The authors have developed a new safety instrumented system (SIS) evaluation method based on reliability analysis. It evaluates the Safety Integrity Level (SIL) using Fault Tree Analysis (FTA). When the SIL falls short of the system's target level, this method can raise it through reliability analysis and system retrofit. A hazard evaluation was carried out on the 415V diesel bus to verify the SIL evaluation method. The availability of the original 415V diesel bus was 99.40%, which comes under the category of SIL 2. After exchanging the diesel generator and the isolator switch using the developed evaluation method, the availability rose to 99.94%, SIL 3.