Which supply fan shutdown event is not a hardwired safety circuit?

Which supply fan shutdown event is not a hardwired safety circuit?

• A. Scheduled On/Off
• B. Overheat
• C. Fault Trip
• D. Emergency Stop

Answer: (A)

Hardwired safety circuits are direct, physical interlocks that can stop equipment without relying on software or the control system. They rely on sensors or switches wired to open the power path when a safety condition is detected, ensuring a fast and reliable shutdown. The events that fit this are those triggered by immediate sensing: an overheat condition activates a high-temperature switch that cuts power; a fault trip opens the circuit when a fault is detected; and an emergency stop is a manual device that physically breaks the power path. These operate independent of the control logic to guarantee a safe stop. Scheduled On/Off, on the other hand, is an operational control driven by a timer or schedule within the building automation system. It decides when the fan runs or stops based on time or occupancy plans, not on a fault or safety condition. It isn’t guaranteed to respond to hazardous conditions and can be affected by controller issues, so it isn’t considered a hardwired safety circuit.

Hardwired safety circuits are direct, physical interlocks that can stop equipment without relying on software or the control system. They rely on sensors or switches wired to open the power path when a safety condition is detected, ensuring a fast and reliable shutdown.

The events that fit this are those triggered by immediate sensing: an overheat condition activates a high-temperature switch that cuts power; a fault trip opens the circuit when a fault is detected; and an emergency stop is a manual device that physically breaks the power path. These operate independent of the control logic to guarantee a safe stop.

Scheduled On/Off, on the other hand, is an operational control driven by a timer or schedule within the building automation system. It decides when the fan runs or stops based on time or occupancy plans, not on a fault or safety condition. It isn’t guaranteed to respond to hazardous conditions and can be affected by controller issues, so it isn’t considered a hardwired safety circuit.