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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 (electricity), thus defining the electrical 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 "hot" (unearthed) 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.
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).
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, but fault currents will usually 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.
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. Examples of devices that use functional earth connections include surge suppressors and electromagnetic interference filters, certain antennas and measurement instruments.
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.
The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):
The second letter indicates the connection between earth and the electrical device being supplied:
In a TN earthing system, one of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system. The body of the electrical device is connected with earth via this earth connection at the transformer.
The conductor that connects the exposed metallic parts of the consumer's electrical installation is called protective earth (PE; see also: Ground). The conductor that connects to the star point in a three-phase system, or that carries the return current in a single-phase system, is called neutral (N). Three variants of TN systems are distinguished:
|TN-S: separate protective earth (PE) and neutral (N) conductors from transformer to consuming device, which are not connected together at any point after the building distribution point.||TN-C: combined PE and N conductor all the way from the transformer to the consuming device.||TN-C-S earthing system: combined PEN conductor from transformer to building distribution point, but separate PE and N conductors in fixed indoor wiring and flexible power cords.|
It is possible to have both TN-S and TN-C-S supplies from the same transformer. For example, the sheaths on some underground cables corrode and stop providing good earth connections, and so homes where "bad earths" are found get converted to TN-C-S.
In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator.
The big advantage of the TT earthing system is that it is clear of high and low frequency noises that come through the neutral wire from connected equipment. TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT does not have the risk of a broken neutral.
In locations where power is distributed overhead and TT is used, installation earth conductors are not at risk should any overhead distribution conductor be fractured by, say, a fallen tree or branch.
In pre-RCD era, the TT earthing system was unattractive for general use because of its worse capability of accepting high currents in case of a live-to-PE short circuit (in comparison with TN systems). But as residual current devices mitigate this disadvantage, the TT earthing system becomes attractive for premises where all AC power circuits are RCD-protected.
The TT earthing system is used throughout Japan, with RCD units in most industrial settings. This can impose added requirements on variable frequency drives and switched-mode power supplies which often have substantial filters passing high frequency noise to the ground conductor.
In an IT network, the electrical distribution system has no connection to earth at all, or it has only a high impedance connection. In such systems, an insulation monitoring device is used to monitor the impedance.
|Earth fault loop impedance||High||Highest||Low||Low||Low|
|Need earth electrode at site?||Yes||Yes||No||No||No|
|PE conductor cost||Low||Low||Highest||Least||High|
|Risk of broken neutral||No||No||High||Highest||High|
|Safety||Safe||Less Safe||Safest||Least Safe||Safe|
|Safety risks||High loop impedance (step voltages)||Double fault, overvoltage||Broken neutral||Broken neutral||Broken neutral|
|Advantages||Safe and reliable||Continuity of operation, cost||Safest||Cost||Safety and cost|
While the national wiring regulations for buildings of many countries follow the IEC 60364 terminology, in North America (United States and Canada), the term "equipment grounding conductor" refers to equipment grounds and ground wires on branch circuits, and "grounding electrode conductor" is used for conductors bonding an earth ground rod (or similar) to a service panel. "Grounded conductor" is the system "neutral". Australian and New Zealand standards use a modified PME earthing system called Multiple Earthed Neutral (MEN). The neutral is grounded(earthed) at each consumer service point thereby effectively bringing the neutral potential difference to zero along the whole length of LV lines.
|This section requires expansion. (October 2013)|
In medium-voltage networks (1 kV to 72.5 kV), which are far less accessible to the general public, the focus of earthing system design is less on safety and more on reliability of supply, reliability of protection, and impact on the equipment in presence of a short circuit. Only the magnitude of phase-to-ground short circuits, which are the most common, is significantly affected with the choice of earthing system, as the current path is mostly closed through the earth. Three-phase HV/MV power transformers, located in distribution substations, are the most common source of supply for distribution networks, and type of grounding of their neutral determines the earthing system.
There are five types of neutral earthing:
In solid or directly earthed neutral, transformer's star point is directly connected to the ground. In this solution, a low-impedance path is provided for the ground fault current to close and, as result, their magnitudes are comparable with three-phase fault currents. Since the neutral remains at the potential close to the ground, voltages in unaffected phases remain at levels similar to the pre-fault ones; for that reason, this system is regularly used in high-voltage transmission networks, where insulation costs are high.
In unearthed, isolated or floating neutral system, as in the IT system, there is no direct connection of the star point (or any other point in the network) and the ground. As result, ground fault currents have no path to be closed and thus have negligible magnitudes. However, in practice, the fault current will not be equal to zero: conductors in the circuit — particularly underground cables — have an inherent capacitance towards the earth, which provides a path of relatively high impedance.
Systems with isolated neutral may continue operation and provide uninterrupted supply even in presence of a ground fault. However, while the fault is present, the potential of other two phases relative to the ground reaches of the normal operating voltage, creating additional stress for the insulation; insulation failures may inflict additional ground faults in the system, now with much higher currents.
Presence of uninterrupted ground fault may pose a significant safety risk: if the current exceeds 4–5 A an electric arc develops, which may be sustained even after the fault is cleared. For that reason, they are chiefly limited to underground and submarine networks, and industrial applications, where the reliability need is high and probability of human contact relatively low. In urban distribution networks with multiple underground feeders, the capacitive current may reach several tens of amperes, posing significant risk for the equipment.
The benefit of low fault current and continued system operation thereafter is offset by inherent drawback that the fault location is hard to detect.