Design and Development of Rack-in Mechanism of Air Circuit Breaker

Abstract

Demand of electrical energy without interruption has increased nowadays. To meet the requirement, the reliability of each individual component should also be high. Circuit breakers are the last link in chain to protect equipment for faulty power supply. Air circuit breaker (ACB) is an automatically operated electrical switch, which is mainly used for low voltage power distribution. It is intended to protect an electrical circuit from damage induced by a short circuit or overload. Its basic role is to detect the defective condition and interrupt current flow, which can be reset (either automatically or manually) to resume normal functioning. ACB is made in various sizes, according to current required from the range 400 to 6300 Amps. For easy maintenance and inspection, draw-out types ACB is used. Breaker is assembled on a cradle. Rack-in mechanism is used to connect and disconnect the breaker with in cradle by applying torque manually using rack-in handle. Lesser torque is desirable in manual operation to reduce fatigue. The main objective of the dissertation is torque reduction of rack-in assembly in draw-out type ACB. The existing ACB has been analyzed for torque. The factors contributing to torque have been identified, using analytical methods. Three alternate mechanisms have been thought and direct translation type rack-in mechanism has been proposed based on grid analysis. It results in reduction of torque by 29.93 percentages comparing to exiting mechanism. A prototype was fabricated to validate the results. Moreover, a portable rack-in device has been design and fabricated for power operated rack-in system. An AC electric motor has been used to apply the torque. It has been performing consistently well. The gearbox casing was designed and FE analysis was carried out for stress prediction and to check safe operation.