Earthing system
An earthing system or grounding system connects specific parts of an electric power system, such as the conductive surfaces of equipment, with the ground 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 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.
Purpose
Protection against Electric Shock
System earthing serves as a key component of one of the most commonly used forms of protection against electric shock. International standard IEC 61140 Protection against electric shock sets out requirements for shock protection under both normal conditions, and fault conditions. Basic protection involves concepts such as insulation. Fault protection concerns the concept of accessible conductive parts within an installation which were never intended to be electrically energised, unintentionally becoming energised and hazardous.One of the methods described by IEC 61140 for addressing fault protection, and one of the most commonly used, is automatic disconnection of supply . This involves the use of an earthing system and protective devices which will automatically break circuits when a fault is detected, removing the hazard. Under this method any accessible conductive parts of an installation which could develop a fault and become hazardous are connected to an earthing system such that in the event of such a fault developing, a circuit is completed, allowing current to flow, and that current flow can cause a protective device to be triggered.
As shown later, only in some earthing system arrangements does the Earth form part of this fault path. In all cases one or more literal connections to Earth are utilised, which acts as a reference point, restricting the degree to which voltage of any part of the earthing system can stray from that of the surrounding ground.
Static build-up prevention
A connection to Earth prevents the build-up of static electricity, as induced by friction for example, such as when wind blows onto a radio mast.Surge protection
Electrical surges can occur due to switching events or lightning strikes and can potentially cause damage to equipment, property, or even pose a threat to life. While in some cases surge protection devices will simply divert surges between electrical conductors, surges will sometimes be diverted to Earth.Functional earthing
Functional earthing serves a purpose other than electrical safety. Example purposes include electromagnetic interference filtering in an EMI filter, and the use of the Earth as a return path in a single-wire earth return distribution system.Other earthing systems
This article only concerns grounding for electrical power. Examples of other earthing systems are:- As part of a single-wire earth return power and signal lines, such as were used for low wattage power delivery and for telegraph lines.
- In radio, as a ground plane for large monopole antenna.
- As ancillary voltage balance for other kinds of radio antennas, such as dipoles.
- As the feed-point of a ground dipole antenna for VLF and ELF radio.
Low-voltage systems
Whilst historically there has been considerable national variation, most developed countries introduced 220 V, 230 V, or 240 V AC sockets with earthed contacts either just before or soon after World War II. 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 pin. In the developing world, local wiring practices may or may not provide a connection to an earth conductor.
On low-voltage electricity networks with a phase-to-neutral voltage, which are mostly used in industry, mining equipment, and machines rather than publicly accessible networks, the earthing system design is equally important from a safety point of view as for domestic users.
The US National Electrical Code permitted the use of the supply neutral wire as the equipment enclosure connection to ground from 1947 to 1996 for ranges and from 1953 to 1996 for clothes dryers, whether plug-in or permanently fixed, provided that the circuit originated in the main service panel. Normal imbalances in the circuit would create small equipment voltages with respect to Earth; a failure of the neutral conductor or connections would allow the equipment to go to full 120 volts, an easily lethal situation. The 1996 and newer editions of the NEC no longer permit this practice. For similar reasons, most countries now mandate dedicated protective earth connections in consumer wiring, a practice that has become nearly universal. In distribution networks however, where connections are fewer and less vulnerable, many countries do permit earth and neutral functions to share a conductor.
If the fault path between exposed conductive parts and the supply has sufficiently low impedance, then should such a part accidentally become energized, the fault current will cause the circuit overcurrent protection device to open, clearing the fault. However, if the impedance of the fault path is too high, then fault currents may not trip the overcurrent protection device quickly enough to meet the requirements of local electrical regulations. This is often the case with a TT-type earthing system. In such cases use of a residual-current device may allow the required disconnection times to be met.
IEC terminology
distinguishes three families of earthing arrangements, using the two-letter codes — TN, TT, and IT.The first letter indicates the relationship between the power-supply and Earth:
The second letter indicates the relationship between the exposed-conductive-parts of the installation, and Earth:
Any subsequent letter indicate:
The letters can reasonably be assumed to derive from French wording, with T deriving from terre ; N from neutre ; S from séparé ; C from combiné ; and I from both isolé and impédance.
Types of TN system
In a TN earthing system, one of the points in the supply transformer is directly connected with Earth, usually the neutral-star-point in a star-connected supply transformer, the same point from which a neutral connection would be provided. Exposed-conductive-parts within a consumer installation are connected with Earth via this connection at the transformer, and thus via the supply cable. The conductor that connects an exposed-conductive-part of the consumer's electrical installation to Earth is called the protective earth conductor.This arrangement is a current standard for residential and industrial electric systems particularly in Europe.
Three variants of TN systems are distinguished:
; TN−S: PE and N are entirely separate conductors. If a neutral conductor is provided, and if the point from which the transformer connects to Earth is the neutral-star-point, then PE and N conductors will be connected at this one and only point within the system. Note that the armoring of the supply cable is commonly used as the PE conductor between the transformer and installation rather than a dedicated conductor within the supply cable.
; TN−C: A single combined PEN conductor fulfils the functions of both a PE and an N conductor. This is not only so for the supply cable but within the consumer installation also. In 230/400 V consumer systems this is normally only used with distribution circuits.
; : Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N conductors. The combined PEN conductor typically spans between the transformer and the consumer installation, with separate earth and neutral conductors used within the installation.
In the UK, a common practice with TN-C-S is to connect the combined PEN supply conductor to Earth at multiple points along its length between the source transformer and the consumer installation. This is known as protective multiple earthing . This is so common that consequently PME is often incorrectly used as a synonym. Similar systems in Australia and New Zealand are designated as multiple earthed neutral and, in North America, as multi-grounded neutral .
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. 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 convert the installation to TT.
The main attraction of a TN system is that the low impedance earth path means that overcurrent protection devices can usually cut off the supply suitably quickly in the event of a earth fault. This is not typically the case for TT systems. The invention of residual current devices provided another means of protection from earth faults, which can be critical for a TT system as an RCD is often the only means of achieving suitable quick disconnection times, but is simply used as a secondary layer of protection in a TN system.
A danger of TN-C-S systems, especially for installations in rural locations where supplies are more likely to be provided with overhead cables exposed to the elements, or certain kinds of installations such as supplies to caravans or boats, is the risk of an open or broken PEN fault whereby the supply PEN conductor is severed or significantly corroded. In such a scenario current will take any alternate path available, and since extraneous-conductive-parts like water and gas pipes should be bonded to an installation's earthing, and the earthing is tied to the neutral, neutral current can still flow via the Earth, potentially passing through neighbouring properties, and voltage-to-Earth can rise significantly, especially should the break occur upstream of properties on different supply phases, in which case the floating neutral could cause voltage to rise as high as three-phase line-to-line voltage. Hypothetically if no complete path existed for current to flow, then exposed-conductive-parts would rise to line voltage. PME helps mitigate risk somewhat. The danger is serious enough that the UK Electricity Safety, Quality and Continuity Regulations 2002 forbids use of PEN conductors to supply caravans and boats where simultaneous contact with Earth is especially high.
TN-C systems are not permitted in some countries. The UK for instance forbids it in the Electricity Safety, Quality and Continuity Regulations 2002. Note that an RCD cannot work on a TN-C system.