Electrical breakdown
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The term electrical breakdown has several similar but distinctly different meanings. The term may apply to a failure of an electric circuit with consequent lack of function. Alternately, it may refer to the application of current to a gas.
The most common meaning is related to automobiles and is the failure of an electric circuit or associated device resulting in a loss of vehicle function (a breakdown). Common problems include battery discharge, alternator failure, broken wires, blown fuses, and a failed fuel pump.
The second meaning is more specifically a reference to the breakdown of the insulation of an electrical wire or other electrical component. Such breakdown usually results in a short circuit and/or a blown fuse, but as breakdown of insulation is more generally found in extremely high voltage applications, it may cause a trip of a circuit breaker. In particular, electrical breakdown is often associated with high voltage transformers in the electricity distribution grid. Another type of electrical breakdown is found in capacitors where the applied voltage can exceed the dielectric's capability to resist and breakdown occurs. Electrical breakdown can also occur across the strings of insulators that suspend power lines or within underground power cables.
Electrical breakdown can also occur in a gas (or mixture of gases, such as air) when the voltage that's applied across a gap exceeds the dielectric strength of the gas(es). The high electrical stress causes the gas to partially ionize and begin conducting. This is done deliberately in low pressure discharges such as in fluorescent lights. (see also Electrostatic Discharge). Electrical breakdown of the air causes a "fresh air" smell of ozone during thunderstorms or around high-voltage equipment. Although air is normally an excellent insulator, when stressed by a sufficiently high voltage, air can begin to break down, becoming partially conductive. This first occurs as a corona discharge on high voltage conductors at points with the highest electrical stress (smaller radius or sharp points). Corona is sometimes seen as a bluish glow around high voltage wires and heard as a sizzling sound along high voltage power lines. Corona can also occur naturally at high points (such as church spires, treetops, or ship masts) during thunderstorms as St. Elmo's Fire. Although corona is usually undesirable, it is essential in the operation of photocopiers (Xerography) and laser printers. If the voltage is sufficiently high, complete electrical breakdown of the air can result in an electrical spark that bridges the entire gap in a blue-white flash, accompanied by a loud snap or bang. Small sparks can also be generated by static electricity. Lightning is an example of an immense spark that can be miles long. If a fuse or circuit breaker doesn't stop the current flowing through a spark in a power circuit, electricity may continuue to flow across the gap, forming a hot electric arc. Although sparks and arcs are usually undesireable, sparks can be quite useful in everyday applications such as spark plugs for gasoline engines, and arcs are used for electrical welding of metals or for creating special purpose alloys in an electric arc furnace.