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Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (less than 2â3 mm (0.079â0.118 in)). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 38,000 volts. Also indicates the status of the circuit breaker (On or Off/tripped). 2  Most breakers are designed so they can still trip even if the lever is held or locked in the on position. 3  This is sometimes referred to as free trip or positive trip operation. 4  Actuator mechanism-forces the contacts together or apart.
Main DefinitionsThe main definitions below are from the UL 489, UL 1066, ANSI C37.13 and ANSI C37.17 Standards.Definitions from the Standard UL 489 (MCCB) 1 - ADJUSTABLE CIRCUIT BREAKER: a circuit breaker that has adjustable time/current tripping characteristics.