Grounding, Electrical, the connecting of electrical equipment and wiring systems to the earth by a wire or other conductor The primary purpose of grounding is to reduce the risk of serious electric shock from current leaking into uninsulated metal parts of an appliance, power tool, or other electrical device. In a properly grounded system, such leaking current (called fault current) is carried away harmlessly. Grounding is also used in manufacturing industries to prevent accumulation of hazardous static electrical charges.
Although most electrical systems have fuses or circuit breakers for protection against a major fault current or short circuit, the human body may be fatally shocked by a current of less than one amperewell below the point at which a fuse or breaker will operate. Grounding helps prevent such a hazard from occurring. In some cases, however, as when a person handles an electrical device while standing on a wet surface, there is a risk of fatal shock from a leaking current even from a properly grounded electrical circuit. For protection against this danger, a safety device called a ground-fault interrupter can be installed in the circuit. This device, so sensitive that it can detect leakages as small as 5 milliamperes, immediately disconnects the circuit when a leakage occurs.
In most homes, the wiring system is permanently grounded to a metal pipe extending into the house from an underground water-supply system or to a metal rod driven into the ground. A copper conductor connects the pipe or rod to a set of terminals for ground connections in the home's electrical service panel. In wiring systems that use electrical cable sheathed in metal, the metal sheathing usually serves as the ground conductor between wall outlets and the service panel. In wiring systems that use plastic-sheathed cable, an extra wire is used for grounding.
An appliance designed to be grounded is equipped with a three-wire cord and a plug having three prongs that is intended to be plugged into a matching outlet. The third wire and prong provide the ground link between the metal frame of the appliance and the ground of the wiring system. Toasters and other appliances having exposed heating coils should not be grounded, because grounding actually increases the risk of electric shock. Double-insulated power tools and small appliances have specially insulated housings that eliminate the need for grounding. These are designed so that no exposed part of the device will be electrically live even if the internal insulation fails.
Protection against lightning by lightning rods and other means is a form of grounding.