In electricity supply systems, an
earthing system defines the electrical potential of the conductors relative to the Earth's conductive surface. The choice of earthing system can affect the
safetySafety is the state of being "safe" , the condition of being protected against physical, social, spiritual, financial, political, emotional, occupational, psychological, educational or other types or consequences of failure, damage, error, accidents, harm or any other event which could be...
and
electromagnetic compatibilityElectromagnetic compatibility is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects that such energy may induce...
of the power supply, and regulations can vary considerably among countries. Most electrical systems connect one supply conductor to
earthIn electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
(ground). 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 (e.g., by standing on it, or touching an earthed sink) will complete a circuit back to the earthed supply conductor and receive an
electric shockElectric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
.
A
protective earth, 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 dropVoltage drop is the reduction in voltage in the passive elements of an electrical circuit. Voltage drops across conductors, contacts, connectors and source internal resistances are undesired as they reduce the supplied voltage while voltage drops across loads and other electrical and electronic...
no current is allowed to flow in this conductor under normal circumstances, but fault currents will usually trip or blow the
fuseIn electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
or
circuit breakerA circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
protecting the circuit. A high
impedanceImpedance may refer to:*Electrical impedance, the ratio of the voltage phasor to the electric current phasor, a measure of the opposition to time-varying electric current in an electric circuit**Characteristic impedance of a transmission line...
line-to-ground fault insufficient to trip the overcurrent protection may still trip a
residual-current deviceA Residual Current Device is a generic term covering both RCCBs and RCBOs.A Residual-Current Circuit Breaker is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor...
(
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. Examples of devices that use functional earth connections include surge suppressors and
electromagnetic interferenceElectromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
filters, certain
antennasAn antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
and measurement instruments. But 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 earth currents. The NEC calls it a
groundED supply conductor to distinguish it from the
equipment groundING conductor.
Until the mid 1900s, power outlets generally lacked protective earth terminals. Devices needing an earth connection often used the supply neutral. Some used dedicated
ground rodsA groundbed is an array of electrodes, known as ground rods, installed in the ground to provide a low resistance electrical path to ground or earth. A groundbed is a component in an earthing system.-Grounding systems:...
. Many appliances had polarized plugs to maintain a distinction between "live" and "neutral", but using the supply neutral for equipment earthing was 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 mandated dedicated protective earth connections that are now almost universal.
IEC terminology
International standardInternational standards are standards developed by international standards organizations. International standards are available for consideration and use, worldwide...
IEC 60364IEC 60364 Electrical Installations for Buildings is the International Electrotechnical Commission's international standard on electrical installations of buildings. This standard is an attempt to harmonize national wiring standards in an IEC standard...
distinguishes three families of earthing arrangements, using the two-letter codes
TN,
TT, and
IT.
The first letter indicates the connection between
earthIn electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the 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
In a
TN earthing system, one of the points in the
generatorIn electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
or
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
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 is called
protective earth (
PE). The conductor that connects to the star point in a
three-phaseIn electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are radians offset in time...
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 the current standard for most residential and industrial electric systems in North America and Europe.
TN−C: A combined PEN conductor fulfills 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 broken neutrals - with a similar system in Australia being designated as
multiple earthed neutral (MEN).
| |
 |
 |
| 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.
TT network
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 the fact that it is clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it. This is why 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-
RCDA Residual Current Device is a generic term covering both RCCBs and RCBOs.A Residual-Current Circuit Breaker is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor...
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 devicesA Residual Current Device is a generic term covering both RCCBs and RCBOs.A Residual-Current Circuit Breaker is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor...
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 drivesA variable-frequency drive is a system for controlling the rotational speed of an alternating current electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive...
and
switching power suppliesA switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
which often have substantial filters passing high frequency noise to the ground conductor.
IT network
In an
IT network, the distribution system has no connection to earth at all, or it has only a high
impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
connection. In such systems, an
insulation monitoring deviceAn insulation monitoring device monitors the ungrounded system between an active phase conductor and earth. It is intended to give an alert or disconnect the power supply when the impedance between the two conductors drops below a set value, usually 50 kΩ.The main advantage of the device that it...
is used to monitor the impedance.
Other terminologies
While the national wiring regulations for buildings of many countries follow the
IEC 60364IEC 60364 Electrical Installations for Buildings is the International Electrotechnical Commission's international standard on electrical installations of buildings. This standard is an attempt to harmonize national wiring standards in an IEC standard...
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 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 netral pd to zero along the whole length of LV lines.
Cost
- TN networks save the cost of a low-impedance earth connection at the site of each consumer. Such a connection (a buried metal structure) is required to provide protective earth in IT and TT systems.
- TN-C networks save the cost of an additional conductor needed for separate N and PE connections. However, to mitigate the risk of broken neutrals, special cable types and lots of connections to earth are needed.
- TT networks require proper RCD protection.
Fault path impedance
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 detectorA Residual Current Device is a generic term covering both RCCBs and RCBOs.A Residual-Current Circuit Breaker is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor...
is installed to detect the current leaking to ground and interrupt the circuit.
Safety
- In TN, an insulation fault is very likely to lead to a high short-circuit current that will trigger an overcurrent circuit-breaker or fuse and disconnect the L conductors. With TT systems, the earth fault loop impedance can be too high to do this, or too high to do it quickly, so an RCD (or formerly ELCB) is usually employed. The provision of a Residual-current device
A Residual Current Device is a generic term covering both RCCBs and RCBOs.A Residual-Current Circuit Breaker is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor...
(RCD) or ELCB to ensure safe disconnection makes these installations EEBAD (Earthed Equipotential Bonding and Automatic Disconnection). Earlier TT installations may lack this important safety feature, allowing the CPC (Circuit Protective Conductor) to become energized for extended periods under fault conditions, which is a real danger.
- In TN-S and TT systems (and in TN-C-S beyond the point of the split), a residual-current device can be used as an additional protection. In the absence of any insulation fault in the consumer device, the equation IL1+IL2+IL3+IN = 0 holds, and an RCD can disconnect the supply as soon as this sum reaches a threshold (typically 10-500 mA). An insulation fault between either L or N and PE will trigger an RCD with high probability.
- In IT and TN-C networks, residual current devices are far less likely to detect an insulation fault. In a TN-C system, they would also be very vulnerable to unwanted triggering from contact between earth conductors of circuits on different RCDs or with real ground, thus making their use impracticable. Also, RCDs usually isolate the neutral core. Since it is unsafe to do this in a TN-C system, RCDs on TN-C should be wired to only interrupt the live conductor.
- In single-ended single-phase systems where the Earth and neutral are combined (TN-C, and the part of TN-C-S systems which uses a combined neutral and earth core), if there is a contact problem in the PEN conductor, then all parts of the earthing system beyond the break will rise to the potential of the L conductor. In an unbalanced multi-phase system, the potential of the earthing system will move towards that of the most loaded live conductor. Therefore, TN-C connections must not go across plug/socket connections or flexible cables, where there is a higher probability of contact problems than with fixed wiring. There is also a risk if a cable is damaged, which can be mitigated by the use of concentric cable construction and/or multiple earth electrodes. Due to the (small) risks of the lost neutral, use of TN-C-S supplies is banned for caravans and boats in the UK, and it is often recommended to make outdoor wiring TT with a separate earth electrode.
- In IT systems, a single insulation fault is unlikely to cause dangerous currents to flow through a human body in contact with earth, because no low-impedance circuit exists for such a current to flow. However, a first insulation fault can effectively turn an IT system into a TN system, and then a second insulation fault can lead to dangerous body currents. Worse, in a multi-phase system, if one of the live conductors made contact with earth, it would cause the other phase cores to rise to the phase-phase voltage relative to earth rather than the phase-neutral voltage. IT systems also experience larger transient overvoltages than other systems.
- In TN-C and TN-C-S systems, any connection between the combined neutral-and-earth core and the body of the earth could end up carrying significant current under normal conditions, and could carry even more under a broken neutral situation. Therefore, main equipotential bonding conductors must be sized with this in mind; use of TN-C-S is inadvisable in situations such as petrol stations, where there is a combination of lots of buried metalwork and explosive gases.
Electromagnetic compatibility
- In TN-S and TT systems, the consumer has a low-noise connection to earth, which does not suffer from the voltage that appears on the N conductor as a result of the return currents and the impedance of that conductor. This is of particular importance with some types of telecommunication and measurement equipment.
- In TT systems, each consumer has its own connection to earth, and will not notice any currents that may be caused by other consumers on a shared PE line.
Regulations
- In the United States National Electrical Code
The National Electrical Code , or NFPA 70, is a regionally adoptable standard for the safe installation of electrical wiring and equipment...
and Canadian Electrical CodeThe Canadian Electrical Code, CE code, or CSA C22.1 is a standard published by the Canadian Standards Association pertaining to the installation and maintenance of electrical equipment in Canada....
the feed from the distribution transformer uses a combined neutral and grounding conductor, but within the structure separate neutral and protective earth conductors are used (TN-C-S). The neutral must be connected to the earth (ground) conductor only on the supply side of the customer's disconnecting switch. Additional connections of neutral to ground within the customer's wiring are prohibited.
- In Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
, FranceThe French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
(TT) and AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
(TN-C-S), the customers must provide their own ground connections.
- Japan is governed by PSE law
The is the law that governs electrical appliance safety in Japan. PSE stands for Product Safety Electrical Appliance & Material. The formal name is . The most recent revision comes April 1, 2001. The PSE symbol can be found on appliances that are approved for use in Japan....
, and uses TT earthing in most installations.
- In Australia, the Multiple Earthed Neutral (MEN) earthing system is used and is described in Section 5 of AS 3000. For an LV customer, it is a TN-C system from the transformer in the street to the premises, (the neutral is earthed multiple times along this segment), and a TN-S system inside the installation, from the Main Switchboard downwards. Looked at as a whole, it is a TN-C-S system.
Application examples
- Most modern homes in Europe have a TN-C-S earthing system. The combined neutral and earth occurs between the nearest transformer substation and the service cut out (the fuse before the meter). After this, separate earth and neutral cores are used in all the internal wiring.
- Older urban and suburban homes in the UK
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
tend to have TN-S supplies, with the earth connection delivered through the lead sheath of the underground lead-and-paper cable.
- Older homes in Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
uses the IT system while newer homes use TN-C-S.
- Some older homes, especially those built before the invention of residual-current circuit breakers and wired home area networks, use an in-house TN-C arrangement. This is no longer recommended practice.
- Laboratory rooms, medical facilities, construction sites, repair workshops, mobile electrical installations, and other environments that are supplied via engine-generators where there is an increased risk of insulation faults, often use an IT earthing arrangement supplied from isolation transformer
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety...
s. To mitigate the two-fault issues with IT systems, the isolation transformers should supply only a small number of loads each and/or should be protected with an insulation monitoring deviceAn insulation monitoring device monitors the ungrounded system between an active phase conductor and earth. It is intended to give an alert or disconnect the power supply when the impedance between the two conductors drops below a set value, usually 50 kΩ.The main advantage of the device that it...
(generally used only by medical, railway or military IT systems, because of cost).
- In remote areas, where the cost of an additional PE conductor outweighs the cost of a local earth connection, TT networks are commonly used in some countries, especially in older properties or in rural areas, where safety might otherwise be threatened by the fracture of an overhead PE conductor by, say, a fallen tree branch. TT supplies to individual properties are also seen in mostly TN-C-S systems where an individual property is considered unsuitable for TN-C-S supply.
- In Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, and IsraelThe State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...
the TN-C-S system is in use; however, the wiring rules currently state that, in addition, each customer must provide a separate connection to earth via both a water pipe bond (if metallic water pipes enter the consumer's premises) and a dedicated earth electrode. In Australia, new installations must also bond the foundation concrete re-enforcing under wet areas to the earth conductor (AS3000), typically increasing the size of the earthing, and provides an equipotential plane in areas such as bathrooms. In older installations, it is not uncommon to find only the water pipe bond, and it is allowed to remain as such, but the additional earth electrode must be installed if any upgrade work is done. The protective earth and neutral conductors are combined until the consumer's neutral link (located on the customer's side of the electricity meter's neutral connection) - beyond this point, the protective earth and neutral conductors are separate.
Comparison of Earthing systems
|
TT |
IT |
TNS |
TNC |
TNCS |
MEN |
| Earth fault loop impedance |
High |
Highest |
Low |
Low |
Low |
Low |
| RCD preferred? |
Yes |
No |
Yes |
No |
No |
After N and PE separates |
| Need earth electrode at site? |
Yes |
Yes |
No |
No |
No |
No |
| PE conductor cost |
Low |
Low |
Highest |
Least |
High |
Least Cost. Earth conductor size is 6mm^2 |
| Risk of broken neutral |
No |
No |
No |
Highest |
High |
No |
| Safety |
Safe |
Less Safe |
Safest |
Least Safe |
Safe |
Intergrity of earth at customer installation needs periodic checking. |
| Electromagnetic interference |
Least |
Least |
Low |
High |
Low |
Low |
| Safety risks |
High loop impedance |
Double fault, overvoltage |
Broken PE |
Broken neutral |
Broken neutral |
Can result in electrocution if earth is broken polarity is reversed. |
| Advantages |
Safe and reliable |
Continuity of operation, cost |
Safest |
Cost |
Safety and cost |
Inexpensive. It is effective in keeping neutral pd at zero potention along the line. |
See also
- Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
- Ground and neutral
Since the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding of equipment and structures...
- Electrical wiring
Electrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring as used to provide power in buildings and structures, commonly referred to as building wiring. This article is intended to...
- Single-wire earth return
- Soil resistivity
Soil resistivity is a measure of how much the soil resists the flow of electricity. It is a critical factor in design of systems that rely on passing current through the Earth's surface. Knowledge of the soil resitivity and how it varies with depth in the soil is necessary to design the grounding...