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Insulation and grounding are two recognized means of preventing injury during electrical equipment operation. Placing non-conductive material such as plastic around the conductor may provide conductor insulation. Grounding may be achieved through the use of direct connection to a known ground such as a metal cold water pipe.
The metal housing or enclosure around a motor or the metal box in which electrical switches, circuit breakers and controls are placed protect the equipment from dirt and moisture and prevent accidental contact with exposed wiring.
There is, however, a hazard associated with housings and enclosures. A malfunction within the equipment, such as deteriorated insulation, may create an electrical shock hazard. Many metal enclosures are connected to a ground to eliminate the hazard, but if a “hot” wire contacts a grounded enclosure, a ground fault results which normally will trip a circuit breaker or blow a fuse.
Metal enclosures and containers are usually grounded by connecting them with a wire going to ground. This wire is called an equipment-grounding conductor. Most portable electric tools and appliances are grounded by this means. There is one disadvantage to grounding. A break in the grounding system may occur without the user’s knowledge.
Insulation may be damaged by hard usage on the job or simply by aging. If this damage causes the conductors to become exposed, the hazards of shocks, burns, and fire will exist. Double insulation may be used as additional protection on the live parts of a tool, but double insulation does not provide protection against defective cords and plugs or against heavy moisture conditions. The use of a ground fault circuit interrupter (GFCI) is one method used to overcome grounding and insulation deficiencies.
The ground fault circuit interrupter is a fast-acting circuit breaker which senses small imbalances in the circuit caused by current leakage to ground and, in a fraction of a second, shuts off the electricity. The GFCI continually matches the amount of current going to an electrical device against the amount of current returning from the device along the electrical path. Whenever the amount “going” differs from the amount “returning” by approximately 5 milliamps, the GFCI interrupts the electric power within as little as one fortieth of a second.
The GFCI will not protect a person from line to line contact hazards, such as a person holding two “hot” wires, or a hot and a neutral wire in each hand. It does provide protection against the most common form of electrical shock hazard, the ground fault. It also provides against fires, overheating and destruction of insulation on wiring.
The provisions of 1910.331 through 1910.335 cover electrical safety-related work practices for both qualified persons (those who have training in avoiding the electrical hazards of working on or near exposed energized parts) and unqualified persons (those with little or no such training) working on, near, or with the following installations:
Grounding requirements are covered in 1910.304(g)(1) through (g)(9):