Electric arc
An electric arc (or arc discharge) is an
Techniques for arc suppression can be used to reduce the duration or likelihood of arc formation.
In the late 19th century, electric arc lighting was in wide use for public lighting. Some low-pressure electric arcs are used in many applications. For example, fluorescent tubes, mercury, sodium, and metal-halide lamps are used for lighting; xenon arc lamps have been used for movie projectors. Electric arcs can be utilized for manufacturing processes, such as electric arc welding, and electric arc furnaces for steel recycling.
History
Sir
The first continuous arc was discovered independently in 1802 and described in 1803
In the late nineteenth century,
Overview
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.
An arc between two electrodes can be initiated by ionization and glow discharge, when the current through the electrodes is increased. The breakdown voltage of the electrode gap is a combined function of the pressure, distance between electrodes 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 similar temperatures of the electrons and the positive ions; in a glow discharge, the ions are much colder 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 until a stable arc is formed. Another example is separation of electrical contacts in switches, relays or circuit breakers; in high-energy circuits arc suppression may be required to prevent damage to contacts.[11]
Electrical resistance along the continuous electric arc creates heat, which ionizes more gas molecules (where the degree of ionization is determined by temperature), and as per this sequence: solid-liquid-gas-plasma; the gas is gradually turned into a thermal plasma. A thermal plasma is in thermal equilibrium; the temperature is relatively homogeneous throughout the atoms, molecules, ions, and electrons. The energy given to electrons is dispersed rapidly to the heavier particles by elastic collisions, due to their great mobility and large numbers.
Current in the arc is sustained by
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 or cathode, when 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.[11]
The various shapes of electric arcs are
An electric arc has a non-linear relationship between current and voltage. Once the arc is established (either by progression from a glow discharge[12] 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 (as 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.
Uses
Industrially, electric arcs are used for
Formation of an intense electric arc, similar to a small-scale arc flash, is the foundation of exploding-bridgewire detonators.
Electric arcs have been studied for
They are used in the laboratory for spectroscopy to create spectral emissions by intense heating of a sample of matter.
Protection of electrical equipment
Arc is still being used in high voltage switchgear for protection of
An undesirable arc can also occur when a high-voltage switch is opened and is extinguished in similar ways. Modern devices use
Visual entertainment
A Jacob's ladder (more formally, a high voltage traveling arc) is a device for producing a continuous train of electric arcs that rise upwards. Similarly to the protecting circuit, the spark gap is formed by two wires diverging at the top.
When high voltage is applied to the gap, a spark forms across the bottom of the wires where they are nearest each other, rapidly changing to an electric arc. Air breaks down at about 30 kV/cm,[13] depending on humidity, temperature, etc. Apart from the anode and cathode voltage drops, the arc behaves almost as a short circuit, drawing as much current as the electrical power supply can deliver, and the heavy load dramatically reduces the voltage across the gap.
The heated ionized air rises, carrying the current path with it. As the trail of ionization gets longer, it becomes more and more unstable, finally breaking. The voltage across the electrodes then rises and the spark re-forms at the bottom of the device.
This cycle leads to an exotic-looking display of electric white, yellow, blue or purple arcs, which is often seen in films about
Media related to Jacob's ladder at Wikimedia Commons
Guiding the arc
Scientists have discovered a method to control the path of an arc between two electrodes by firing laser beams at the gas between the electrodes. The gas becomes a plasma and guides the arc. By constructing the plasma path between the electrodes with different laser beams, the arc can be formed into curved and S-shaped paths. The arc could also hit an obstacle and reform on the other side of the obstacle. The laser-guided arc technology could be useful in applications to deliver a spark of electricity to a precise spot.[16][17]
Undesired arcing
Undesired or unintended electric arcing can have detrimental effects on electric power transmission, 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 snubber circuits 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 flash describes an explosive electrical event that presents a hazard to people and equipment.
Undesired arcing in electrical contacts of
- immersion in transformer oil, dielectric gas or vacuum
- arc chutes
- magnetic blowouts
- pneumatic blowouts
- sacrificial ("arcing") contacts
- damping materials to absorb arc energy, either thermally or through chemical decomposition
Arcing can also occur when a low resistance channel (foreign object, conductive
An electric arc over the surface of
Arcing over some types of printed circuit boards, possibly due to cracks of the traces or the failure of a solder joint, renders the affected insulating layer conductive as the dielectric is combusted due to the high temperatures involved. This conductivity prolongs the arcing due to cascading failure of the surface.
Arc suppression
Arc suppression is a method of attempting to reduce or eliminate an 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 uses 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, the second of which can be detected by its distinctive sharp smell. These chemicals can be produced by high-power contacts in relays and motor commutators, and they are corrosive to nearby metal surfaces. Arcing also erodes the surfaces of the contacts, wearing them down and creating high contact resistance when closed.[21]
Health hazards
Exposure to an arc-producing device can pose health hazards. An arc formed in air will ionize oxygen and nitrogen, which then can re-form into reactive molecules such as
Arcs can also produce a broad spectrum of wavelengths spanning the visible light and the invisible ultraviolet and infrared spectrum. Very intense arcs generated by means such as arc welding can produce significant amounts of ultraviolet radiation which is damaging to the cornea of the observer. These arcs should only be observed through special dark filters which reduce the arc intensity and shield the observer's eyes from the ultraviolet rays.
See also
- Arc transmitter
- List of light sources
- Marx generator
- Spark gap
- Vacuum arc
- Paschen's law
References
- ^ "The Arc Species "Zoo"". Arc Suppression Technologies. 15 December 2020. Retrieved March 28, 2023.
- S2CID 11047670.
- ISBN 978-1330187593.
- ^ The Electric Arc, by Hertha Ayrton, page 20
- S2CID 4135392.
- LCCN 63020205.
- ISBN 978-0-217-88947-6. This is the likely origin of the term "arc".
- ^ a b "Tracking down the origin of arc plasma Science-II. Early continuous discharges". by André ANDERS. IEEE Xplore, ieee.org. IEEE Transactions on Plasma Science. Volume: 31, issue: 5, Oct 2003.
- ISBN 978-90-277-1402-2.
- doi:10.1093/ref:odnb/37136. (Subscription or UK public library membershiprequired.)
- ^ S2CID 37226480.
- ISBN 978-81-219-2450-4.
- S2CID 250794264.
- ^ "Resonant High Voltage Supplies". Archived from the original on 2015-05-18. Retrieved 2015-05-07.
- ^ "20kV DC power supply (homemade/DIY) using flyback with built in diodes". rimstar.org.
- ^ "Laser beams make lightning tunnels". Retrieved 2015-06-20.
- PMID 26601188.
- ^ "Arc Suppression". Retrieved December 6, 2013.
- ISBN 9780471456032.
- ^ Harper & Petrie 2003, p. ???[page needed]
- ^ "Lab Note #106 Environmental Impact of Arc Suppression". Arc Suppression Technologies. April 2011. Retrieved October 10, 2011.
External links
- "High Voltage Arcs and Sparks" Videos of 230 kV 3-phase "Jacobs Ladder" and unintentional 500 kV power arc
- High Voltage Arc Gap Calculator to calculate the length of an arc knowing the voltage or vice versa