Circuit Protection Unit 9

Objectives Explain how fuses and circuit breakers work. Explain what a short circuit is. Explain the purpose of a ground-fault interrupter

When wires carry current, they become hot When wires carry current, they become hot. The greater the current, the hotter the wire becomes. Each wire can safely carry only a certain amount of current. If it carries more than that amount, the wire may burn. A cord is designed to carry alternating current (AC) has two sets of wires, side by side. The two sets are separated from each other by plastic insulation. If the insulation were to crack open, the two sets of wire could touch. Then the current would bypass the appliance at the end of the cord, and go directly through the crack in the insulation. The resistance of the circuit would be much less and the current would be much greater than normal. The overheated cord could then cause a fire.

Short Circuit When part of a circuit with the greatest resistance is bypassed, a short circuit exists. Short circuits are very dangerous because of the overheating of wires.

Circuits in buildings contain safety devices designed to immediately open the circuit if the current becomes to great. For example, a fuse is a safety device that contains a strip of metal with a low melting point. The strip of metal is called a fuse ribbon. A fuse is a one time use element. If the current in the circuit becomes to great (the circuit is overloaded), the heat melts the fuse ribbon. The circuit is broken and a potential fire is avoided.

Some circuits us a 15-amp fuse (breaker) Some circuits us a 15-amp fuse (breaker). A kitchen or garage circuit may use a 20-fuse (breaker). Assume that the supply line in the kitchen is connected to a toaster that draws 5 amps, an electric gridle that draws 5 amps, and a refrigerator that draws 5 amps. If all three output devices are turned on at the same time, the line current is 15 amps. Adding any more devices would increase the current and could overload the circuit. If the fuse (breaker) is rated for 20 amps, any current above 20 amps will melt the fuse or trip the breaker. For example, suppose you add an iron to the circuit that draws 10 amps. If all four devices are turned on at the same time, the circuit will draw 25 amps. The fuse will “blow out” (a breaker will trip) and break the circuit. Before a blown fuse is replaced, the cause of the overload should be determined and one of the devices disconnected from the circuit.

What would happen if you replace the blown 20 amp fuse with a30 amp fuse (or breaker)? A fuse (or breaker) with too high a rating for the wiring would not prevent the wires from overheating and a fire could result. The MB100 Kit has a 1 amp fuse. Any current above 1 amp will cause the fuse to blow out and break the circuit. The MB100 is designed for use with low currents and will not produce enough current to blow the fuse. Circuit in buildings are protected by circuit breakers. You will also have circuit breakers in your house. Some circuit breakers use magnets. These work because a current produces a force on a magnet. When the current become to great, the force is great enough to operate a switch which opens the circuit.

Other circuit breakers use bimetallic strips-strips made of two metals placed one right next to another. Each metal expands by different amount when heated. As a result, when the strip is heated by current flowing through it, the strip bends. If too much current flows through the strip, it bends so much that it opens the circuit. The advantages of a circuit breakers over fuses is that you do not have to replace the circuit breakers each time a circuit is overloaded. Instead, you can simply flip a switch to reclose the circuit. Of course, if you do not correct the overload problem before reclosing the circuit, the circuit breaker will open the circuit again. The MB100 Kit has a ½ amp circuit breaker. In the Experiment, you will create a circuit to trip the breaker in order to see how it operates.

A ground-fault occurs when there is a break in the low-resistance grounding path from a tool or electrical system. The electrical current may then take an alternative path to the ground through the user, resulting in serious injuries or death. The ground-fault circuit interrupter, or GFCI, is a fast-acting circuit breaker designed to shut off electric power in the event of a ground-fault within as little as 1/40 of a second. It works by comparing the amount of current going to and returning from equipment along the circuit conductors. When the amount going differs from the amount returning by approximately 5 milliamperes, the GFCI interrupts the current.

The GFCI is rated to trip quickly enough to prevent an electrical incident. If it is properly installed and maintained, this will happen as soon as the faulty tool is plugged in. If the grounding conductor is not intact or of low-impedance, the GFCI may not trip until a person provides a path. In this case, the person will receive a shock, but the GFCI should trip so quickly that the shock will not be harmful. The GFCI will not protect you from line contact hazards (i.e. a person holding two "hot" wires, a hot and a neutral wire in each hand, or contacting an overhead power line). However, it protects against the most common form of electrical shock hazard, the ground-fault. It also protects against fires, overheating, and destruction of wire insulation.