- - AGRICULTURE CORE CURRICULUM - - (CLF2000) Advanced Core Cluster: AGRICULTURAL MECHANICS (CLF2650) Unit Title: ELECTRICITY ___________________________________________________________________________ (CLF2657) Topic: Testing Electric Circuits Time Year(s) 2 hours 1 / 2 / 3 / 4 ___________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the students will be able to: Learning Outcome #: (N-10) - Trouble shoot electrical circuits in a safe manner. Special Material and Equipment: Test light, voltage meter References: Cooper, E. L. (1987). AGRICULTURAL MECHANICS: FUNDAMENTALS AND APPLICATIONS. Albany, NY: Delmar Publishers. Erpelding, L. H. (1971). AGRIBUSINESS ELECTRICAL LESSON PLANS. Danville, IL: Interstate Printers & Publishers. Evaluation: Quiz by instructor. TOPIC PRESENTATION: Testing Electric Circuits A. Types of Electrical Circuit Testers 1. There are two general types of electrical test meters: a. Digital - Meter displays numbers when measuring volts, ohms, amperes, or other electrical measurements. b. Analog - Meter uses a needle to indicate measurement reading of volts, ohms, amperes, or other electrical measurements. 2. Types of Electrical Test Devices a. Ammeter 1) Measures the flow of current in an electrical circuit. 2) Units of measurement are amperes or milliamperes (milliamps). 3) The meter must be installed in the circuit in series in order to measure current properly. b. Voltmeter 1) Measures the potential of electricity in a circuit. 2) Units of measurement are volts. 3) The meter must be installed in the circuit in parallel in order to measure voltage properly. c. Ohmmeter 1) Measures the resistance of a circuit component or circuit section. 2) Units of measurement are ohms. 3) The meter must be installed in the circuit in parallel across the component or circuit section for which the resistance is to be measured. 4) It is important that the circuit not be energized (i.e., no current should be flowing through the circuit) when the measurement is being made. 5) An ohmmeter should be calibrated with each use and with each change of meter scale. d. Multitester 1) A meter with variable scales capable of measuring volts and ohms and additionally giving an audible indication of circuit continuity. e. Multimeter 1) A meter with variable scales capable of measuring volts, ohms, and milliamperes and additionally giving an audible indication of circuit continuity. f. Test Light 1) A light bulb with two wires attached can serve as a test light. 2) It can be used to troubleshoot circuits for shorts, opens, grounds, and voltage. 3) When connected in series in a circuit, the light bulb lights to indicate presence of current. g. Continuity Test Light 1) This is similar to a test light except that it contains a battery along with a light bulb. 2) It can be used to troubleshoot circuits for shorts, opens, grounds, and voltage. 3) The continuity test light will "light" whenever it is connected to both ends of a circuit wire that has "continuity," i.e., is not broken. h. Three-prong Circuit Tester 1) This is a commercially available device that plugs into the outlet of the circuit. 2) A three-prong circuit tester indicates: a) Correct wiring b) Open ground c) Reverse polarity d) Open neutral e) Hot and ground wire reversed B. Troubleshooting Electric Circuits with a Test Light 1. Short Circuits a. A short circuit occurs when direct contact is made between the two wires of a circuit or some other part which is conducting electricity as a result of damage or failure of the insulation. b. When a short circuit occurs, the fuse or circuit breaker will open the circuit. c. A bulb and/or test light can be used to locate short circuits. 1) Shut off the power to the circuit by opening the master switch before attaching any leads from the test light. 2) Replace the blown fuse with a 150- or 200-watt bulb in the fuse holder. If a breaker protects the electrical circuit and it is tripped, disconnect the ungrounded (black) wire from the breaker and connect one lead of the test light to the screw terminal of the breaker and the other lead to the ungrounded wire. 3) Close the master switch or the breaker. The test light will glow brightly if a short circuit or a large overload exists. 4) To find the location of the short, turn off all light fixtures, wall switches, and disconnect all cords, lamps, and appliances on the circuit. If the bulb glows with nothing plugged in, there is a short in the circuit or an outlet box. 5) If the bulb does not light in Step 2, there is a short in one of the items switched off or disconnected. If the bulb glows, that item is shorted. If the bulb does not light, continue switching on and plugging in items, one at a time, until the short is located. 6) If the test light glows brightly when a light fixture is being tested, remove the bulbs from the fixture one at a time. If the test lamp goes out when one of the bulbs is removed, the short is in the bulb. However, the test lamp will remain glowing if the short is in the light fixture. 2. Open Circuits a. Sometimes "opens" or breaks in electrical connections in appliances will occur. These most generally occur in the cord near a point where the cord enters the electrical device. b. When an open occurs, the machine or appliance will not operate. However, before assuming the problem is an open, check to make sure that the attachment plug is making good contact with the convenience outlet. c. The test lamp may be used to locate opens. 1) Connect the clips on the test lamp to the terminals of the electrical device. Most likely the casing or a portion of it will have to be removed from the device. 2) Plug the attachment cord on the test lamp into a convenience outlet and turn the appliance on. If the device operates,the open exists in the service cord. If, it still does not operate, the open is in the switch or some other part of the device. 3. Accidental Grounds a. An accidental ground or grounding fault can occur when insulation becomes damaged or is removed by some cause. The conductor can then electrically charge that with which it makes contact. If a person touches the electrically charged part, the electron flow travels through him to the ground. b. To prevent accidental grounds, manufacturers often attach a ground wire to the frames of tools, appliances, and machines. If the frame is accidentally charged, the electron flow will travel to the ground through the ground wire. c. The test lamp may be used to check for accidental grounds. 1) Disconnect the device being tested and insulate it from the ground. 2) Attach one of the clips on the test lamp to one of the flat prongs (not the round prong) on the attachment plug of the device being tested and the other clip to the frame of the device. 3) Plug in the test lamp. If the bulb glows, the frame of the appliance is electrically charged and can cause a serious accident. 4) Each appliance that has been repaired should be checked for accidental grounds before being returned to the owner or to use. 4. The test lamp may be used for other purposes. a. The test lamp may be used to determine whether an outlet is 120 or 240 volts. The 240-volt lamp in the test lamp will glow brightly if the circuit is 240 and dimly if 120. b. The test lamp can be used to determine whether an outlet is properly grounded for the use of equipment grounds. 1) Use a pigtail socket and a 240-volt bulb. Hold one of the leads on the metal screw, holding the cover plate in place, and the other lead in one or the other of the slots in the outlet. 2) The test lamp will glow when the lead to the outlet is in the slot connected to the fuse or breaker back at the panel if the outlet is grounded. _________________________________________________________ ACTIVITY: 1. Construct a test light and use it to troubleshoot a circuit that has been bugged. 2. Have students wire a circuit and then test their wiring with an outlet tester. _________________________________________________________ 6/26/91 OLR/tf #%&C