Ground (electricity)
In electrical engineering, ground or earth may refer to reference ground – a reference point in an electrical circuit from which voltages are measured, earth ground – an electrically neutral node that has a lot of available charges, or common ground – a common return path for electric current,.
Ground 'wire, or earth wire, is a wire that connects an electrical equipment from its conductive but normally-unenergized parts to earth ground or common ground. To ground or to earth' an object is to electrically connect the object to earth ground or common ground.
Electrical circuits may be grounded for several reasons. Exposed conductive parts of electrical equipment are connected to neutral to protect users from electrical shock hazards. If internal insulation fails, dangerous voltages may appear on the exposed conductive parts. Connecting exposed conductive parts to a ground wire which provides a low-impedance path for current to flow back to the incoming neutral will allow circuit breakers to interrupt power supply in the event of a fault. In electric power distribution systems, a protective earth conductor is an essential part of the safety provided by the earthing system. There is usually a large conductor attached to one side of the power supply, which serves as the common ground, the common return path for current from many different components in the circuit.
Connection to earth ground limits the build-up of static electricity when handling flammable products or electrostatic-sensitive devices. In some telegraph and power transmission circuits, the physical ground itself can be used as one conductor of the circuit, saving the cost of installing a separate return conductor.
For measurement purposes, the ground of the Earth, having a reasonably constant potential, serves as a reference ground against which other potentials can be measured. An electrical ground system should have an appropriate current-carrying capability to serve as an adequate zero-voltage reference level. Circuits in portable electronic devices, such as cell phones and media players, as well as circuits in vehicles, have a reference ground.
In electronic circuit theory, a ground usually refers to earth ground, an idealized infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential. Where a physical ground connection has a significant resistance, the approximation of zero potential is no longer valid. Stray voltages or earth potential rise effects will occur, which may create noise in signals or produce an electric shock hazard if large enough.
History
Long-distance electromagnetic telegraph systems from 1820 onwardsused two or more wires to carry the signal and return currents. It was discovered by German scientist Carl August von Steinheil| in 1836–1837, that the ground could be used as the return path to complete the circuit, making the return wire unnecessary. Steinheil was not the first to do this, but he was not aware of earlier experimental work, and he was the first to do it on an in-service telegraph, thus making the principle known to telegraph engineers generally. 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.
In the late nineteenth century, when telephony began to replace telegraphy, it was found that the currents in the earth induced by power systems, electric 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.
Building wiring installations
Electrical power distribution systems are often connected to earth ground to limit the voltage that can appear on distribution circuits. A distribution system insulated from earth ground may attain a high potential due to transient voltages caused by static electricity or accidental contact with higher potential circuits. An earth ground connection of the system dissipates such potentials and limits the rise in voltage of the grounded system.In a mains electricity wiring installation, the term ground conductor typically refers to two different conductors or conductor systems as listed below:
Equipment bonding conductors or equipment ground conductors provide a low-impedance path between normally non-current-carrying metallic parts of equipment and one of the conductors of that electrical system's source. If any exposed metal part should become energized, such as by a frayed or damaged insulator, it creates a short circuit, causing the overcurrent device to open, clearing the fault. It is important to note this action occurs regardless of whether there is a connection to the physical ground; the physical ground itself has no role in this fault-clearing process since current must return to its source; however, the sources are very frequently connected to the physical ground.. By bonding all exposed non-current carrying metal objects together, as well as to other metallic objects such as pipes or structural steel, they should remain near the same voltage potential, thus reducing the chance of a shock. This is especially important in bathrooms where one may be in contact with several different metallic systems such as supply and drain pipes and appliance frames. When a conductive system is to be electrically connected to the physical ground, one puts the equipment bonding conductor and the grounding electrode conductor at the same potential.
A is used to connect the system grounded 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 US 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 is to limit the voltage imposed by lightning events and contact with higher voltage lines. 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 a specific grounding electrode is often mandated by regulating authorities. The same 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. The size of power grounding conductors is usually regulated by local or national wiring regulations.
Bonding
Strictly speaking, the terms grounding or earthing are meant to refer to an electrical connection to ground. 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 eliminate foreign voltages.Earthing systems
In electricity supply systems, an earthing system or 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. In a high-impedance grounded system, the fault current is limited to a few amperes ; a low-impedance grounded system will permit several hundred amperes to flow on a fault. A large solidly grounded distribution system may have tens of thousands of amperes of ground fault current.In a polyphase AC system, the instantaneous vector sum of the phases is zero. This neutral point is commonly used to refer the phase voltages to earth ground instead of connecting one of the phase conductors to earth. Any Δ-Y connected transformer may be used for the purpose. A nine winding transformer may be used to balance the phase currents of a delta connected source with an unbalanced load.
Low-resistance grounding systems use a neutral grounding resistor to limit the fault current to 25 A or greater. Low resistance grounding systems will have a time rating 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 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, 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 and transient overvoltages could occur.