IMPROVEMENT IN DEFECTS PER MILLION OF MOULDED CASE CIRCUIT BREAKER

Abstract

Circuit breakers are used for electrical protection for the devices. The equipment(s) are protected from short circuit currents, overloads, and fault currents by circuit breakers. Moulded Case Circuit Breaker (MCCB) is one of the circuit breakers among different types of them. MCCBs are used in applications in which the current rating ranges from 16A to 1600A. MCCBs can clear the fault up to 5kA to 200kA. The purpose of the project was to analyze the problem that occurs on the integration line and remove the problem by improving the design. In this project, there were two problems analyzed and improved. The first problem was the mechanical contact system not trip. From the experiment thermal magnetic release spring was found which having less margin to overcome the tripping force of mechanical contact system. Two concepts were proposed for the same. In first concept improvement in TMR spring which gives force to the lever to strike on trip-plate. In existing compression spring had the force of 6.86N and modified conical spring has been the force of 20.601N in trip condition. The conical spring is better than the existing compression spring. The second concept was to reduce the force required to trip the mechanical contact system. For that, the existing trip-plate design was modified that an increase in mechanical advantage. The modification in trip-plate, it was required to modify TMR lever accordingly. The modification in lever helped to increase the force exerted on trip-plate. For the design of the trip-plate and lever, PTC creo and PTC creo simulation software tools were used. The second problem was high differential contact loads in the MCCB shaft. In the study of the shaft, the moving contact was shifted concerning the center pin which was creating an in-balance force on contact load. To design the optimum moving contact full factorial design of experiment method was used. To analyze the data Minitab software tool was used. Based on the design of experiments the optimum moving contact was designed. In the existing design of moving contact the difference between contact loads was 11.17N. The modified moving contact with a combination of 80 slot angle, 11mm PCD of spring hole, and clearance of 1mm. the average difference between contact loads is reduced to 4.48N.