This study aims to analyze the condition of a 150 kV circuit breaker (PMT) at Seduduk Putih Substation, focusing on the quality of SF6 gas as an insulating medium and the impact of gas leakage on system reliability. Data were collected by testing the main SF6 gas parameters, including pressure, purity, moisture content, dew point, and decomposition products, under three conditions: before maintenance, during sudden leakage, and after corrective actions. Observations show that before maintenance, gas pressure ranged from 4.0–4.3 bar with purity of 91–93%, moisture content of 2000– 2300 ppmv, dew point between -2°C and 0°C, and decomposition products of 380–420 ppmv, indicating initial degradation of the insulating medium. During leakage, pressure dropped sharply to 1.9–2.3 bar, gas purity decreased to 83–85%, moisture content increased to 4000 ppmv, dew point rose to 3–5°C, and decomposition products reached 1700–1900 ppmv, indicating severely impaired insulation and significantly increased
leakage current. After corrective measures, including evacuation of the insulation chamber, cleaning, and refilling with pure SF6 gas, all parameters returned to optimal values: pressure 7 bar, purity 100%, moisture content 327 ppmv, dew point -31°C, and decomposition products 0 ppmv, with estimated insulation resistance increasing from
10 MΩ during leakage to 1000 MΩ. These results confirm that SF6 gas quality is crucial for the reliability of 150 kV PMTs. Gradual gas degradation can trigger leakage and reduce insulation capability, while proper maintenance can restore the system to operational standards. This study emphasizes the importance of regular monitoring and
preventive maintenance based on SF6 gas quality parameters to ensure the safety and reliability of high-voltage power distribution systems.