Application of Fishbone Diagram & Pareto Analysis for Improving Quality and Reliability of MCCB – A Case Study

Abstract

The moulded case circuit breaker (MCCB) is employed in electrical systems to safeguard the circuit and its constituent parts from excessive current, which must be isolated when necessary to avoid overload or short circuit. Its trustworthiness is essential for ensuring that the electrical networks run smoothly. During the overload conditions of an MCCB, tripping characteristics should perform as per the applicable standard. Also, the operational efficiency of the MCCB with the accessories its compatible with is very crucial. An accessory – under voltage relay is used to protect the circuit from the damage done through the low-voltage condition. Here, the concerned MCCB was experiencing the thermal inconsistency under overload condition and operational inefficiency with the UVR. So, the objective of this research is to improve its overall quality and enhance its reliability by mitigating these issues. Hence to investigate the problem, cause and effect analysis is carried out and subsequently various tests have been carried out on a batch of MCCB. Further, pareto analysis is carried out and based on the data of test results and the main cause for the issue has been identified as the ‘trip plate not rotating sufficiently’. To resolve the issue, a modification i.e. removal of the slot in latch bracket has been done in the operating mechanism of the breaker. Hence, after modification in the latch bracket, constant trip travel is achieved. So, the modification resulted in achieving operational efficiency with UVR and improving the thermal characteristics of the breaker. Thus, the goal to mitigate the issues and achieve the improved performance from the circuit breaker is achieved by implementing a design change in the mechanism of the circuit breaker.