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 ms and 150 ms depending upon the age and construction of the device. Gas Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 gas to quench the stretched arc. Hybrid Hybrid switchgear is a type which combines the components of traditional air-insulated switchgear (AIS) and SF6 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 interrupters 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 (<2–8 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 (SF6) breaker. Because SF6 is a greenhouse gas more potent than CO2, by switching from SF6 to CO2 it is possible to reduce the greenhouse gas emissions by 10 tons during the product lifecycle. Protective circuitry Circuit breakers and fuses Circuit breakers