Analysis of Fuse Failure in a 6 kV 5-Cubicle Circuit Breaker Based on Motor Starting Current Characteristics in a Steam Turbine Of Block II at PT PLN Indonesia Power UBP Grati

Abstract

The reliability of a power plant electrical system is essential to ensure continuous operation, particularly in the 6 kV distribution system supplying auxiliary equipment such as the Circulating Water Pump (CWP) motor. A problem was identified in Cubicle 5 at PT PLN Indonesia Power UBP Grati, where a 250 A fuse frequently failed during motor operation, raising concerns about its suitability. This study aims to analyze the cause of the fuse failure and evaluate the protection system using a descriptive method based on field data, including motor specifications, fuse ratings, and operating conditions such as starting current. The analysis shows that the motor starting current reaches approximately 325 A, exceeding the fuse rating and causing thermal stress due to I²t energy accumulation from repeated operations. As a result, the fuse operates near its limit and eventually fails under normal conditions. It can be concluded that the failure is caused by a mismatch between the fuse characteristics and the motor starting profile, as well as improper protection coordination, thus requiring reevaluation of fuse selection and protection settings to ensure reliable and selective system operation.