What is the fail-safe principle in safety instrumentation?

Fail-safe design ensures that when a component fails, it fails to a safe state that protects personnel and equipment. Examples: fail-closed valve on loss of air/signal (for isolation applications), fail-open valve (for relief or cooling applications), transmitters outputting below 4 mA or above 20 mA on failure (burnout detection), and de-energize to trip (loss of power causes safe shutdown). The philosophy depends on process hazards - the safe state may be shutdown, vented, or another predetermined condition. Fail-safe design is fundamental to SIS reliability.