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In electricity supply systems, an earthing system or grounding system is circuitry which connects parts of the electric circuit with the ground, thus defining the electric potential of the conductors relative to the Earth's conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the power supply. In particular, it affects the magnitude and distribution of short circuit currents through the system, and the effects it creates on equipment and people in the proximity of the circuit. If a fault within an electrical device connects a live supply conductor to an exposed conductive surface, anyone touching it while electrically connected to the earth will complete a circuit back to the earthed supply conductor and receive an electric shock. A ''protective earth'' (PE), known as an ''equipment grounding conductor'' in the US National Electrical Code, avoids this hazard by keeping the exposed conductive surfaces of a device at earth potential. To avoid possible voltage drop no current is allowed to flow in this conductor under normal circumstances. In the event of a fault, currents will flow that should trip or blow the fuse or circuit breaker protecting the circuit. A high impedance line-to-ground fault insufficient to trip the overcurrent protection may still trip a residual-current device (''ground fault circuit interrupter'' or GFCI in North America) if one is present. This disconnection in the event of a dangerous condition before someone receives a shock, is a fundamental tenet of modern wiring practice and in many documents is referred to as automatic disconnection of supply (ADS). The alternative is defence in depth, where multiple independent failures must occur to expose a dangerous condition - reinforced or double insulation come into this latter category. In contrast, a ''functional earth'' connection serves a purpose other than shock protection, and may normally carry current. The most important example of a functional earth is the neutral in an electrical supply system. It is a current-carrying conductor connected to earth, often, but not always, at only one point to avoid flow of currents through the earth. The NEC calls it a ''groundED supply conductor'' to distinguish it from the ''equipment groundING conductor''. Other examples of devices that use functional earth connections include surge suppressors and electromagnetic interference filters, certain antennas and measurement instruments. Regulations for earthing system vary considerably among countries and among different parts of electric systems. Most low voltage systems connect one supply conductor to the earth (ground). People use an earthing system mainly for these applications: * To protect a structure from lightning strike, directing the lightning through the earthing system and into the ground rod rather than passing through the structure. * Part of the safety system of mains electricity, preventing problems associated with floating ground. * The most common ground plane for large monopole antenna and some other kinds of radio antenna. Other, less common applications of earthing systems include: * single-wire earth return. * one at each end of a ground dipole ELF antenna. ==Low-voltage systems== In low-voltage distribution networks, which distribute the electric power to the widest class of end users, the main concern for design of earthing systems is safety of consumers who use the electric appliances and their protection against electric shocks. The earthing system, in combination with protective devices such as fuses and residual current devices, must ultimately ensure that a person must not come into touch with a metallic object whose potential relative to the person's potential exceeds a "safe" threshold, typically set at about 50 V. In most developed countries, 220/230/240 V sockets with earthed contacts were introduced either just before or soon after World War II, though with considerable national variation in popularity. In the United States and Canada, 120 volt power outlets installed before the mid-1960s generally did not include a ground (earth) pin. In the developing world, local wiring practice may not provide a connection to an earthing pin of an outlet. In the absence of a supply earth, devices needing an earth connection often used the supply neutral. Some used dedicated ground rods. Many 110 V appliances have polarized plugs to maintain a distinction between "live" and "neutral", but using the supply neutral for equipment earthing can be highly problematical. "Live" and "neutral" might be accidentally reversed in the outlet or plug, or the neutral-to-earth connection might fail or be improperly installed. Even normal load currents in the neutral might generate hazardous voltage drops. For these reasons, most countries have now mandated dedicated protective earth connections that are now almost universal. If the fault path between accidentally energized objects and the supply connection has low impedance, the fault current will be so large that the circuit over current protection device (fuse or circuit breaker) will open to clear the ground fault. Where the earthing system does not provide a low-impedance metallic conductor between equipment enclosures and supply return (such as in a TT separately earthed system), fault currents are smaller, and will not necessarily operate the over current protection device. In such case a residual current detector is installed to detect the current leaking to ground and interrupt the circuit. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Earthing system」の詳細全文を読む スポンサード リンク
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