In electrical engineering, ground or earth is the reference point in
an electrical circuit from which voltages are measured, a common
return path for electric current, or a direct physical connection to
Electrical circuits may be connected to ground (earth) for several
reasons. In mains powered equipment, exposed metal parts are connected
to ground so that if, due to any fault conditions, a "Line" supply
voltage connection occurs to any such conductive parts, the current
flow will then be such that any protective equipment installed for
either overload or "leakage" protection will operate and disconnect
the "Line" voltage. This is done to prevent harm resulting to the user
from coming in contact with any such dangerous voltage in a situation
where the user may, at the same time, also come in contact with an
object at ground/earth potential. In electrical power distribution
systems, a Protective
1 History 2 Radio communications 3 Building wiring installations
3.1 Earthing systems 3.2 Impedance grounding 3.3 Ungrounded systems
4 Power transmission 5 Electronics
5.1 Circuit ground versus earth 5.2 Functional grounds 5.3 Separating low signal ground from a noisy ground
6 Lightning protection systems 7 Bonding 8 Ground (earth) mat 9 Isolation 10 See also 11 Notes 12 References 13 External links
Long-distance electromagnetic telegraph systems from 1820 onwards
used two or more wires to carry the signal and return currents. It was
then discovered, probably by the German scientist Carl August
Steinheil in 1836–1837, that the ground could be used as the
return path to complete the circuit, making the return wire
unnecessary. However, there were problems with this system,
exemplified by the transcontinental telegraph line constructed in 1861
by the Western Union Company between St. Joseph, Missouri, and
Sacramento, California. During dry weather, the ground connection
often developed a high resistance, requiring water to be poured on the
ground rod to enable the telegraph to work or phones to ring.
Later, when telephony began to replace telegraphy, it was found that
the currents in the earth induced by power systems, electrical
railways, other telephone and telegraph circuits, and natural sources
including lightning caused unacceptable interference to the audio
signals, and the two-wire or 'metallic circuit' system was
reintroduced around 1883.
An electrical connection to earth can be used as a reference potential
for radio frequency signals for certain kinds of antennas. The part
directly in contact with the earth - the "earth electrode" - can be as
simple as a metal rod or stake driven into the earth, or a connection
to buried metal water piping (the pipe must be conductive). Because
high frequency signals can flow to earth due to capacitative effects,
capacitance to ground is an important factor in effectiveness of
signal grounds. Because of this, a complex system of buried rods and
wires can be effective. An ideal signal ground maintains a fixed
potential (zero) regardless of how much electric current flows into
ground or out of ground. Low impedance at the signal frequency of the
electrode-to-earth connection determines its quality, and that quality
is improved by increasing the surface area of the electrode in contact
with the earth, increasing the depth to which it is driven, using
several connected ground rods, increasing the moisture content of the
soil, improving the conductive mineral content of the soil, and
increasing the land area covered by the ground system.
Some types of transmitting antenna systems in the VLF, LF, MF and
lower SW range must have a good ground to operate efficiently. For
example, a vertical monopole antenna requires a ground plane that
often consists of an interconnected network of wires running radially
away from the base of the antenna for a distance about equal to the
height of the antenna. Sometimes a counterpoise is used as a ground
plane, supported above the ground.
Building wiring installations
See also: Earthing system
Electrical power distribution systems are often connected to ground to
limit the voltage that can appear on distribution circuits. A
distribution system insulated from ground may attain a high potential
due to transient voltages caused by arcing, static electricity, or
accidental contact with higher potential circuits. A ground connection
of the system dissipates such potentials and limits the rise in
voltage of the grounded system.
In a mains electricity (AC power) wiring installation, the term ground
conductor typically refers to three different conductors or conductor
systems as listed below.
Equipment earthing conductors provide an electrical connection between
the physical ground (earth) and the grounding/bonding system, which
connects (bonds) the normally non-current-carrying metallic parts of
equipment. According to the U.S.
National Electrical Code
Metal water pipe used as grounding electrode
A grounding electrode conductor (GEC) is used to connect the system grounded ("neutral") conductor, or the equipment to a grounding electrode, or a point on the grounding electrode system. This is called "system grounding" and most electrical systems are required to be grounded. The U.S. NEC and the UK's BS 7671 list systems that are required to be grounded.  According to the NEC, the purpose of connecting an electrical system to the physical ground (earth) is to limit the voltage imposed by lightning events and contact with higher voltage lines, and also for voltage stabilization. In the past, water supply pipes were used as grounding electrodes, but due to the increased use of plastic pipes, which are poor conductors, the use of an actual grounding electrode is required. This type of ground applies to radio antennas and to lightning protection systems. Permanently installed electrical equipment, unless not required to, has permanently connected grounding conductors. Portable electrical devices with metal cases may have them connected to earth ground by a pin on the attachment plug (see Domestic AC power plugs and sockets). The size of power grounding conductors is usually regulated by local or national wiring regulations. Earthing systems Main article: Earthing system In electricity supply systems, an earthing (grounding) system defines the electrical potential of the conductors relative to that of the Earth's conductive surface. The choice of earthing system has implications for the safety and electromagnetic compatibility of the power supply. Regulations for earthing systems vary considerably between different countries. A functional earth connection serves more than protecting against electrical shock, as such a connection may carry current during the normal operation of a device. Such devices include surge suppression, electromagnetic-compatibility filters, some types of antennas, and various measurement instruments. Generally the protective earth system is also used as a functional earth, though this requires care. Impedance grounding Distribution power systems may be solidly grounded, with one circuit conductor directly connected to an earth grounding electrode system. Alternatively, some amount of electrical impedance may be connected between the distribution system and ground, to limit the current that can flow to earth. The impedance may be a resistor, or an inductor (coil). In a high-impedance grounded system, the fault current is limited to a few amperes (exact values depend on the voltage class of the system); a low-impedance grounded system will permit several hundred amperes to flow on a fault. A large solidly grounded distribution system may have thousands of amperes of ground fault current. In a polyphase AC system, an artificial neutral grounding system may be used. Although no phase conductor is directly connected to ground, a specially constructed transformer (a "zig zag" transformer) blocks the power frequency current from flowing to earth, but allows any leakage or transient current to flow to ground. Low-resistance grounding systems use a neutral grounding resistor (NGR) to limit the fault current to 25 A or greater. Low resistance grounding systems will have a time rating (say, 10 seconds) that indicates how long the resistor can carry the fault current before overheating. A ground fault protection relay must trip the breaker to protect the circuit before overheating of the resistor occurs. High-resistance grounding (HRG) systems use an NGR to limit the fault current to 25 A or less. They have a continuous rating, and are designed to operate with a single-ground fault. This means that the system will not immediately trip on the first ground fault. If a second ground fault occurs, a ground fault protection relay must trip the breaker to protect the circuit. On an HRG system, a sensing resistor is used to continuously monitor system continuity. If an open-circuit is detected (e.g., due to a broken weld on the NGR), the monitoring device will sense voltage through the sensing resistor and trip the breaker. Without a sensing resistor, the system could continue to operate without ground protection (since an open circuit condition would mask the ground fault) and transient overvoltages could occur. Ungrounded systems Where the danger of electric shock is high, special ungrounded power systems may be used to minimize possible leakage current to ground. Examples of such installations include patient care areas in hospitals, where medical equipment is directly connected to a patient and must not permit any power-line current to pass into the patient's body. Medical systems include monitoring devices to warn of any increase of leakage current. On wet construction sites or in shipyards, isolation transformers may be provided so that a fault in a power tool or its cable does not expose users to shock hazard. Circuits used to feed sensitive audio/video production equipment or measurement instruments may be fed from an isolated ungrounded technical power system to limit the injection of noise from the power system. Power transmission In single-wire earth return (SWER) AC electrical distribution systems, costs are saved by using just a single high voltage conductor for the power grid, while routing the AC return current through the earth. This system is mostly used in rural areas where large earth currents will not otherwise cause hazards. Some high-voltage direct-current (HVDC) power transmission systems use the ground as second conductor. This is especially common in schemes with submarine cables, as sea water is a good conductor. Buried grounding electrodes are used to make the connection to the earth. The site of these electrodes must be chosen carefully to prevent electrochemical corrosion on underground structures. A particular concern in design of electrical substations is earth potential rise. When very large fault currents are injected into the earth, the area around the point of injection may rise to a high potential with respect to distant points. This is due to the limited finite conductivity of the layers of soil in the earth. The gradient of the voltage (changing voltage within a distance) may be so high that two points on the ground may be at significantly different potentials, creating a hazard to anyone standing on the ground in the area. Pipes, rails, or communication wires entering a substation may see different ground potentials inside and outside the substation, creating a dangerous touch voltage. Electronics
Signal grounds serve as return paths for signals and power (at extra
low voltages, less than about 50 V) within equipment, and on the
signal interconnections between equipment. Many electronic designs
feature a single return that acts as a reference for all signals.
Power and signal grounds often get connected, usually through the
metal case of the equipment. Designers of printed circuit boards must
take care in the layout of electronic systems so that high-power or
rapidly switching currents in one part of a system do not inject noise
into low-level sensitive parts of a system due to some common
impedance in the grounding traces of the layout.
Circuit ground versus earth
Busbars are used for ground conductors in high-current circuits.
Lightning protection systems are designed to mitigate the effects of lightning through connection to extensive grounding systems that provide a large surface area connection to earth. The large area is required to dissipate the high current of a lightning strike without damaging the system conductors by excess heat. Since lightning strikes are pulses of energy with very high frequency components, grounding systems for lightning protection tend to use short straight runs of conductors to reduce the self-inductance and skin effect. Bonding Main article: Electrical bonding Strictly speaking, the terms grounding or earthing are meant to refer to an electrical connection to ground/earth. Bonding is the practice of intentionally electrically connecting metallic items not designed to carry electricity. This brings all the bonded items to the same electrical potential as a protection from electrical shock. The bonded items can then be connected to ground to bring them to earth potential. Ground (earth) mat Main article: Ground mat In an electrical substation a ground (earth) mat is a mesh of conductive material installed at places where a person would stand to operate a switch or other apparatus; it is bonded to the local supporting metal structure and to the handle of the switchgear, so that the operator will not be exposed to a high differential voltage due to a fault in the substation. In the vicinity of electrostatic sensitive devices, a ground (earth) mat or grounding (earthing) mat is used to ground static electricity generated by people and moving equipment. There are two types used in static control: Static Dissipative Mats, and Conductive Mats. A static dissipative mat that rests on a conductive surface (commonly the case in military facilities) are typically made of 3 layers (3-ply) with static dissipative vinyl layers surrounding a conductive substrate which is electrically attached to ground (earth). For commercial uses, static dissipative rubber mats are traditionally used that are made of 2 layers (2-ply) with a tough solder resistant top static dissipative layer that makes them last longer than the vinyl mats, and a conductive rubber bottom. Conductive mats are made of carbon and used only on floors for the purpose of drawing static electricity to ground as quickly as possible. Normally conductive mats are made with cushioning for standing and are referred to as "anti-fatigue" mats.
3 ply static dissipative vinyl grounding mat shown at macro scale
For a static dissipative mat to be reliably grounded it must be attached to a path to ground. Normally, both the mat and the wrist strap are connected to ground by using a common point ground system (CPGS). In computer repair shops and electronics manufacturing workers must be grounded before working on devices sensitive to voltages capable of being generated by humans. For that reason static dissipative mats can be and are also used on production assembly floors as "floor runner" along the assembly line to draw static generated by people walking up and down. Isolation See also: Galvanic isolation Isolation is a mechanism that defeats grounding. It is frequently used with low-power consumer devices, and when electronics engineers, hobbyists, or repairmen are working on circuits that would normally be operated using the power line voltage. Isolation can be accomplished by simply placing a "1:1 wire ratio" transformer with an equal number of turns between the device and the regular power service, but applies to any type of transformer using two or more coils electrically insulated from each other. For an isolated device, touching a single powered conductor does not cause a severe shock, because there is no path back to the other conductor through the ground. However, shocks and electrocution may still occur if both poles of the transformer are contacted by bare skin. Previously it was suggested that repairmen "work with one hand behind their back" to avoid touching two parts of the device under test at the same time, thereby preventing a circuit from crossing through the chest and interrupting cardiac rhythms/ causing cardiac arrest. Generally every AC power line transformer acts as an isolation transformer, and every step up or down has the potential to form an isolated circuit. However, this isolation would prevent failed devices from blowing fuses when shorted to their ground conductor. The isolation that could be created by each transformer is defeated by always having one leg of the transformers grounded, on both sides of the input and output transformer coils. Power lines also typically ground one specific wire at every pole, to ensure current equalization from pole to pole if a short to ground is occurring. In the past, grounded appliances have been designed with internal isolation to a degree that allowed the simple disconnection of ground by cheater plugs without apparent problem (a dangerous practice, since the safety of the resulting floating equipment relies on the insulation in its power transformer). Modern appliances however often include power entry modules which are designed with deliberate capacitive coupling between the AC power lines and chassis, to suppress electromagnetic interference. This results in a significant leakage current from the power lines to ground. If the ground is disconnected by a cheater plug or by accident, the resulting leakage current can cause mild shocks, even without any fault in the equipment. Even small leakage currents are a significant concern in medical settings, as the accidental disconnection of ground can introduce these currents into sensitive parts of the human body. As a result, medical power supplies are designed to have low capacitance. Class II appliances and power supplies (such as cell phone chargers) do not provide any ground connection, and are designed to isolate the output from input. Safety is ensured by double-insulation, so that two failures of insulation are required to cause a shock. See also
Appliance classes Ground constants Ring ground Ground loop (electricity) Ground wire (transmission line) Isolated ground Phantom circuit Floating ground Soil resistivity Ufer Ground Virtual ground
^ An 'electrochemical telegraph' created by physician, anatomist and
Samuel Thomas von Sömmering
Federal Standard 1037C in support of MIL-STD-188
Wikimedia Commons has media related to Earthing.
Circuit Grounds and Grounding Practices
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Devices Application Note
An IC Amplifier User’s Guide to Decoupling, Grounding, and Making
Things Go Right for a Change (PDF) —
GND: 4140443-9 N