Ground (electricity)

From Academic Kids

The term ground (or earth) usually means a common return in circuits.

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Meanings

In electrical engineering, the term ground or earth has the following meanings:

  1. An electrical connection to earth. 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 (usually copper), or a connection to buried metal water piping. Or it can be a complex system of buried rods and wires. The resistance of the electrode-to-earth connection determines its quality, and 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. This type of ground applies to radio antennas and to lightning protection systems.
  2. In a mains (AC power) wiring installation, the ground is the wire that carries currents away under fault conditions. This power ground ground 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 ground 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 pipes often used to be used as ground electrodes but this was banned in some countries when plastic pipe became popular.
  3. In an electrical circuit operating at signal voltages (usually less than 50V or so), a common return path that is the zero voltage reference level for the equipment or system. 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.
  4. An electrical connection to the inside surface of a Faraday Cage. 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 e-fields within the enclosure. For this reason, the inside surface of a Faraday Cage behaves like an infinite sink for electrical charge. 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.
  5. A ground on a lightning protection system used to dissipate the strike into the earth.

Uses

A power ground serves to provide a return path for fault currents and therefore allow 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 the filters protect, rather than a direct use of the power ground.

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 50V) 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 dissipate the huge currents from lightning strikes.

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] (http://www.du.edu/~jcalvert/tel/morse/morse.htm), 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 which required pouring water 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.

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