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Circuit breakers, fuses, ground-fault circuit interrupters (GFCIs), arc-fault circuit interrupters (AFCIs), transient-voltage surge suppressors (TVSSs) and other protective devices are a home's first line of defense against electrical accidents. Equally important is the grounding system, which normally doesn't carry current but must always be on guard to provide a safe path for abnormal current flow and to ensure protective devices' quick operation.
The basics. The principles of grounding are simple but often misunderstood, even by experienced electricians. The term grounded is defined in the code as meaning “connected to earth or to some conducting body that serves in place of earth,” such as a grounded metal electrical box. The earth as a whole is a conductor and is assumed to have a voltage potential of zero, but it's made of different types of soil that resist cur rent flow and don't conduct electricity very well.
Electricity always seeks the path of least resistance to return to its source, usually the utility's transformer. Instead of the earth itself, grounding wires are used to ground all metal objects and provide an effective low-resistance, highly conductive path for abnormal current. Grounding all metal objects also ensures they are at the same safe zero-voltage potential as the earth. If a hot wire comes in contact with a grounded metal object, the low-resistance grounding wires carry the maximum current necessary to quickly trip the circuit breaker or protective device. The grounding wires try to keep the metal object close to the earth's zero-voltage potential to reduce the shock hazard.
Electrical systems and equipment are connected to ground rods, underground metal water pipes and other electrodes to limit and stabilize the voltage to ground during normal operation and to prevent excessive voltages from lightning strikes, power surges or accidental contact with high-voltage utility lines.
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Service equipment is usually grounded to the underground metal water pipe within 5 ft. (1.5 m) of the point of entrance into the building. Because of the growing use of plastic water pipe, the code requires one or more supplemental electrodes, such as a ground rod. Copper-coated ground rods must be at least 8 ft. (2.5 m) in length, 1/2 in. (13 mm) in diameter and driven flush with or below the earth. Interior metal piping systems must be grounded by installing a wire called a bonding jumper around water meters and filters. Use only approved clamps for ground rod and metal pipe connections.
Making Older Receptacles Safer
You can increase the safety of your old two-prong outlets in two ways. Installing GFCI outlets will protect against a deadly shock even if the outlet isn't connected to a grounding wire. If you have grounded metal conduit and boxes, new three-prong outlets can be installed.
GFCI. Turn off power to the receptacle outlet at the service panel. Check to make sure the power is off. Care fully unscrew the outlet and pull it out of the box. (You may need to install a larger box to safely contain the wires and new GFCI). Disconnect wires from the old outlet and splice them to 6-in. (15-cm) pigtails of white or black wire. Connect the new wires to the GFCI terminals marked “Line,” attaching the black wire to the brass screw and the white wire to the silver screw. Fold wires neatly into the box, fasten the GFCI and plate, and attach the stickers that say “No Equipment Ground.”
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Three-prong. Three-prong grounding-type outlets can be installed only as replacements where a grounding means is available, such as metal conduit, metal armored cable, flexible- metal conduit and their metal boxes. One can also install a ground from elsewhere on the grounding electrode system by using a wire of the same gauge as the branch-circuit conductors. Turn off power to the receptacle outlet at the service panel. Check to make sure the power is off. Carefully unscrew the outlet and pull it out of the box. Connect the black wire to the brass screw and the white wire to the silver screw. Install a bare or green-insulated bonding jumper from the green ground screw on the outlet to the metal box using a green ground clip or green ground screw threaded into the back of the box. Fold wires neatly into box and install the outlet and plate.
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Last modified: Friday, 2020-05-01 11:05 PST