- Electric arc
An electric arc is an
electrical breakdownof a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. The phenomenon was first described by Vasily V. Petrov, a Russian scientist who discovered it in 1802. An archaic term is voltaic arc as used in the phrase "voltaic arc lamp".
The various shapes of electric arc are emergent properties of nonlinear patterns of current and
electric field. The arc occurs in the gas-filled space between two conductive electrodes(often made of carbon) and it results in a very high temperature, capable of meltingor vaporizing most materials. An electric arc is a continuous discharge, while a similar electric sparkdischarge is momentary. An electric arc may occur either in direct-current circuits or in alternating current circuits. In the latter case, the arc may re-strike on each half cycle of the current. An electric arc differs from a glow dischargein that the current density is quite high, and the voltage drop within the arc is low; at the cathodethe current density may be as high as one million amps per square centimeter. [ A. H. Howatson, "An Introduction to Gas Discharges", Pergamon Press, Oxford pgs. 80-95]
An electric arc has a non-linear relationship between current and voltage. Once the arc is established (either by progression from a glow discharge [Princples of Electronics By V.K. Mehta ISBN 8121924502 pages 101-107] or by momentarily touching the electrodes then separating them), increased current results in a lower voltage between the arc terminals. This negative
impedanceeffect requires that some positive form of impedance to be placed in the circuit, if it is desired to maintain a stable arc. This property is the reason uncontrolled electrical arcs in apparatus become so destructive, since once initiated an arc will draw more and more current from a fixed-voltage supply until the apparatus is destroyed.
Industrially, electric arcs are used for
welding, plasma cutting, for electrical discharge machining, as an arc lampin movie theater projectors, and Followspots in stage lighting. Electric arc furnaces are used to produce steeland other substances. Calcium carbideis made in this way as it requires a large amount of energy to promote an endothermicreaction (at temperatures of 2500 °C).
Electric arcs have been studied for
electric propulsionof spacecraft.
Undesired or unintended electric arcing can have detrimental effects on
electric power transmissionand distribution systems and electronic equipment. Devices which may cause arcing include switches, circuit breakers, relay contacts, fuses and poor cable terminations. When an inductive circuit is switched off the current cannot instantaneously jump to zero; a transient arc will be formed across the separating contacts. Switching devices susceptible to arcing are normally designed to contain and extinguish an arc, and snubbercircuits can supply a path for transient currents, preventing arcing. If a circuit has enough current and voltage to sustain an arc formed outside of a switching device, the arc can cause damage to equipment such as melting of conductors, destruction of insulation, and fire. An arc flashdescribes an explosive electrical event that presents a hazard to people and equipment.
Undesired arcing in electrical
contactors can be suppressed by various devices, including:
* immersion in oil,
inert gasor vacuum
* [http://www.arcadvisor.com/index.html Arc Analysis]
* [http://www.rmcybernetics.com/research/resonance/plasma.htm Unusual Arcing Photos]
* [http://arcmelt.velp.info/ Some more info about making electric arcs] using a
* [http://188.8.131.52/frames/longarc.htm Videos of 230,000 volt 3-phase "Jacobs Ladder" and unintentional 500,000 volt power arc]
* [http://www.cirris.com/testing/voltage/arc.html High Voltage Arc Gap Calculator] to calculate the length of an arc knowing the voltage or vice versa
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