What is the purpose of a fail-safe in fault detection and diagnostics (FDD) for DDC?

• A. To ensure safe operation or safe shutdown in case of sensor/actuator failure.
• B. To log events without changing operation.
• C. To maximize speed during fault conditions.
• D. To bypass faults to keep processes running.

Answer: (A)

Fail-safes in fault detection and diagnostics for DDC are all about safety: when a sensor or actuator fails, the system moves to a safe state or shuts down safely to prevent unsafe operation. The control should respond to a detected fault by limiting or reversing dangerous actions, or by tripping the process to a secure condition, so people and equipment aren’t put at risk and equipment isn’t damaged. Logging the fault without changing how the system operates doesn’t prevent unsafe states. Maximizing speed during fault conditions is risky and undermines safety, and bypassing faults to keep the process running can hide problems and create hazardous situations. So the purpose is to ensure safe operation or safe shutdown in case of sensor/actuator failure.