404 Electrical Systems 21.3.2 Continuity Tester A continuity tester is similar to a test light with a power source added to the lamp. See Figure 21-8. When the leads of the continuity tester are connected by an electrical path, the lamp lights to indicate a complete circuit. Like a test light, a continuity tester has limited application. 21.3.3 Receptacle Testers Normal residential 120 VAC wiring consists of three wires. Th e “hot” wire (black), the “neutral” wire (white), and the “ground” conductor (uninsulated or green). Th e neutral wire is, by code requirements, bonded (con- nected) to ground at the point where the electrical service enters the building and again at the circuit breaker panel. In order for a receptacle (electrical outlet) to be wired correctly, each conductor is connected to a specifi c termi- nal. However, in some cases, conductors are attached to incorrect terminals and the receptacle is not properly wired. A common troubleshooting task for electricians is to check receptacle wiring. One device for testing a receptacle to determine the “hot” slot is a screwdriver voltage tester. Th is device consists of a pocket screwdriver that has a neon lamp and a current-limiting resistor connected between the screwdriver blade and the metal pocket clip. Th e elec- trician inserts the screwdriver into one slot of the recep- tacle while his hand is in contact with the metal pocket clip. If the screwdriver is inserted into the hot side of the receptacle, the neon lamp lights. Receptacle testers, Figure 21-9, have replaced most screwdriver voltage testers. A receptacle tester has three indicator lamps. It is plugged into a receptacle and if the two yellow lamps illuminate, the outlet is wired correctly. Any other combination of illuminated lamps indicates a wiring error that must be corrected. A legend is included on the device to indicate the type of error encountered. In addition to the three indicator lights, the recep- tacle tester has a test button that is used to trip a GFCI (ground-fault circuit interrupter). GFCIs trip almost instantaneously when a miniscule amount of current fl ows to ground. GFCIs are employed as a safety mea- sure and are required by the NEC (National Electrical Code) in certain instances. GFCI protection can be pro- vided by GFCI receptacles or by GFCI circuit breakers. Th e modern version of the screwdriver voltage tester is the noncontact voltage tester, Figure 21-10. A noncontact voltage tester employs a Hall eff ect sensor (a semiconduc- tor device that is sensitive to magnetic fi elds) and operates in a similar fashion to the electrician’s screwdriver, except that no direct contact with a live conductor is required. Th is allows the user to verify the presence of voltage on a con- ductor even through the conductor insulation. Indicator lamps Legend for analyzing lamp pattern GFCI test button Goodheart-Willcox Publisher Figure 21-9. A receptacle tester for analyzing the wiring connected to an electrical outlet. Goodheart-Willcox Publisher Figure 21-10. A noncontact voltage tester lights in the presence of voltage. Interestingly, if you drag the detector tip of a non- contact voltage tester along a power cord between an electronic device and a receptacle, you will notice that the detector senses the presence of voltage for a certain distance, then senses nothing. Th is occurs because the conductors in the cord are twisted. Th e twist places the hot conductor next to the sensor for a certain distance, and then the ground and neutral conductors next to the sensor for the remaining distance. 21.3.4 Digital Multimeter (DMM) A digital multimeter (DMM) is an instrument used to measure a variety of electrical properties. A DMM can serve as a voltmeter for measuring electrical potential, an ohmmeter for measuring resistance, and an ammeter for measuring current. Due to its ease-of-use and versatility, a DMM is one of an industrial maintenance technician’s most valuable tools. Copyright Goodheart-Willcox Co., Inc.