Brush discharge
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A brush discharge is an electrical
disruptive discharge Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. All insulating mate ...
similar to a
corona discharge A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown ...
that takes place at an
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
with a high
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
applied to it, embedded in a nonconducting fluid, usually air. It is characterized by multiple luminous writhing sparks,
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...
streamers composed of
ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
air molecules, which repeatedly strike out from the electrode into the air, often with a crackling sound. The streamers spread out in a fan shape, giving it the appearance of a "brush". Corona and brush discharges are sometimes called ''one-electrode discharges'' because they occur in the vicinity of a single electrode, and don't extend as far as the electrode carrying opposite polarity voltage in the circuit, as an
electric arc An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (p ...
(a ''two-electrode discharge'') does. *''Corona discharge'' — occurs at sharp points and edges (radius < 1 mm). It is a uniform ionization (
glow discharge A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the str ...
) visible as a dim stationary blue glow, fading out as it extends from the conductor. *''Brush discharge'' — occurs at a curved electrode (radius between 5 and 50 mm) in the vicinity of a flat electrode. It consists of a short ionization channel which breaks up into a fan of multiple moving streamers which strike toward the other electrode. If the electrode is too sharp, a corona discharge will usually occur instead of a brush discharge. *''Arc'' or ''spark discharge'' — A "two electrode" discharge that occurs when an ionized channel extends all the way from one electrode to the other. This allows a large current to flow, releasing a large amount of energy. Both brush and corona discharges represent local regions next to conductors where due to the high voltage the air has undergone
electrical breakdown Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. All insulating mate ...
: it has
ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
and become conductive, allowing current to leak into the air. They occur when the
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
at the conductor exceeds the
dielectric strength In physics, the term dielectric strength has the following meanings: *for a pure electrically insulating material, the maximum electric field that the material can withstand under ideal conditions without undergoing electrical breakdown and becom ...
of the air, the "disruptive potential gradient", roughly 30 kilovolts per centimeter. At that voltage,
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
s in the air are accelerated by the electric field to a high enough velocity that they knock other electrons off gas molecules when they hit them, creating
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s and additional electrons, which go on to ionize additional molecules in a chain reaction. The electric field is highest at sharp points on the conductor, so discharges tend to form at these points. Because the electric field decreases as the distance from the conductor increases, it eventually drops below the value needed for ionization, so corona and brush discharges have a limited extent and are localized near the conductor. Occurring in very
high voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant spec ...
equipment like EHV
power transmission line Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a ''transmission network''. This is d ...
s,
radio transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
s and their
antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...
s, CRT power supplies, and power supplies for scientific equipment like
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
s and
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particle ...
s, a brush discharge represents a serious failure of electrical insulation, and may be a fire hazard. Like other
electric arc An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (p ...
s, brush discharges produce
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
gas, which can be noxious to nearby people in an enclosed space and over time can cause embrittlement of some plastics.
Tesla coil A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low-current, high-frequency alternating-current electricity. Tesla experimented with a number of different ...
s producing brush discharges and
streamer discharge A streamer discharge, also known as ''filamentary discharge'', is a type of transient electrical discharge which forms at the surface of a conductive electrode carrying a high voltage in an insulating medium such as air. Streamers are luminous w ...
s are displayed for entertainment at
science fair Science and engineering fairs, hosted by schools worldwide, offer students the opportunity to experience the practices of science and engineering for themselves. In the United States, the Next Generation Science Standards makes experiencing the ...
s and rock concerts. The ability of an electrical discharge to cause an explosion in flammable atmospheres is measured by the ''effective energy'' of the discharge. The effective energy of brush discharges is 10-20 mJ, much larger than that of corona discharges 0.1 mJ. Therefore, brush discharges are considered an explosion hazard, while corona discharges are not. Brush discharges can occur from charged insulating plastics (for example polythene) to a conductor.


See also

*
Lichtenberg figure A Lichtenberg figure (German ''Lichtenberg-Figuren''), or Lichtenberg dust figure, is a branching electric discharge that sometimes appears on the surface or in the interior of insulating materials. Lichtenberg figures are often associated wit ...
*
Partial discharge In electrical engineering, partial discharge (PD) is a localized dielectric breakdown (DB) (which does not completely bridge the space between the two conductors) of a small portion of a solid or fluid electrical insulation (EI) system under hig ...
*
Glow discharge A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the str ...
*
Electrostatic discharge Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two electrically charged objects caused by contact, an short circuit, electrical short or dielectric breakdown. A buildup of static electricity can be caused ...
*
Electric spark An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures. Michael F ...


References

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External links


Explosions and ESD, Niels JonassenBrush discharge factors
Electrical breakdown de:Büschelentladung