2. Electrical Safety

2013 alexander ganago page 13 of 28 last printed

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Unformatted text preview: utlet) ensures that electric current flows through the load, which is shown as a resistor in Figure 2- 7. This load can be a heater, a lighting fixture, a motor, etc. The wiring ensures that electric power is transferred from the source of power to the load. However, for the sake of electrical safety we use 3 wire outlets and equip them with the Ground Fault Circuit Interrupter (GFCI) as shown in Figure 2- 8. Figure 2- 8. Standard US wall outlet has 3- prong connectors (the colors are according to the NEC). The Test and Reset buttons belong to the Ground Fault Circuit Interrupter (GFCI). Why is it necessary? © 2013 Alexander Ganago Page 16 of 28 Last printed 2013- 07- 07 12:28 AM File: 2013 EEW 1- 02 Electrical safety.docx Engineering in the Electrical World (EEW) Part 1 Unit 2: Electrical safety Suppose that the appliance has a metal case, and the insulation on one of the wires fails. Consider two scenarios: 1. The insulation fails on the neutral wire so it accidentally contacts the metal case of the appliance. If a user who is electrically connected to the ground also establishes electric connection with the metal case, there is no problem, because both points of contact on the user’s body are at the same electric potential, at zero volts. This is shown in the left panel of Figure 2- 9. 2. On the contrary, the right panel of Figure 2- 9 presents a dangerous scenario: now, the hot wire is connected to the metal case thus electric current can flow through the body of a user who touches the metal case and is also connected to the ground. This current is shown with a wavy arrow in the right panel of Figure 2- 9. <Sidebar> An electric appliance with faulty insulation may deliver a deadly shock to the user. Figure 2- 9. The outcome of insulation failure within an electric appliance depends on which of the two wires makes an accidental contact with the metal case of the appliance. The left panel shows a harmless situation where the neutral wire contacts the case. On the contrary, the right panel presents a dangerous situation, in which the live wire makes electric contact with the metal case: electric current flows through the body of the user who is connected to the ground and touches the case of the appliance. There are several ways to reduce the risk of electric shock due to faulty insulation within an ap...
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This note was uploaded on 12/06/2013 for the course EECS 314 taught by Professor Ganago during the Fall '07 term at University of Michigan.

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