Fuses and Breakers


Fuses and breakers limit the current which can flow in a circuit. The metal filament in the fuse melts and breaks the connection, whereas in a breaker, the heating effect on a bimetallic strip causes it to bend and trip a spring-loaded switch.

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The Circuit-Breaker

Circuit breakers act to limit the current in a single circuit in most household applications. Typically a single circuit is limited to 20 amperes, although breakers come in many sizes. This means that 20 amps of current will heat the bimetallic strip to bend it downward and release the spring-loaded trip-lever. Since the heating is fairly slow, another mechanism is employed to handle large surges from a short circuit. A small electromagnet consisting of wire loops around a piece of iron will pull the bimetallic strip down instantly in case of a large current surge.
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The Ground Wire

The term "ground" refers to a connection to the earth, which acts as a reservoir of charge. A ground wire provides a conducting path to the earth which is independent of the normal current-carrying path in an electrical appliance. Attached to the case of an appliance, it holds the voltage of the case at ground potential (usually taken as the zero of voltage). This protects against electric shock. The ground wire and a fuse or breaker are the standard safety devices used with standard electric circuits.

Is the ground wire necessary? The appliance will operate normally without the ground wire because it is not a part of the conducting path which supplies electricity to the appliance. In fact, if the ground wire is broken or removed, you will normally not be able to tell the difference. But if high voltage has gotten in contact with the case, there may be a shock hazard. In the absence of the ground wire, shock hazard conditions will often not cause the breaker to trip unless the circuit has a ground fault interrupter in it. Part of the role of the ground wire is to force the breaker to trip by supplying a path to ground if a "hot" wire comes in contact with the metal case of the appliance.

Appliance ground exampleWhat is a ground fault interrupter?
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Appliance Ground Wire

Three electrical connections are made to a standard appliance like a clothes washing machine. The "hot" wire carries an effective voltage of 120 volts to the appliance and the neutral serves as a return path. The third wire is the electrical ground which is just connected to the metal case of the appliance.

If the hot wire shorts to the case of the appliance, the 120 volt supply will be applied to the very low resistance path through the ground wire to the earth. This will cause an extremely high current to flow and will cause the breaker or fuse to interrupt the circuit.

One problem with this arrangement is that if the ground wire is broken or disconnected, it will not be detectable from the operation of the appliance since the ground wire is not a part of the circuit for electric current flow. In that case, if the hot wire shorts to the case and the neutral wire does not, then the breaker may not trip and the entire 120 volts will be applied to the metal case of the appliance, representing a shock hazard. The ground wire of an appliance is the main protection against shock hazard.

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