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The three-wire system replaced the older two-wire system, which lacks an earth/ground wire. Under ordinary circumstances, insulation on the live/hot and neutral wires prevents the case from being directly in the circuit, so that the earth/ground wire may seem like double protection. Grounding the case solves more than one problem, however. The simplest problem is worn insulation on the live/hot wire that allows it to contact the case, as shown in [link] . Lacking an earth/ground connection (some people cut the third prong off the plug because they only have outdated two hole receptacles), a severe shock is possible. This is particularly dangerous in the kitchen, where a good connection to earth/ground is available through water on the floor or a water faucet. With the earth/ground connection intact, the circuit breaker will trip, forcing repair of the appliance. Why are some appliances still sold with two-prong plugs? These have nonconducting cases, such as power tools with impact resistant plastic cases, and are called doubly insulated . Modern two-prong plugs can be inserted into the asymmetric standard outlet in only one way, to ensure proper connection of live/hot and neutral wires.

Part a of the figure describes an appliance connected to an AC source. One end of the AC circuit is connected to a circuit breaker. The other end of the circuit breaker is connected to the appliance. The appliance is shown as a resistance enclosed in a rectangular metal case. The case of application is shown to have a point where it is in contact with the hot wire from AC source due to lack of insulation. The other end of the resistance is connected back to the AC source through a connecting wire. The connecting wire and the A C source are grounded. The ground terminal at the appliance case is shown as broken. A person is shown to hold one hand on the appliance case and another hand on tap connection pipeline carrying water. The high voltage is shown to flow from the insulation damaged live wire to the metal case of the appliance to the person in contact with the appliance case, then through him and to the pipe line and then back to ground. The person is shown to receive a severe shock.
Worn insulation allows the live/hot wire to come into direct contact with the metal case of this appliance. (a) The earth/ground connection being broken, the person is severely shocked. The appliance may operate normally in this situation. (b) With a proper earth/ground, the circuit breaker trips, forcing repair of the appliance.

Electromagnetic induction causes a more subtle problem that is solved by grounding the case. The AC current in appliances can induce an emf on the case. If grounded, the case voltage is kept near zero, but if the case is not grounded, a shock can occur as pictured in [link] . Current driven by the induced case emf is called a leakage current , although current does not necessarily pass from the resistor to the case.

The figure describes an appliance connected to an AC source. One end of the AC circuit is connected to a circuit breaker. The other end of the circuit breaker is connected to an appliance. The appliance is shown as a resistance enclosed in a rectangular metal case known as the case of appliance. The other end of the resistance is connected back to the AC source through a connecting wire. The connecting wire and the A C source are grounded. The ground terminal at the appliance case is shown as broken. A person is shown holding one hand on the appliance case and the other hand free. Due to the current in the appliance I app, there is an induced e m f in the appliance case. This is shown as leakage current which is shown to flow through the person in contact with the appliance and back to the ground. This current is termed I leak.
AC currents can induce an emf on the case of an appliance. The voltage can be large enough to cause a shock. If the case is grounded, the induced emf is kept near zero.

A ground fault interrupter (GFI) is a safety device found in updated kitchen and bathroom wiring that works based on electromagnetic induction. GFIs compare the currents in the live/hot and neutral wires. When live/hot and neutral currents are not equal, it is almost always because current in the neutral is less than in the live/hot wire. Then some of the current, again called a leakage current, is returning to the voltage source by a path other than through the neutral wire. It is assumed that this path presents a hazard, such as shown in [link] . GFIs are usually set to interrupt the circuit if the leakage current is greater than 5 mA, the accepted maximum harmless shock. Even if the leakage current goes safely to earth/ground through an intact earth/ground wire, the GFI will trip, forcing repair of the leakage.

The figure describes a ground fault interrupter device connected across the hot or live and neural wires of an AC circuit. The ground fault interrupter device is shown as a rectangular block connected to a coil wound on a ring shaped iron core. The terminals of AC source are connected to an appliance shown as a resistance in a appliance case. The grounding of the appliance is shown broken. A person in contact with the appliance case is also shown. A leakage current I leak is shown to flow through him to the ground. The current I minus I leak flows back to the A C terminals. The leakage current here follows a hazardous path.
A ground fault interrupter (GFI) compares the currents in the live/hot and neutral wires and will trip if their difference exceeds a safe value. The leakage current here follows a hazardous path that could have been prevented by an intact earth/ground wire.
Practice Key Terms 3

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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