
A brush discharge is an electrical
disruptive discharge 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 (material), conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone ...
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 a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...
with a high
voltage
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
applied to it, embedded in a nonconducting fluid, usually air. It is characterized by numerous luminous writhing sparks,
plasma 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 i ...
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 ( ...
(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 (physics), 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 va ...
) 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
In electronics, electrical breakdown or dielectric breakdown is a process that occurs when an electrically insulating material (a dielectric), subjected to a high enough voltage, suddenly becomes a conductor and current flows through it. All ...
: 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 i ...
and become conductive, allowing current to leak into the air. They occur when the
electric field
An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...
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 in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
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 convent ...
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 sp ...
equipment like
EHV power transmission lines,
radio transmitter
In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna with the purpose of signal transmissio ...
s and their
antennas,
CRT
CRT or Crt most commonly refers to:
* Cathode-ray tube, a display
* Critical race theory, an academic framework of analysis
CRT may also refer to:
Law
* Charitable remainder trust, United States
* Civil Resolution Tribunal, Canada
* Columbia ...
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'' originated as an acronym for light amplification by stimulated emission of radi ...
s and
particle accelerator
A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies to contain them in well-defined particle beam, beams. Small accelerators are used for fundamental ...
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 ( ...
s, brush discharges produce
ozone
Ozone () (or trioxygen) is an Inorganic compound, 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 , break ...
gas, which can be noxious to nearby people in an enclosed space and over time can cause embrittlement of some plastics.
Tesla coils producing brush discharges and
streamer discharge
In electromagnetism, a streamer discharge, also known as filamentary discharge, is a type of transient electric discharge which forms at the surface of a conductive electrode carrying a high voltage in an Insulator (electricity), insulating medi ...
s are displayed for entertainment at
science fair
A science fair or engineering fair is an event hosted by a school that offers students the opportunity to experience the practices of science and engineering for themselves. In the United States, the Next Generation Science Standards makes ex ...
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
polyethylene
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...
) to a conductor.
See also
*
Lichtenberg figure
A Lichtenberg figure (German: ''Lichtenberg-Figur''), or Lichtenberg dust figure, is a branching electric discharge that sometimes appears on the surface or in the interior of Electrical insulation, insulating materials. Lichtenberg figures a ...
*
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 hi ...
*
Glow discharge
A glow discharge is a Plasma (physics), 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 va ...
*
Electrostatic discharge
Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two differently-charged objects when brought close together or when the dielectric between them breaks down, often creating a visible electric spark, spark as ...
*
Electric spark
An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an Ionization, ionized, Electric current, electrically conductive channel through a normally-insulating medium, often air or other ga ...
References
{{reflist
External links
Explosions and ESD, Niels JonassenBrush discharge factors
Electrical breakdown
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