The demand state refers to the condition when a safety function must act to bring a process to a safe state such as when a hazardous event or fault occurs.
In this state, the process moves from normal operation (no demand) to performing its safety action, such as de-energizing outputs, shutting down motion, or cutting power (demand).
The demand state can be either energized or de-energized, depending on how the process is designed to reach its safe condition:
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De-energize to trip: The system becomes safe when power is removed (e.g., closing a filling valve to prevent tank overflow or stopping gas injection during a leak).
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Energize to trip: The system requires energy to achieve safety (e.g., energizing a valve to release fire suppression water).
While the PLC safe state is always de-energized, the process demand state may differ. Ideally, both should align, but this isn’t always possible depending on the application design.
For information regarding demand states and different application used, please reference the Modicon M580, Safety Manual | Schneider Electric.
