The design, operation and most typical defects of widely used tank oil circuit breakers of types VM-35 and MKP-35 are considered. Repair of circuit breakers of both types is considered in detail and works on elimination of defects of separate units are described. Information is given on measurement of resistance, speed of switching on and off of movable contacts and other values characterizing the state of circuit breakers. The brochure is designed for electricians, electricians and foremen involved in the operation, adjustment and repair of circuit breakers.