Open circuit may refer to: * Open circuit breathing apparatus, any type of breathing apparatus where the exhaled gas is discharged to the surroundings without recycling any of it ** Open-circuit scuba, a type of Scuba-diving equipment where the user breathes from the set and then exhales to the surroundings without recycling the exhaled breathing gas ** Open circuit surface-supplied diving equipment, a type of surface-supplied diving equipment where the user breathes from the supplied gas and exhales to the surroundings without recycling the exhaled gas * Open-circuit test, a method used in electrical engineering to determine the impedance in the excitation branch of a real transformer * Open-circuit voltage, the difference of electrical potential between two terminals of a device when there is no external load connected *An open electrical circuit is an electrical network that lacks a complete path between the terminals of its power source

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