Editing Switchgear (section)

==Circuit breaker types ==
A switchgear may be a simple open-air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is the gas-insulated switchgear (GIS), where the conductors and contacts are insulated by pressurized sulfur hexafluoride [[gas]] (SF<sub>6</sub>). Other common types are oil or vacuum insulated switchgear.

The combination of equipment within the switchgear enclosure allows them to interrupt fault currents of thousands of amps. A [[circuit breaker]] (within a switchgear enclosure) is the primary component that interrupts fault currents. The quenching of the arc when the circuit breaker pulls apart the contacts (disconnects the circuit) requires careful design. Circuit breakers fall into these six types:

===Oil===
[[File:TMW 50981 Schnittmodell Hochspannung-Leistungschalter HPF500F.jpg|thumb|Cutaway model of an oil-filled high-voltage circuit breaker]]
Oil circuit breakers rely upon the vaporization of some of the oil to blast a jet of oil along the arc's path. The vapor released by the arcing consists of [[hydrogen gas]].
[[Mineral oil]] has better insulating properties than air. Whenever there is a separation of current-carrying contacts in the oil, the arc in the circuit breaker is initialized at the moment of separation of contacts, and due to this arc the oil is vaporized and decomposed to mostly [[hydrogen gas]] and ultimately creates a hydrogen bubble around the [[electric arc]]. This highly compressed gas bubble around the turn prevents the re-striking of the arc after the current reaches zero crossing of the cycle. The oil circuit breaker is one of the oldest types of circuit breakers.

===Air===
Air circuit breakers may use compressed air (puff) or the magnetic force of the arc itself to elongate the arc. As the length of the sustainable arc is dependent on the available voltage, the elongated arc will eventually exhaust itself. Alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current flow very quickly: typically between 30&nbsp;ms and 150&nbsp;ms depending upon the age and construction of the device.

===Gas===
{{main article|Sulfur hexafluoride circuit breaker}}
Gas (SF<sub>6</sub>) circuit breakers sometimes stretch the arc using a [[magnetic field]], and then rely upon the dielectric strength of the SF<sub>6</sub> gas to quench the stretched arc.

===Hybrid===
{{main article|Hybrid switchgear modules}}
Hybrid switchgear is a type which combines the components of traditional air-insulated switchgear (AIS) and SF<sub>6</sub> gas-insulated switchgear (GIS) technologies. It is characterized by a compact and modular design, which encompasses several different functions in one module.

===Vacuum===
Circuit breakers with [[vacuum interrupter]]s have minimal arcing characteristics (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched by a small amount (&lt;2–8&nbsp;mm). Near zero current the arc is not hot enough to maintain a plasma, and current ceases; the gap can then withstand the rise of voltage. Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 40,500 volts. Unlike the other types, they are inherently unsuitable for interrupting DC faults. The reason vacuum circuit breakers are unsuitable for breaking high DC voltages is that with DC there is no "current zero" period. The plasma arc can feed itself by continuing to gasify the contact material.

===Carbon dioxide ===
Breakers that use [[carbon dioxide]] as the insulating and arc extinguishing medium work on the same principles as a [[sulfur hexafluoride]] (SF<sub>6</sub>) breaker. Because SF<sub>6</sub> is a [[greenhouse gas]] more potent than CO<sub>2</sub>, by switching from SF<sub>6</sub> to CO<sub>2</sub> it is possible to reduce the greenhouse gas emissions by 10 tons during the product lifecycle.<ref>{{cite web |title= Switzerland : ABB breaks new ground with environment friendly high-voltage circuit breaker. |url= http://www.thefreelibrary.com/Switzerland+%3A+ABB+breaks+new+ground+with+environment+friendly...-a0300987882 |access-date= 9 July 2013 |archive-date= 24 December 2019 |archive-url= https://web.archive.org/web/20191224163718/https://www.thefreelibrary.com/Switzerland+:+ABB+breaks+new+ground+with+environment+friendly...-a0300987882 |url-status= dead }}</ref>