How a circuit works
Service panels, whether they have breakers or fuses, divide household current into several circuits. Each circuit carries power from the service panel via hot (usually black or red) wires to various outlets in the house, and then back to the service panel via a neutral (usually white) wire.
TYPES OF CIRCUITS. Most household circuits carry 120 volts; some may be 240-volt circuits. Circuits are rated according to amps. If the outlets on a circuit draw too many amps, the circuit overloads. When this happens, a fuse will blow or a breaker will trip, preventing an unsafe condition. A 120-volt circuit usually serves a number of outlets. For instance, it may supply power to a series of lights, a series of receptacles, or some of each. A heavy-use item, such as a dishwasher or refrigerator, may have its own dedicated circuit. A 240-volt circuit is always dedicated to one outlet. A standard 120-volt 15-amp circuit uses #14 wire; a 20-amp circuit uses thicker #12 wire. Older 240-volt circuits use three wires; two hot and one neutral. Recent codes require four wires, as shown below; the added wire is for grounding. Circuits provide convenience as well as safety. If you are making a repair or new installation, you can shut off power to an individual circuit rather than having to shut down power to the entire house.
A SERVICE PANEL HAS 120-AND 240-VOLT CIRCUITS. Your service panel distributes power according to the needs of a circuit. For example, a 240-volt circuit is designed to supply electricity to a heavy-duty user of power, such as an electric range or a dryer. The single receptacle on a dedicated 120-volt circuit might feed a refrigerator or a large microwave, while another 120-volt circuit feeds a series of receptacles and overhead light fixture switches.
Grounding and polarization
Normally electricity travels through insulated wires and exits through a fixture such as a light bulb. If a wire comes loose or if a device cracks, a short circuit (ground fault) results, releasing electricity where you don’t want it. A short can occur, for example, if a loose wire inside a dryer touches the dryer’s frame or if cracked insulation allows bare wire to touch a metal electrical box. If you touch electrified metal, you’ll get a dangerous shock. Grounding and polarization protect against this. Here’s how they work:
GROUNDING minimizes the possibility that a short circuit will cause a shock. A grounded device, fixture, or appliance is usually connected to a grounding wire—either bare or green— that leads to the neutral bar in the service panel. This bar is connected to the earth by a heavy-gauge copper grounding wire running to one or a combination of the following:
• cold-water pipe (If the water meter is installed on the cold-water pipe, the ground must be connected on the street side of the meter or the water meter must be jumped with a grounding wire tightly clamped to both sides.)
• grounding rods driven at least 10 feet in the ground
• metal plate sunk in a footing
Another method uses the metal sheathing of armored cable or conduit instead of a ground wire as the ground path to the service panel. When a ground fault occurs, the ground path carries the power to the service panel. This extra path lowers resistance, causing a great deal of power to flow back to the panel. This in turn trips a circuit breaker or blows a fuse. At the same time, power is directed harmlessly into the earth. Whether your system uses grounding wires or sheathing as the ground path, it must be unbroken. A single disconnected ground wire or a loose connection in the sheathing or conduit can make the grounding system useless. To check whether a receptacle is grounded, plug in a receptacle analyzer.
POLARIZATION ensures that electricity goes where you want it to go. Because a polarized plug has one prong wider than the other, it can be inserted into a polarized receptacle only one way. If the receptacle is wired correctly and an appliance plug is polarized, the hot wire, not the neutral wire, will always be controlled by the appliance switch. If the receptacle or plug isn’t polarized, the neutral wire might be connected to the appliance switch instead, and power would be present in the appliance even when it is switched off. For extra protection against shock, install GFCI protection.
HOW A GROUNDED RECEPTACLE WORKS. To ground a receptacle, a ground wire (either bare copper or green-clad copper) is attached to the receptacle (and to the box, if it is metal) and leads to the neutral bus bar in the service panel. The panel itself is grounded. This receptacle is also polarized.
HOW A POLARIZED RECEPTACLE WORKS. The black wire is connected to the receptacle's brass terminal at one end and to the circuit breaker or fuse at the other end. The white wire runs from the silver terminal screw to the service panel’s neutral bus bar.
A CONTINUITY TESTER TELLS YOU WHETHER A DEVICE IS DEFECTIVE. Disconnect the device from all household wires. Attach the tester's alligator dip to one terminal and touch the probe to the other terminal. If the device switch is working, the tester light will glow when the switch is turned on and go out when the switch is turned off. To test the wiring in an appliance or lamp, touch both ends of each wire. The tester light will glow if the wire is unbroken. To test a fuse follow the same procedure.
A VOLTAGE DETECTOR SENSES POWER EVEN THROUGH WIRE AND CABLE INSULATION. This handy tester lets you check whether wires are live before you work on them. The probe doesn't need to touch a bare wire or terminal. Press the detector button and hold it on or near an insulated wire or cable to see if power is present. If it is, a light comes on.
A VOLTAGE TESTER INDICATES THE PRESENCE OF POWER. A four-level voltage tester is safer and more reliable than one-level versions. Always confirm that a voltage tester is working by trying it on a circuit you know to be live. Touch the tester's probes to a hot wire and a grounded box or to a hot wire and a neutral wire, or insert them into the slots of a receptacle. If the tester light doesn't come on, the circuit is shut off.
A RECEPTACLE ANALYZER TELLS YOU WHETHER YOUR RECEPTACLES ARE SAFE. When you plug this analyzer into a receptacle, one or more of three lights will glow, telling you whether the receptacle is working, grounded, and polarized. Several styles are available, including some that vibrate or make noise to indicate which slot is hot. Red analyzers test ground fault circuit interrupter (GFCI) receptacles as well as standard receptacles. Yellow analyzers test standard receptacles only.
TEST FOR VOLTAGE - Multi-testers have negative and positive probes. Test for voltage by touching each probe to a wire, terminal, or receptacle slot. You also can touch one probe to the black wire and the other to a ground, such as a metal box. The display should show between 108 and 132 volts for a 120-volt circuit, and between 216 and 264 volts for a 240-volt circuit. Low-voltage circuitry can register as low as 4 volts.
TEST FOR CONTINUITY - To test a switch with a multi-tester, shut off the power and disconnect the wires from the switch. Set the dial on the multi-tester to any ohms setting and touch a test probe to each terminal. Turn the switch on. Zero resistance shows the switch works; infinity means it is defective. The tester should indicate infinity when you turn the switch off.