What is equipotential zone

Grounding system - Earthing system

Physical equipment that creates a low-resistance connection between a circuit or electrical system and ground
This article looks at power grounding systems. For information on grounding pits / grounding, see Grounding (electricity).

A Grounding system (UK and IEC) or Grounding system (US) connects certain parts of a power supply system with the earth, typically the conductive earth's surface, for safety and functional reasons. The choice of the earthing system can impair the safety and electromagnetic compatibility of the system. The regulations for earthing systems vary considerably from country to country, although most follow the recommendations of the International Electrotechnical Commission. Special cases for earthing in mines, in patient care areas or in potentially explosive areas of industrial plants can be specified in the regulations.

In addition to power systems, other systems may need to be grounded for safety or functional reasons. Tall structures can have lightning rods as part of a system to protect them from lightning strikes. Telegraph lines can use earth as a conductor of a circuit, saving the cost of installing a return wire over a long circuit. Wireless antennas may require specific grounding to operate, control static electricity, and protect against lightning.

Goals of electrical grounding

System ground

A main component of grounding systems is static dissipation (system grounding), regardless of whether it is caused by lightning or friction (like wind blowing against an antenna mast). System grounding is required for use in systems such as utility distribution systems, telecommunications systems, and in commercial / residential buildings where an essential metal system must be connected at one point and referenced to earth. With system grounding, any static discharge that builds up is conducted to ground through a heavy ground electrode conductor and then into a ground electrode. The system ground is not to be confused with the device ground.

Device grounding

The earthing of devices is a component of electrical systems that protects against residual currents. Residual currents are mainly caused by insulation faults in a conductor and subsequent contact with a conductive surface. Technically speaking, this type of grounding is not a ground connection. It is a low-resistance connection between the neutral and the ground busbar in the main service console (and nowhere else). If a fault occurs and contact is made with a grounded surface, a large amount of current will flow to the ground rod, through the earth neutral link, and back to the power source. The overcurrent protection devices recognize this as a short-circuit condition and open the circuit in order to safely remedy the fault. The grounding standards for US equipment are set out in the National Electric Code.

Functional grounding

A working earth connection serves a purpose other than electrical safety and can carry electricity during normal operation. For example, in a single wire ground return distribution system, ground forms one conductor of the circuit and carries all of the load current. Other examples of devices that use functional ground connections include surge protectors and electromagnetic interference filters.

Low voltage systems

In low voltage networks that distribute power to the broadest class of end users, the primary concern in the design of grounding systems is the safety of the loads using the electrical appliances and their protection from electric shock. In combination with protective devices such as fuses and residual current devices, the earthing system must ultimately ensure that a person does not come into contact with a metallic object whose potential is one in relation to the potential of the person safe Threshold, which is typically set to about 50V.

In most industrialized countries, 220 V, 230 V, or 240 V sockets with grounded contacts were introduced either shortly before or shortly after World War II, albeit with significant national variations. In the United States and Canada, where the supply voltage is only 120V, sockets installed before the mid-1960s generally did not have a ground pin. In developing countries, local wiring practices may or may not connect to an earth.

In low voltage networks with a phase-neutral voltage of more than 240 V to 690 V, which are mainly used in industrial / mining machines / machines and not in publicly accessible networks, the design of the grounding system is just as important for safety reasons as it is for household users .

For a time, the US National Electrical Code allowed certain main devices that are permanently connected to the power supply to use the neutral cable of the power supply to connect the device housing to ground. This was not allowed for plug-in devices because the neutral and live conductor could easily be accidentally replaced, which was a serious hazard. If the neutral conductor were to be interrupted, the device housing would no longer be connected to ground. Normal imbalances in a split-phase distribution system can lead to undesirable earth voltages. Recent editions of the NEC no longer allow this practice. For similar reasons, most countries have now prescribed special protective earth connections for consumer cabling, which are now almost universal. In the distribution networks, where connections are less and less vulnerable, many countries allow the earth and the neutral conductor to share a conductor.

If the fault path between accidentally energized objects and the utility connection has a low impedance, the fault current is so large that the overcurrent protection device (fuse or circuit breaker) is opened to clear the earth fault. If the earthing system does not provide a low-resistance metal conductor between the device housing and the supply return (e.g. in a separately earthed TT system), the fault currents are lower and do not necessarily operate the overcurrent protection device. In this case, a residual current device is installed to detect the current leaking to ground and break the circuit.

IEC terminology

The 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 the earth and the power supply device (generator or transformer):

"T" - Direct connection of a point with the earth (French: terre)
"I" - No point is connected to earth (French: isolé), except perhaps via a high impedance.

The second letter indicates the connection between the earth or the network and the electrical device to be supplied:

"T" - The earth connection is made via a local direct connection to earth (French: terre), usually via an earthing rod.
"N" - The earth connection is supplied from the power supply network either separately to the neutral conductor (TN-S) in combination with the neutral conductor (TN-C) or to both (TN-CS). These are discussed below.

Types of TN networks

TN-S : Disconnect the protective conductor (PE) and the neutral conductor (N) from the transformer to the consumer device, which are not connected to each other at any time after the building distribution point.

TN-C : Combined PE and N conductors from the transformer to the consumer device.

TN-CS : Combined PEN conductor from the transformer to the building distribution point, but separate PE and N conductors in fixed internal cabling and flexible mains cables.

In one TN- Earthing system is one of the points in the generator or transformer connected to earth, usually the star point in a three-phase system. The body of the electrical device is connected to earth through this earth connection on the transformer. This arrangement is a current standard for electrical residential and industrial systems, especially in Europe.

The conductor that connects the exposed metal parts of the consumer electrical installation is called the Protective earth ( PE , see also: Boden). The conductor that is connected to the neutral point in a three-phase system or that carries the reverse current in a single-phase system is called Neutral conductor ( N ) designated. A distinction is made between three variants of TN systems:

TN - S.
PE and N are separate conductors that are only connected to one another near the power source.
TN - C.
A combined PEN conductor fulfills the functions of a PE and an N conductor. (for 230/400 V systems that are normally only used for distribution networks)
TN-C-S
Part of the system uses a combined PEN conductor, which at some point will be split into separate PE and N lines. The combined PEN conductor is usually located between the substation and the entry point into the building, and the earth and neutral conductors are separated in the maintenance head. In the UK this system is also called protective multiple earthing (PME) , because the combined neutral and earth conductor takes the shortest practicable route with local Earthing rods at the source and at intervals along the distribution networks to each location to provide both system grounding and equipment grounding at each of those locations. Similar systems in Australia and New Zealand are called multiply earthed neutral conductor (MEN) and in North America as multi-level neutral conductor (MGN) .

It is possible that both TN-S and TN-CS supplies are drawn from the same transformer. For example, the jackets of some underground cables corrode and no longer provide good ground connections. Therefore, houses in which high-resistance "bad groundings" are found can be converted to TN-CS. This is only possible in a network if the neutral conductor is sufficiently robust against failures and a conversion is not always possible. The PEN must be suitably reinforced against failure as an open circuit PEN can apply full phase voltage to any exposed metal connected to system ground after the interruption. The alternative is to provide a local earth and switch to TT. The main attraction of a TN network is that the low impedance ground path allows for easy automatic disconnection (ADS) of a high current circuit in the event of a short between line and PE, as the same circuit breaker or fuse is operated for either LN or L-PE fault , and a residual current circuit breaker is not required to detect earth faults.

TT network

The grounding system TT (French: terre-terre)

In one TT -Earthing system (French: terre-terre) the protective earth connection for the consumer is provided by a local earth electrode (sometimes also referred to as a Terra-Firma connection), and another independent earth electrode is installed on the generator. There is no ground wire between the two. The fault loop impedance is higher, and if the lead impedance is not very low, a TT installation should always have an RCD as the first isolator.

The great advantage of the TT grounding system is the lower level of conducted interference from the connected devices of other users. TT has always been preferred for special applications such as telecommunications locations that benefit from interference-free grounding. In addition, TT grids do not pose any serious risks in the event of a defective neutral conductor. In addition, in places where electricity is distributed overhead, there is no risk of earth conductors being live if an overhead line conductor is broken, for example by a fallen tree or branch .

In the pre-RCD era, the TT grounding system was unattractive for general use as it was difficult to arrange or ADS reliable automatic shutdown (ADS) in the event of a short circuit between line and PE (compared to TN systems with the same circuit breaker) Fuse works either with LN or L-PE errors). Because residual current devices mitigate this disadvantage, the TT grounding system has become much more attractive, provided all AC circuits are RCD protected. In some countries (e.g. the UK), TT is recommended for situations where maintaining a low impedance equipotential zone by gluing is impractical, where there is significant outdoor wiring, e.g. In the supply of mobile homes and some agricultural environments or where there is a high error. Electricity can present other hazards, e.g. B. in petrol stations or marinas.

The TT grounding system is used across Japan, with RCD units in most industrial settings. This can place additional demands on frequency converters and switched-mode power supplies, which often have significant filters that conduct high-frequency noise to the earth conductor.

IT network

In one IT Network (isolé-terre), the electrical distribution system has no connection to earth at all or only a high-resistance connection.

comparison

TT IT TN-S TN-C TN-CS
Earth fault loop impedance High Highest Low Low Low
RCD preferred? Yes N / A Optional No Optional
Do you need a grounding electrode on site? Yes Yes No No Optional
PE conductor costs Low Low Highest at least High
Risk of a neutral break No No High Highest High
security For sure Less secure The safest Least sure For sure
Electromagnetic interference at least at least Low High Low
Security risks High loop impedance (step voltages) Double fault, overvoltage Neutral broken Neutral broken Neutral broken
advantages Safe and reliable Business continuity, costs The safest costs Security and Cost

Other terminology

While national building wiring regulations in many countries follow the terminology of IEC 60364, in North America (United States and Canada) the term "equipment ground wire" refers to branch circuit ground and ground wire and "ground electrode wire". is used for conductors that connect a grounding rod, an electrode or the like to a maintenance plate. The "local" grounding / grounding electrode provides "system grounding" in every building in which it is installed.

The "earthed" live conductor is the "neutral" system. A modified Protective Multiple Earthing (PME) system called Multiple Earthed Neutral (MEN) is used in Australian and New Zealand standards. The neutral conductor is earthed (earthed) at every consumer service point, which effectively brings the neutral potential difference towards zero over the entire length of the low-voltage lines. In North America, the term "Multigrounded Neutral" (MGN) system is used.

In the UK and some Commonwealth of Nations, the term "PNE", which means "phase neutral earth", is used to indicate that three (or more for non-single-phase connections) conductors are being used, i.e. PN-S.

Resistance Grounded Neutrality (India)

Resistance grounding system is used for mining in India as per Central Electricity Authority regulations. Instead of a fixed connection from neutral to earth, a neutral earth resistance (NGR) is used to limit the current to earth to less than 750 mA. Because of the fault current limitation, it is safer for gaseous mines. Because earth leakage is limited, leakage protectors can be set to less than 750 mA. In comparison, in a firmly grounded system, the earth fault current can be as much as the available short circuit current.

The neutral earth resistance is monitored to detect a broken earth connection and to turn off the power supply if a fault is detected.

Earth fault protection

To avoid accidental electric shock, current sense circuits are used at the source to disconnect power if the leakage current exceeds a certain limit. Residual current devices (RCDs, RCCBs or GFCIs) are used for this purpose. So far, a residual current circuit breaker has been used. In industrial applications, earth fault relays with separate symmetrical core current transformers are used. This protection operates in the milliamp range and can be set from 30 mA to 3000 mA.

Verification of earth connectivity

In addition to the ground wire, a separate pilot wire is run from the distribution / equipment supply system to monitor the continuity of the wire. This is used in the tow cables of mining machines. If the ground wire is broken, the pilot wire allows a meter at the source end to cut power to the machine. This type of circuit is a must for portable heavy electrical equipment (such as LHD (Load, Haul, Dump Machine)) used in underground mines.

properties

costs

  • TN networks save the costs of a low-resistance earth connection at the location of each consumer. Such a connection (a buried metal structure) is required in IT and TT systems Protective earth provide.
  • TN-C networks save the cost of an additional conductor required for separate N and PE connections. However, to reduce the risk of the neutral conductor breaking, special types of cables and many connections to earth are required.
  • TT networks require proper Ground Fault Interrupter (RCD) protection.

security

  • In TN, an insulation fault very likely leads to a high short-circuit current, which trips an overcurrent circuit breaker or a fuse and disconnects the L conductors. In TT systems, the ground fault loop impedance may be too high to do this or too high to do it within the required time. Therefore, an RCD (formerly ELCB) is usually used. Previous TT installations may not have this important safety feature, so that the CPC (Circuit Protective Conductor or PE) and possibly associated metal parts that are within reach of people (exposed conductive parts and externally conductive parts) will be subject to faults with electricity for long periods of time Can be taken care of in conditions that is a real danger.
  • In TN-S and TT systems (and in TN-CS beyond the point of division) a residual current device can be used for additional protection. If there is no insulation fault in the consumer device, the equation applies I. L1 + I. L2 + I. L3 + I. N = 0, and an FI can interrupt the supply as soon as this sum reaches a threshold value (typically 10 mA - 500 mA). An insulation fault between L or N and PE has a high probability of triggering an FI.
  • In IT and TN-C networks, it is far less likely that residual current devices will detect an insulation fault. In a TN-C system, they would also be very susceptible to unwanted tripping due to contact between earth conductors of circuits on different RCDs or with real ground, making their use impractical. In addition, RCDs usually isolate the neutral core. Since this is not safe in a TN-C system, RCDs on TN-C should be wired in such a way that only the line conductor is interrupted.
  • In single-phase single-phase systems that combine earth and neutral (TN-C and the part of TN-CS systems that uses a combined neutral and earth core), there is a contact problem in the PEN conductor on all parts of the earth system after the break rise to the potential of the L conductor. In an asymmetrical multi-phase system, the potential of the earthing system moves in the direction of the most heavily loaded line conductor. Such a rise in the potential of the neutral conductor beyond the interruption is called known neutral inversion . Therefore, TN-C connections must not be routed over plug connections or flexible cables, which are more likely to have contact problems than fixed cables. There is also a risk of cable damage, which can be reduced by using a concentric cable construction and multiple grounding electrodes. Due to the (low) risk that the lost neutrality will raise "grounded" metalwork to a dangerous potential, as well as the increased risk of shock from being close to good contact with true earth, the use of TN-CS consumables for caravan sites is prohibited in the UK and shore supply for boats, highly recommended for use on farms and outdoor construction sites. In such cases, it is recommended that all external TT cabling be provided with a residual current circuit breaker and a separate grounding electrode.
  • In IT systems, it is unlikely that a single insulation fault will cause dangerous currents to flow through a human body in contact with the earth, since there is no low-resistance circuit for such a current. However, a first insulation fault can effectively transform an IT system into a TN system, and a second insulation fault can then lead to dangerous body currents. Worse still, in a multi-phase system, contact of the ground conductors with one of the line conductors would cause the other phase cores to rise to phase phase voltage relative to earth rather than phase neutral voltage. IT systems also experience greater transient overvoltages than other systems.
  • In TN-C and TN-CS systems, any connection between the combined neutral and earth core and the earth's body can carry a significant current under normal conditions and even more under an interrupted neutral situation. The most important equipotential bonding conductors must therefore be dimensioned with this in mind. The use of TN-CS is not advisable in situations like gas stations where there is a combination of heavily buried metalwork and explosive gases.

Electromagnetic compatibility

  • In TN-S and TT systems, the consumer has a low-noise connection to earth that does not suffer from the voltage that occurs on the N conductor due to the return currents and the impedance of this conductor. This is of particular concern with some types of telecommunications and measurement equipment.
  • In TT systems, each consumer has its own connection to earth and does not notice any currents that could be caused by other consumers on a shared PE line.

Regulations

  • In the United States National Electrical Code and Canadian Electrical Code, a combined neutral and grounding conductor is used to feed from the distribution transformer, but separate neutral and protective grounding conductors (TN-CS) are used within the structure. The neutral conductor may only be connected to ground on the supply side of the customer's isolating switch.
  • In Argentina, France (TT) and Australia (TN-CS) customers must provide their own ground connections.
  • Equipment in Japan must comply with PSE law, and building wiring uses TT grounding in most installations.
  • Australia uses the Multiple Earthed Neutral (MEN) grounding system described in Section 5 of AS / NZS 3000. For an LV customer it is a TN-C system from the transformer on the street to the premises (the neutral conductor is grounded several times along this segment) and a TN-S system in the installation from the main switchboard downwards. Overall, it is a TN-CS system.
  • In Denmark, the High Voltage Regulation (Stærkstrømsbekendtgørelsen) and in Malaysia the Electricity Ordinance of 1994 stipulate that all consumers must use TT grounding, although in rare cases TN-CS may be allowed (in the same way as in the US). Different rules apply to larger companies.
  • In India, according to the provisions of the Central Electricity Authority, CEAR, 2010, Rule 41, a grounding, a neutral conductor of a 3-phase 4-wire system and the additional third wire of a 2-phase 3-wire system are provided. The grounding is done with two separate connections. The grounding system must also have at least two or more grounding pits (electrodes) to ensure proper grounding. According to Rule 42, an installation with a connected load greater than 5 kW over 250 V must have a suitable earth fault protection device to isolate the load in the event of an earth fault or leakage.

Application examples

  • The TN-S system is common in areas of the UK where underground power cables are predominant.
  • In India, LT supply is generally provided via the TN-S system. The neutral conductor is double grounded on each distribution transformer. Neutral and earth conductors run separately on overhead lines. Separate conductors for overhead lines and armoring of cables are used for grounding. Additional ground electrodes / pits are installed at each end of the user to provide a redundant ground path.
  • Most modern homes in Europe have a TN-CS grounding system. The combined neutral and earth occur between the nearest substation and the business interruption (the fuse in front of the meter). Thereafter, separate ground and neutral conductors are used throughout all internal wiring.
  • Older urban and suburban homes in the UK typically have TN-S coverage, with the ground connection being made through the cable sheath of an underground lead-and-paper cable.
  • Older households in Norway use the IT system while newer houses use TN-CS.
  • Some older homes, especially those built prior to the invention of residual current circuit breakers and wired home networks, use an internal TN-C arrangement. This is no longer recommended.
  • Laboratories, medical facilities, construction sites, repair shops, mobile electrical installations and other environments that are powered by motor-generators, in which there is an increased risk of insulation failure, often use an IT grounding arrangement that is provided by isolating transformers. In order to reduce the two-fault problems with IT systems, the isolating transformers should only supply a small number of loads and be protected with an insulation monitoring device (which is generally only used by medical, railway or military IT systems for reasons of cost ).
  • In remote areas, where the cost of an additional PE conductor outweighs the cost of a local earth connection, TT networks are widely used in some countries, especially in older buildings or in rural areas where safety is otherwise due to the breakage of one Earth conductor could be endangered overhead PE conductor, for example from a fallen branch. TT deliveries to individual objects are also mainly seen in TN-CS systems, in which a single object is viewed as unsuitable for TN-CS supply.
  • The TN-CS system is used in Australia, New Zealand and Israel. However, the wiring rules stipulate that each customer must also establish a separate connection to earth using a special earth electrode. (All metallic water pipes that enter the consumer's premises must also be "connected" to the grounding point on the switchboard / switchboard.) In Australia and New Zealand, the connection between the protective earth rod and the neutral rod on the main switchboard / switchboard is called a multiple Earthed Neutral Link or MEN Link. This MEN Link is removable for installation test purposes, but is connected by either a locking system (e.g. lock nuts) or two or more screws during normal operation. In the MEN system, the integrity of the neutral is of the utmost importance. In Australia, new installations also need to connect the foundation concrete reinforcing under wet areas to the ground wire (AS3000), which typically increases the ground size (i.e., decreases resistance) and provides an equipotential plane in areas such as bathrooms. In older installations, it is not uncommon to find only the plumbing connection and it is allowed to remain as such. However, the additional ground electrode must be installed when upgrading. The incoming protective earth / neutral conductor is connected to a neutral rail (on the customer side of the neutral connection of the electricity meter), which is then connected to the earth rail via the customer's MEN connection - beyond this point the protective earth and the neutral conductors are separated.

High voltage systems

Simulation of multiple earthing in a layered floor

In high voltage networks (above 1 kV), which are far less accessible to the general public, the focus of the design of grounding systems is less on safety than on reliability of supply, reliability of protection and the effects on equipment in the presence of a short circuit. Only the size of the phase-to-ground short-circuits, which occur most frequently, is significantly influenced when choosing the earthing system, since the current path is usually closed through the earth. Three-phase HV / MV power transformers, in distribution substations, are the most common source of supply for distribution networks, and the type of grounding of their neutral determines the grounding system.

There are five types of neutral grounding:

  • Solidly earthed neutral
  • Unearthed neutral
  • Resistance-earthed neutral conductor
    • Low resistance grounding
    • High resistance grounding
  • Reactance-grounded neutral conductor
  • Use of grounding transformers (such as the zigzag transformer)

Solidly earthed neutral

in the firm or directly earthed neutral conductor, the neutral point of the transformer is directly connected to earth. In this solution, a low resistance path is provided to close the earth fault current and, as a result, its magnitudes are comparable to three phase fault currents. Since the neutral conductor remains at the potential near the ground, the voltages in unaffected phases remain at a similar level as before the fault. For this reason, this system is regularly used in high-voltage transmission networks where insulation costs are high.

Resistance-earthed neutral conductor

To limit the earth fault of the short circuit, an additional neutral earth resistance (NER) is added between the neutral of the transformer's star point and earth.

Low resistance grounding

If the resistance is low, the fault current limitation is relatively high. In India, it is limited to 50A for open pit mines under the Central Electricity Authority Regulations, CEAR, 2010, Rule 100.

High resistance grounding

The high-resistance grounding system grounds the neutral conductor through a resistor that limits the ground fault current to a value that is equal to or slightly greater than the capacitive charging current of this system

Unearthed neutral

In one excavated , isolated or floating neutrals As in the IT system, there is no direct connection between the star point (or any other point in the network) and the ground. As a result, earth fault currents have no path to be closed and are therefore negligible. In practice, however, the fault current is not zero: conductors in the circuit - especially underground cables - have an inherent capacitance to earth that provides a path with a relatively high impedance.

Systems with an isolated neutral conductor can continue operation and guarantee an uninterrupted supply even in the event of an earth fault. While the fault is present, however, the potential of the other two phases relative to ground reaches the normal operating voltage, which creates an additional load on the insulation. Insulation faults can cause additional earth faults in the system, now with much higher currents.

The presence of an uninterrupted earth fault can pose a significant safety risk: if the current exceeds 4 A - 5 A, an arc is created that can be maintained even after the fault has been rectified. For this reason, they are mainly limited to underground and submarine networks as well as industrial applications where the need for reliability is high and the likelihood of human contact is relatively low. In urban distribution networks with several underground branches, the capacitive current can reach tens of amps, which poses a significant risk to the equipment.

The benefit of low fault current and continued system operation thereafter is offset by the inherent disadvantage that the fault location is difficult to identify.

Grounding rods

According to IEEE standards, ground rods are made from materials such as copper and steel. When choosing an earthing rod, there are various selection criteria such as: corrosion resistance, diameter depending on the fault current, conductivity and others. There are different types of copper and steel: copper-bonded stainless steel, solid copper, galvanized steel. In the last few decades, chemical grounding rods for low-resistance grounding that contain natural electrolytic salts have been developed. and ground rods made of nano-carbon fiber.

Ground connections

Earthing installation connectors are a means of communication between the various components of earthing and lightning protection installations (earthing rods, earthing conductors, power cables, busbars, etc.).

In high voltage installations, exothermic welding is used for underground connections.

Soil resistance

Vertical stress on a floor

Soil resistance is an essential aspect when planning and calculating an earthing system / earthing system. Its resistance depends on the efficiency of removing unwanted currents to zero potential (ground). The persistence of a geological material depends on several components: the presence of metal ores, the temperature of the geological layer, the presence of archaeological or structural features, the presence of dissolved salts and impurities, porosity and permeability. There are several basic methods of measuring soil resistance. The measurement is carried out with two, three or four electrodes. The measuring methods are: pole-pole, dipole-dipole, pole-dipole, Wenner method and the Schlumberger method.

See also

References

General