What is the difference between SIS and BPCS?

BPCS (Basic Process Control System) controls the process during normal operation to achieve production goals - controlling temperatures, pressures, flows to maintain optimal operation. SIS (Safety Instrumented System) protects against hazardous conditions when the process deviates beyond safe limits - it is the last line of automated defense. Key differences: SIS has higher reliability requirements, SIS should be independent from BPCS (separate hardware, power, communications), SIS only acts when BPCS fails to control process, and SIS has specific standards (IEC 61511) and testing requirements. SIS is not for normal process control.