Reliability improvement of fork biasing spring in MCCB mechanism

Trips are used in moulded case circuit breaker (MCCB) to provide protection to low voltage distribution systems. Trip free can be achieved by using the fork biasing spring, that helps the fork to gets back to its intended trip position after the breaker gets tripped and helps avoiding intermediate position. Mechanical endurance testing of MCCBs mechanism for trip operation showed that fork biasing spring fails before the desired number of trip operation. In this study an improved design for fork biasing spring is proposed in order to increase the reliability of fork biasing spring. The springs are assembled in the MCCB mechanism and are subject to mechanical endurance test for trip operation. The results obtained from mechanical endurance test are used to determine the reliability of the springs using Weibull, Normal, Exponential, Lognormal, 3 Parameter Weibull, 3 Parameter Lognormal, 2 Parameter Exponential and 3 Parameter Loglogistic distribution for life data plots. The cause of the failure in present fork biasing spring is due to hook breakage in extension springs and the recommended torsion spring increases reliability.