Book page 116 ee for 21 st century 1 2 avoid

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EE for 21 st century 1-2 Avoid electrocution Unit 1-2-3 GFCI © 2015 Alexander Ganago Page 5 of 9 Last printed 2015-07-24 1:46 PM File: 2015 1-2-3 GFCI.docx <Sidebar> Polarized plugs, which have the two blades of different widths and can be plugged into the receptacle only one way, provide an additional protection against electric shock. B. Another solution requires that a third wire is always connected to the ground and to the metal case of the appliance. If the live wire has a faulty insulation and accidentally contacts the metal case of the appliance, electric current will return to the receptacle through the power cable, and probably trip the circuit breaker. However, the user will not suffer any electric shock because both points of contact connect the user’s body to the ground. This solution is shown in Figure 4, which also lists the colors of insulation for each wire according to the US National Electric Code. <Sidebar> The third wire, which connects the metal case of an electric appliance to the ground, protects the user from electric shock. Figure 4. Three-wire connection of an electric appliance includes the ground wire, which ensures that the metal case of the appliance is always grounded. If the live wire accidentally connects to the metal case, the current will return to the source through the power cable without entering the human body; most likely, the circuit breaker will trip. C. The third solution requires a special device called the Ground Fault Circuit Interrupter (GFCI). Its circuitry includes a transformer whose magnetic core surrounds both the hot and neutral/ground wires, and the electronic circuit (shown with a triangle in Figure 5), which compares the input and output currents. If the difference between these currents approaches the Let-Go threshold, the GFCI automatically and promptly opens the circuit (as shown in the right panel of Figure 5) to ensure that the harm to the user is minimal. Book Page 117
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EE for 21 st century 1-2 Avoid electrocution Unit 1-2-3 GFCI © 2015 Alexander Ganago Page 6 of 9 Last printed 2015-07-24 1:46 PM File: 2015 1-2-3 GFCI.docx <Sidebar> Ground Fault Circuit Interrupter (GFCI) measures the difference between the input and output currents to/from the appliance; it automatically and promptly opens the circuit if their difference approaches the Let-Go threshold. This protects the user from a deadly electric shock. Figure 5. Ground Fault Circuit Interrupter (GFCI) measures the difference between the input and output currents (left panel). If it exceeds 6 mA, the GFCI automatically opens the circuit. Thus, even if the insulation fails and the user touches the metal case of the appliance, the current that enters the human body remains below the Let-Go threshold. Take a second look at Figure 2. Note that every power receptacle equipped with GFCI has two buttons, labeled Reset and Test. You have to press the Reset button in order to restore power after the GFCI opened the circuit. Every month test the operation of the GFCI by pressing the Test button.
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  • Fall '07
  • Ganago
  • Electric charge, Alexander Ganago

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