Ground (electricity)
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Chassis ground |
Earth ground |
The term ground or earth usually means a common return path in electrical circuits. The terms Earth return and ground return are also common.
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[edit] Different types of ground
In electrical engineering, the term ground or earth has a few meanings depending on the specific application areas.
[edit] In radio frequency communications
An electrical connection to earth for as a reference potential for radio frequency antenna signals. 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 (Fig. 1), or a connection to buried metal water piping (though this carries the risk of the water pipe being later replaced with plastic). Because high frequency signals can flow to earth through capacitance, 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 zero voltage regardless of how much electrical current flows into ground or out of ground. The resistance 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 of the soil, improving the conductive mineral content of the soil, and increasing the land area covered by the ground system.
[edit] In AC power wiring installations
In a mains (AC power) wiring installation, the ground is a wire with an electrical connection to earth, that provides an alternative path to the ground for heavy currents that might otherwise flow through a victim of electric shock. This power ground grounding wire is (directly or indirectly) connected to one or more earth electrodes. These may be located locally, be far away in the suppliers network or in many cases both. This grounding wire is usually but not always connected to the neutral wire at some point and they may even share a cable for part of the system under some conditions. The ground wire is also usually bonded to pipework to keep it at the same potential as the electrical ground during a fault. Water supply pipes often used to be used as ground electrodes but this was banned in some countries when plastic pipe such as PVC became popular. This type of ground applies to radio antennas and to lightning protection systems.
[edit] In electronics
[edit] Circuit ground versus earth
In an electrical circuit operating at signal voltages (usually less than 50 V or so), a common return path that is the zero voltage reference level for the equipment or system. Voltage is a differential quantity, which appears between two points having some electrical potentials. In order to deal only with a voltage (an electrical potential) of a single point, the second point has to be connected to a reference point (ground) having usually zero voltage. This signal ground may or may not actually be connected to a power ground. A system where the system ground is not actually connected to earth is often referred to as a floating ground.
[edit] Separating low signal ground from a noisy ground
In television stations, recording studios, and other installations where sound quality is critical, a special signal ground known as a "technical ground" (or "technical earth") is often installed. This is basically the same thing as an AC power ground, but no appliance ground wires are allowed any connection to it, as they may carry electrical interference. In most cases, the studio's metal equipment racks are all joined together with heavy copper cables (or flattened copper tubing or busbars) and similar connections are made to the technical ground. Great care has to be taken that nobody places any AC-grounded appliances (heaters etc) on the racks, as a single AC ground connection to the technical ground will destroy its effectiveness. For particularly demanding applications, the main technical ground may consist of a heavy copper pipe, if necessary fitted by drilling through several concrete floors, so they can all be connected by the shortest possible path to a grounding rod in the basement. With the recent proliferation of digital studios with massive audio channel capacity (up to 512 channels), the installation of high-quality audio grounding has become mandatory, as any ground-introduced interference tends to be cumulative. Noise levels that were perfectly satisfactory for old 24-channel studios may become intolerable with ten or twenty times that number.
[edit] In electrical shielding
An electrical connection to the inside surface of a Faraday cage serves as an example of an electrical shielding. Any excess charges deposited on the inner surface of a Faraday cage will migrate to the outer surface of the cage, where they can produce no electric fields within the enclosure. For this reason, the inside surface of a Faraday cage behaves like an infinite sink for electrical charge from the perspective of objects within. Even if the Faraday cage itself is not connected to the Earth, the inner surface of the cage can be used in place of an earth connection.
[edit] In lightning protection
A ground conductor on a lightning protection system is used to dissipate the strike into the earth.
[edit] Uses
A power ground serves to provide a return path for fault currents and therefore allows the fuse or breaker to disconnect the circuit. The power ground is also often bonded to the house's incoming pipework, and pipes and cables entering the bathroom are sometimes cross-bonded. This is done to try to reduce the voltage between objects that can be touched simultaneously. Filters also connect to the power ground, but this is mainly to stop the power ground carrying noise into the systems which the filters protect, rather than as a direct use of the power ground. See: NEC (Article 250).
In Single Wire Earth Return (SWER) 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.
Signal grounds serve as return paths for signals and power at 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 together, usually through the metal case of the equipment.
Lightning protection is a very specialised form of grounding used in an attempt to divert the huge currents from lightning strikes.
Grounding is primarily used for safety to prevent electric shock or fires caused by a voltage potential between the earth and a conductor such as an appliance cabinet or chassis. Grounding is often used to conduct lightning strikes harmlessly to earth rather than starting fires and damaging equipment. It is also used to control electrical noise in computer, audio and video, and communications circuits. This illustrates that an electrical ground should have an appropriate current-carrying capability in order to serve as an adequate zero-voltage reference level.
[edit] History
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 [1], 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 Saint 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 system was reintroduced.
[edit] See also
- Domestic AC power plugs and sockets
- Earthing systems
- Ground constants
- Ground loop
- Phantom circuit
- Phantom loop
- Virtual ground
- Ground mat
[edit] Source
- Federal Standard 1037C in support of MIL-STD-188
[edit] External links
- The Electromagnetic Telegraph, by J. B. Calvert
- Grounding for Low- and High- Frequency Circuits (PDF) — Analog Devices Application Note
- An IC Amplifier User’s Guide to Decoupling, Grounding, and Making Things Go Right for a Change (PDF) — Analog Devices Application Note
