Earthing system

An earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary considerably among countries, though most follow the recommendations of the International Electrotechnical Commission. Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants.
In addition to electric power systems, other systems may require grounding for safety or function. Tall structures may have lightning rods as part of a system to protect them from lightning strikes. Telegraph lines may use the Earth as one conductor of a circuit, saving the cost of installation of a return wire over a long circuit. Radio antennas may require particular grounding for operation, as well as to control static electricity and provide lightning protection.
Objectives of electrical earthing.
System Grounding.
A primary component of earthing systems is static dissipation (system grounding), whether it be lightning-induced or friction-induced (like wind blowing against an antenna mast). System grounding is required for use in systems like utility distribution systems, telecommunications systems, and in commercial/residential buildings where any significant metal system must be bonded together and referenced to the earth at one point. System grounding works by sending any built up static discharge to the ground through a heavy grounding electrode conductor and then into an earthing electrode. System Grounding is not to be confused with Equipment Grounding.
Equipment Grounding.
Equipment grounding is a component of electrical systems that protects against fault currents. Fault currents are mainly caused by insulation failure of a conductor and subsequent contact with a conductive surface. This type of grounding is not a grounding connection, technically speaking. It is a low-impedance bonding connection between the neutral and ground bus-bars in the main service panel (and nowhere else). When a fault occurs and contact is made with a grounded surface, a large amount of current rushes to the grounding bar, across the ground-neutral bonding connection, and back to the source of current. The over-current protective devices sense this as a short-circuit condition and open the circuit, safely clearing the fault. US equipment grounding standards are set by the National Electric Code.
Functional earthing.
A "functional earth" connection serves a purpose other than electrical safety, and may carry current as part of normal operation. For example, in a single-wire earth return power distribution system, the earth forms one conductor of the circuit and carries all the load current. Other examples of devices that use functional earth connections include surge suppressors and electromagnetic interference filters.
Low-voltage systems.
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 does not come into contact 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 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. However in the United States and Canada, where the supply voltage is only 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 or may not provide a connection to an earth. 
On low voltage electricity networks with a phase to neutral voltage exceeding 240 V upto 690 V, which are mostly used in industrial / mining equipment / machines rather than publicly accessible networks, the earthing system design is equally important from safety point of view as for domestic users.
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 similar reasons, most countries have now mandated dedicated protective earth connections in consumer wiring that are now almost universal. In the distribution networks, where connections are fewer and less vulnerable, many countries allow the earth and neutral to share a conductor.
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 device is installed to detect the current leaking to ground and interrupt the circuit.
IEC terminology.
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):
The second letter indicates the connection between earth or network and the electrical device being supplied:
Types of TN networks.
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.
This arrangement is a current standard for residential and industrial electric systems particularly in Europe.
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:
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 high resistance "bad earths" are found may be converted to TN-C-S. This is only possible on a network when the neutral is suitably robust against failure, and conversion is not always possible. The PEN must be suitably reinforced against failure, as an open circuit PEN can impress full phase voltage on any exposed metal connected to the system earth downstream of the break. The alternative is to provide a local earth and convert to TT.
The main attraction of a TN network is the low impedance earth path allows easy automatic disconnection (ADS) on a high current circuit in the case of a line-to-PE short circuit as the same breaker or fuse will operate for either L-N or L-PE faults, and an RCD is not needed to detect earth faults.
TT network.
In a TT (French: terre-terre) earthing system, the protective earth connection for the consumer is provided by a local earth electrode, (sometimes referred to as the Terra-Firma connection) and there is another independently installed at the generator. There is no 'earth wire' between the two.
The fault loop impedance is higher, and unless the electrode impedance is very low indeed, a TT installation should always have an RCD (GFCI) as its first isolator.
The big advantage of the TT earthing system is the reduced conducted interference from other users' connected equipment. TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT networks do not pose any serious risks in the case of a broken neutral. In addition, in locations where power is distributed overhead, earth conductors are not at risk of becoming live 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 the difficulty of arranging reliable automatic disconnection (ADS) in the case of a line-to-PE short circuit (in comparison with TN systems, where the same breaker or fuse will operate for either L-N or L-PE faults). But as residual current devices mitigate this disadvantage, the TT earthing system has become much more attractive providing that all AC power circuits are RCD-protected. In some countries (such as the UK) TT is recommended for situations where a low impedance equipotential zone is impractical to maintain by bonding, where there is significant outdoor wiring, such as supplies to mobile homes and some agricultural settings, or where a high fault current could pose other dangers, such as at fuel depots or marinas.
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.
IT network.
In an IT network (isolé-terre), the electrical distribution system has no connection to earth at all, or it has only a high-impedance connection.
Other terminologies.
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, electrode or similar to a service panel. The "local" Earth/Ground electrode provides "System Grounding" at each building where it is installed.
The "Grounded" current carrying conductor is the system "neutral".
Australian and New Zealand standards use a modified Protective Multiple Earthing (PME ) system called Multiple Earthed Neutral (MEN). The neutral is grounded (earthed) at each consumer service point thereby effectively bringing the neutral potential difference towards zero along the whole length of LV lines.
In North America, the term "Multigrounded Neutral" system (MGN) is used.
In the UK and some Commonwealth countries, the term "PNE", meaning Phase-Neutral-Earth is used to indicate that three (or more for non-single-phase connections) conductors are used, i.e., PN-S.
Resistance-earthed neutral (India).
A resistance earth system is used for mining in India as per Central Electricity Authority Regulations. Instead of a solid connection of neutral to earth, a neutral grounding resistor (NGR) is used to limit the current to ground to less than 750 mA. Due to the fault current restriction it is safer for gassy mines.
 Since the earth leakage is restricted, leakage protection devices can be set to less than 750 mA . By comparison, in a solidly earthed system, earth fault current can be as much as the available short-circuit current.
The neutral earthing resistor is monitored to detect an interrupted ground connection and to shut off power if a fault is detected.
Earth leakage protection.
To avoid accidental shock, current sensing circuits are used at the source to isolate the power when leakage current exceed a certain limit. Residual-current devices (RCDs, RCCBs or GFCIs) are used for this purpose. Previously, an earth leakage circuit breaker is used. In industrial applications, earth leakage relays are used with separate core balanced current transformers. This protection works in the range of milli-Amps and can be set from 30 mA to 3000 mA.
Earth connectivity check.
A separate pilot wire is run from distribution/ equipment supply system in addition to earth wire, to supervise the continuity of the wire. This is used in the trailing cables of mining machinery. If the earth wire is broken, the pilot wire allows a sensing device at the source end to interrupt power to the machine. This type of circuit is a must for portable heavy electric equipment (like LHD (Load, Haul, Dump machine)) being used in underground mines.
High-voltage systems.
In high-voltage networks (above 1 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:
Solid-earthed neutral.
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.
Resistance-earthed neutral.
To limit short circuit earth fault an additional neutral earthing resistor (NER) is added between the neutral of transformer's star point and earth.
Low-resistance earthing.
With low resistance fault current limit is relatively high. In India it is restricted for 50 A for open cast mines according to Central Electricity Authority Regulations, CEAR, 2010, rule 100.
High-resistance earthing.
High resistance grounding system grounds the neutral through a resistance which limits the ground fault current to a value equal to or slightly greater than the capacitive charging current of that system
Unearthed neutral.
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 a 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 formula_1 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 A – 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.
Grounding rods.
According to the IEEE standards, grounding rods are made from material such as copper and steel. For choosing a grounding rod there are several selection criteria such as: corrosion resistance, diameter depending of the fault current, conductivity and others. There are several types derived from copper and steel: copper-bonded, stainless-steel, solid copper, galvanized steel ground. In recent decades, there has been developed chemical grounding rods for low impedance ground containing natural electrolytic salts. and Nano-Carbon Fiber Grounding rods.
Grounding connectors.
Connectors for earthing installation are a means of communication between the various components of the earthing and lightning protection installations (earthing rods, earthing conductors, current leads, busbars, etc.).
For high voltage installations, exothermic welding is used for underground connections.
Soil resistance.
Soil resistance is a major aspect in the design and calculation of an earthing system/grounding installation. Its resistance depends on the efficiency of the removal of unwanted currents to zero potential (ground). The resistance of a geological material depends on several components: the presence of metal ores, the temperature of the geological layer, the presence of archeological or structural features, the presence of dissolved salts, and contaminants, porosity and permeability. There are several basic methods for measuring soil resistance. The measurement is performed with two, three or four electrodes. The measurement methods are: pole-pole, dipole-dipole, pole-dipole, Wenner method, and the Schlumberger method.