applications of earthing
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In low-voltage 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.
On electricity networks with a system voltage of 240 V to 1.1 kV, which are mostly used in industrial / mining equipment / machines rather than publicly accessible networks, the earthing system design is as equally important from safety point of view as for domestic users.
In most developed countries, 220 V, 230 V, or 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 V 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.
For a time, US National Electrical Code allowed certain major appliances permanently connected to the supply to use the supply neutral wire as the equipment enclosure connection to ground. This was not permitted for plug-in equipment as the neutral and energized conductor could easily be accidentally exchanged, creating a severe hazard. If the neutral was interrupted, the equipment enclosure would no longer be connected to ground. Normal imbalances in a split phase distribution system could create objectionable neutral to ground voltages. Recent editions of the NEC no longer permit this practice. 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 overcurrent 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 overcurrent protection device. In such case a residual current detector is installed to detect the current leaking to ground and interrupt the circuit.
On electricity networks with a system voltage of 240 V to 1.1 kV, which are mostly used in industrial / mining equipment / machines rather than publicly accessible networks, the earthing system design is as equally important from safety point of view as for domestic users.
In most developed countries, 220 V, 230 V, or 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 V 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.
For a time, US National Electrical Code allowed certain major appliances permanently connected to the supply to use the supply neutral wire as the equipment enclosure connection to ground. This was not permitted for plug-in equipment as the neutral and energized conductor could easily be accidentally exchanged, creating a severe hazard. If the neutral was interrupted, the equipment enclosure would no longer be connected to ground. Normal imbalances in a split phase distribution system could create objectionable neutral to ground voltages. Recent editions of the NEC no longer permit this practice. 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 overcurrent 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 overcurrent protection device. In such case a residual current detector is installed to detect the current leaking to ground and interrupt the circuit.
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