Electrical breakdown
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Electrical breakdown is a process whereby an electrically insulating material begins to fail, and begins to become electrically conductive.
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Breakdown in different materials
Actual breakdown processes differ significantly between a high vacuum, gases, liquids, and solids.
Gas
In gases, the breakdown process may only be partial, resulting in corona discharges in regions having high electrical stress. In uniform electrical fields, complete breakdown can also occur quite suddenly with little advance warning. Electrical breakdown of air can produce both ozone and nitrogen oxides depending of the strength of the electric field. The voltage required to cause breakdown in a gas is given by Paschen's law.
Vacuum
Breakdown within a vacuum usually begins via electron emission from microscopic imperfections on the surface of the negative-most electrode (cathode). Localized heating can then lead to thermionic emmission and even explosive vaporization of these points and subsequent arc-over via the resulting metal vapor.
Liquid
Breakdown within insulating liquids is often preceeded by the injection of charges into the fluid (space charges) and formation of bubbles of vapor at the electrode-fluid interface. Actual breakdown is often initiated within, and progresses from, these gas bubbles.
Solid
Breakdown within solids is usually associated with physical defects or impurities. Unlike dielectric breakdown within a vacuum, gas, or fluid, an internal breakdown within a solid is usually catastrophic - once it occurs, the insulating properties are permanently degraded or destroyed.
Possible results of electrical breakdown
See electrical discharge types for details.
Spark
A spark is a sudden and momentary electric current that flows between two objects at different electrical potentials. Sparks create a shock wave which is rapidly converted to an impulsive noise in the air - sparks make a distinctive snap or bang.
Arc
An arc is a continious current flowing between two objects at different electrical potentials. An arc can occur when a spark bridges a gap and sufficient current is provided to continue flow through the resulting ionized gases and/or ionized electrode materials.
Corona discharge
Corona is a partial breakdown of air or other gas. In air, it can produce significant amounts of ozone.
Streamer discharge
A streamer is a branch of hairlike discharges starting from a very hot plasma leader and spreading outwards.
Brush discharge
A brush discharge is a type of low energy corona discharge that takes place between two electrodes in a nonconducting medium like air and is characterized by nonsparking, faintly glowing branch like structures composed of ionized particles.
