch. The switch is actuated by an electromagnet or a bi-metal strip. The hot wire in the circuit is attached to each side of the switch. When the switch is “on,” current can flow through the switch, uninterrupted. The flow of current in a circuit breaker is from one pole of the switch, through the electromagnet or bi-metal strip, to the moving contact of the switch, to the stationary contact and out the opposite pole of the switch.

When an electromagnet is used in the circuit, the electricity is what magnetizes it. The EMF increases when the current is boosted and decreased when the current drops. If the current elevates to an unsafe level, which is the amperage rating of the circuit breaker, the EMF is strong enough to pull the switch linkage and open the circuit.

When Eaton circuit breakers use a bi-metallic strip to open the circuit, it works on an identical principle. The difference is, the bi-metallic strip bends and in doing so, it moves the linkage and opens the circuit.

In very large circuit breakers, the action is caused by an explosive charge which, when set off, moves a piston which moves the switch linkage, opening the circuit.

Advancements in design:

More advanced designs of circuit breakers use semiconductors to monitor the current levels rather than leave this to electrical devices. Semiconductors are far more concise, and they interrupt an overloaded circuit much faster; however, they are considerably more costly. It is the elevated costs that are the underlying reason that most domestic applications continue to use electromagnetic or bi-metallic operating circuit breakers.


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