What is the role of a fail-safe in DDC Fault Detection and Diagnostics?

• A. To ensure safe operation or safe shutdown in the event of sensor/actuator failure.
• B. To provide a detailed fault log only.
• C. To increase system speed under fault.
• D. To bypass faults to maintain operation.

Answer: (A)

Fail-safe behavior is about safety first. In DDC fault detection and diagnostics, when a sensor or actuator fails or provides unreliable data, the system responds by forcing a safe state or performing a safe shutdown to prevent unsafe conditions. This means the controller either holds the controlled variable at a safe value, moves outputs to safe positions, or shuts the process down if necessary. The aim is to protect occupants, equipment, and the process even when measurements go wrong. This approach works with fault detection: the fault is identified, and the fail-safe enforces a safe condition while the issue is addressed, often accompanied by alarms for operators. That’s why this option best captures the role: ensuring safe operation or safe shutdown in the event of sensor or actuator failure.