Ground (electricity)
The term ground (or earth) usually means a common return in circuits.
Meanings
In electrical engineering, the term ground or earth has the following meanings:
- An electrical connection to earth via an earth-electrode which can be as simple as a metallic ground rod or stake driven into the earth, a connection to buried metal water piping, or an extensive 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.
- In an electrical circuit, a common return path that usually (a) is connected to an earth-electrode subsystem and (b) is extended throughout a facility via a facility ground system consisting of the signal reference subsystem, the fault protection subsystem, and the lightning protection subsystem.
- In an electrical circuit, a common return path that (a) may not necessarily be connected to earth and (b) is the zero voltage reference level for the equipment or system. This is often referred to as a floating ground.
Uses
Grounding is primarily used for safety to prevent electric shock or fires started from a voltage potential existing 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.
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 Karl August Steinheil (1801-1870) in 1836-1837 , that the ground could 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.
See also
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