Earthing system

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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.

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. 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.

People use an earthing system mainly for these applications:

Other, less common applications of earthing systems include:

Low-voltage systems[edit]

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.

IEC terminology[edit]

International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.

The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):

"T" — Direct connection of a point with earth (Latin: terra)
"I" — No point is connected with earth (isolation), except perhaps via a high impedance.

The second letter indicates the connection between earth and the electrical device being supplied:

"T" — Direct connection of a point with earth
"N" — Direct connection to neutral at the origin of installation, which is connected to the earth

TN networks[edit]

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.

TN-S-earthing.svg

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
PE and N are separate conductors that are connected together only near the power source. This arrangement is a current standard for most residential and industrial electric systems particularly in Europe.[citation needed]
TN−C
A combined PEN conductor fulfils the functions of both a PE and an N conductor. Rarely used.
TN−C−S
Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor typically occurs between the substation and the entry point into the building, and separated in the service head. In the UK, this system is also known as protective multiple earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at many locations, to reduce the risk of electric shock in the event of a broken PEN conductor - with a similar system in Australia and New Zealand being designated as multiple earthed neutral (MEN).
TN-S-earthing.svgTN-C-earthing.svgTN-C-S-earthing.svg
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 taken 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 may be converted to TN-C-S.

TT network[edit]

In a TT (Terra-Terra) earthing system, the protective earth connection for the consumer is provided by a local earth electrode, and there is another independently installed at the generator. There is no 'earth wire' between the two. The fault impedance is higher, and unless the electrode impedance is very low indeed, a TT installation should always have an RCD as its first isolator.

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.

TT-earthing.svg

IT network[edit]

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.

IT-earthing.svg

Comparison[edit]

TTITTN-STN-CTN-C-S
Earth fault loop impedanceHighHighestLowLowLow
RCD preferred?YesN/ANoNoNo
Need earth electrode at site?YesYesNoNoNo
PE conductor costLowLowHighestLeastHigh
Risk of broken neutralNoNoHighHighestHigh
SafetySafeLess SafeSafestLeast SafeSafe
Electromagnetic interferenceLeastLeastLowHighLow
Safety risksHigh loop impedance (step voltages)Double fault, overvoltageBroken neutralBroken neutralBroken neutral
AdvantagesSafe and reliableContinuity of operation, costSafestCostSafety and cost

Other terminologies[edit]

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.

Properties[edit]

Cost[edit]

Safety[edit]

Electromagnetic compatibility[edit]

Regulations[edit]

Application examples[edit]

Medium-voltage systems[edit]

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:[2]

Solid-earthed neutral[edit]

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.[2] 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.[3]

Unearthed neutral[edit]

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.[4]

Systems with isolated neutral may continue operation and provide uninterrupted supply even in presence of a ground fault.[2] However, while the fault is present, the potential of other two phases relative to the ground reaches \sqrt{3} 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.[3]

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.[4] 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.[5]

See also[edit]

References[edit]

  1. ^ Gates, B.G. (1936). Neutral inversion in power systems. In Journal of the Institution of Electrical Engineers 78 (471): 317-325. Retrieved 2012-03-20.
  2. ^ a b c Parmar, Jignesh, Types of neutral earthing in power distribution (part 1), EEP - Electrical Engineering Portal 
  3. ^ a b Guldbrand, Anna (2006), System earthing, Industrial Electrical Engineering and Automation, Lund University 
  4. ^ a b Bandyopadhyay, M. N. (2006). "21. Neutral earthing". Electrical Power Systems: Theory and Practice. PHI Learning Pvt. Ltd. pp. 488–491. 
  5. ^ Fischer, Normann; Hou, Daqing (2006), Methods for detecting ground faults in medium-voltage distribution power systems, Schweitzer Engineering Laboratories, Inc., p. 15