۱۳۸۸ دی ۸, سه‌شنبه

Grounding

Grounding is a necessary safety feature of every electrical system. It protects against electric shock, fire, and damage to equipment and reduces the hazards associated with lightning. Proper grounding for both the system and electrical equipment is particularly relevant for farmsteads.System grounding starts at the main service entrance. The neutral of the main service is connected either to an all-metal water pipe or to a metal rod that has been driven into the ground. These act as ground electrodes. If a ground rod is employed, it must be driven deep enough to always be In contact with moist soil (at least 10 feet). To ensure continuity of the ground throughout the system, each branch service entrance should be grounded with its neutral wire connected to the grounded main service neutral.Equipment grounding ensures that a person who comes in contact with electrical equipment does not get a shock because of a ground fault. A ground fault can occur when wiring connections become loose or protective insulation is broken or defective.When current leaks and electrifies a metal box or fixture, it follows a conductive path to complete its circuit. An equipment ground serves as a conductor or path of low resistance.The best way to ground power tools is to use tools with three-wire power cords and three-prong plugs. The third wire is the ground and connects to the system ground when it is plugged into a properly grounded outlet. When using an extension cord, be sure it has a third wire for grounding. To confirm that receptacles have maintained their ground, periodically check them with a circuit tester. Double-insulated power tools are also available, offering protection from ground faults by providing a layer of insulation between the wires and the tool's metal casing. Double insulated power tools are not as effectively grounded as three-wire tools because the insulation can fail if it gets wet.Any metal, system, or equipment component that may become electrified when a fault occurs needs to be grounded. This includes, but is not limited to, junction boxes, motor frames, and fuse boxes. The electrical system ground does not ground these metallic enclosures. A qualified electrician should install all ground connections to make sure the system functions properly.Ground fault circuit interrupters (GFCI) are also used to protect against shocks from ground faults. A GFCI detects current leakage at very low levels (as little as 5 milliamps) and rapidly cuts off the power. Three types of commonly available GFCIs are units built into circuit breakers, units built into receptacles, and portable GFCIs that plug into any receptacle.
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