Electric arc

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An electric arc between two nails



Lightning is a natural electric arc.

The phenomenon was first described by Sir Humphry Davy, in an 1801 paper published in William Nicholson's Journal of Natural Philosophy, Chemistry and the Arts.[1] In the same year Davy publicly demonstrated the effect, before the Royal Society, by transmitting an electric current through two touching carbon rods and then pulling them a short distance apart. The demonstration produced a "feeble" arc, not readily distinguished from a sustained spark, between charcoal points. The Society subscribed for a more powerful battery of 1000 plates and in 1808 he demonstrated the large-scale arc.[2] He is credited with naming the arc.[3] He called it an arc because it assumes the shape of an upward bow when the distance between the electrodes is not small.[4] This is due to the buoyant force on the hot gas. Independently the phenomenon was subsequently rediscovered and described as a "special fluid with electrical properties", by Vasily V. Petrov, a Russian scientist experimenting with a copper-zinc battery consisting of 4200 discs.[5]



An electric arc is the form of electric discharge with the highest current density. The maximum current through an arc is limited only by the external circuit, not by the arc itself. The voltage across an arc decreases as the current increases, giving it a dynamic negative resistance characteristic. Where a sustained arc is required, this characteristic requires some external circuit element to stabilize current, which would otherwise increase bounded only by the supply limit.

An arc between two electrodes can be initiated by ionization and glow discharge, as the current through the electrodes is increased. The breakdown voltage of the electrode gap is a function of the pressure and type of gas surrounding the electrodes. When an arc starts, its terminal voltage is much less than a glow discharge, and current is higher. An arc in gases near atmospheric pressure is characterized by visible light emission, high current density, and high temperature. An arc is distinguished from a glow discharge partly by the approximately equal effective temperatures of both electrons and positive ions; in a glow discharge, ions have much less thermal energy than the electrons.

A drawn arc can be initiated by two electrodes initially in contact and drawn apart; this can initiate an arc without the high-voltage glow discharge. This is the way a welder starts to weld a joint, momentarily touching the welding electrode against the workpiece then withdrawing it till a stable arc is formed. Another example is separation of electrical contacts in switches, relays and circuit breakers; in high-energy circuits arc suppression may be required to prevent damage to contacts.[6]



A low-frequency (less than 100 Hz) alternating current arc resembles a direct current arc; on each cycle, the arc is initiated by breakdown, and the electrodes interchange roles as anode and cathode as current reverses. As the frequency of the current increases, there is not enough time for all ionization to disperse on each half cycle and the breakdown is no longer needed to sustain the arc; the voltage vs. current characteristic becomes more nearly ohmic.[6]

Electric arc between strands of wire.


An electric arc has a non-linear relationship between current and voltage. Once the arc is established (either by progression from a glow discharge [7] or by momentarily touching the electrodes then separating them), increased current results in a lower voltage between the arc terminals. This negative resistance effect requires that some positive form of impedance—an electrical ballast—be placed in the circuit 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.


An electric arc can melt calcium oxide


Spark plugs are used in petrol internal combustion engines of vehicles to initiate the combustion of the fuel in a timed fashion.




Electric arcs have been studied for electric propulsion of spacecraft.

Undesired arcing[edit]

Arc damage to a CEE 7/7 plug. The discharge path was formed by a conductive liquid seeping into the plug-socket assembly.


Undesired arcing in electrical contacts of contactors, relays and switches can be reduced by devices such as contact arc suppressors[8] and RC Snubbers or through techniques including:

Arcing can also occur when a low resistance channel (foreign object, conductive dust, moisture...) forms between places with different potential. The conductive channel then can facilitate formation of an electric arc. The ionized air has high electrical conductivity approaching that of metals, and can conduct extremely high currents, causing a short circuit and tripping protective devices (fuses, circuit breakers). Similar situation may occur when a lightbulb burns out and the fragments of the filament pull an electric arc between the leads inside the bulb, leading to overcurrent that trips the breakers.

Electric arc over the surface of plastics causes their degradation. A conductive carbon-rich track tends to form in the arc path, negatively influencing their insulation properties. The arc susceptibility is tested according to ASTM D495, by point electrodes and continuous and intermittent arcs; it is measured in seconds to form a track that is conductive under high-voltage low-current conditions. Some materials are less susceptible to degradation than others; e.g. polytetrafluoroethylene has arc resistance of about 200 seconds. From thermosetting plastics, alkyds and melamine resins are better than phenolic resins. Polyethylenes have arc resistance of about 150 seconds, polystyrenes and polyvinyl chlorides have relatively low resistance of about 70 seconds. Plastics can be formulated to emit gases with arc-extinguishing properties; these are known as arc-extinguishing plastics.


Arc suppression[edit]

Main article: Arc suppression

Arc suppression is a method of attempting to reduce or eliminate the electrical arc. There are several possible areas of use of arc suppression methods, among them metal film deposition and sputtering, arc flash protection, electrostatic processes where electrical arcs are not desired (such as powder painting, air purification, PVDF film poling) and contact current arc suppression. In industrial, military and consumer electronic design, the latter method generally applies to devices such as electromechanical power switches, relays and contactors. In this context, arc suppression refers to the concept of contact protection.

Part of the energy of an electrical arc forms new chemical compounds from the air surrounding the arc; these include oxides of nitrogen, and ozone, which can be detected by its distinctive sharp smell. These chemicals can be produced by high-power contacts in relays and motor commutators, and are corrosive to nearby metal surfaces. Arcing also erodes the surfaces of the contacts, wearing them down and creating high contact resistance when closed.[9]

See also[edit]


  1. ^ The Electric Arc, By Hertha Ayrton, page 94
  5. ^ Kartsev, V. P. (1983). Shea, William R, ed. Nature Mathematized. Boston, MA: Kluwer Academic. p. 279. ISBN 90-277-1402-9. 
  7. ^ Mehta, V.K. (2005). Principles of Electronics: for Diploma, AMIE, Degree & Other Engineering Examinations (9th ed., multicolour illustrative ed.). New Delhi: S. Chand. pp. 101–107. ISBN 81-219-2450-2. 
  8. ^ "Arc Suppression". Retrieved December 6, 2013. 

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