Demand Mode of Operation Safety Systems

safety system designed to achieve its safety action only when receiving a specific request from its

surrounding environment

Note 1 to entry: Such systems spend most of their time in stand-by position but need nevertheless to be ready to

work as soon as a demand occurs.

Note 2 to entry: Such systems are subject to hidden failures. Diagnostic and periodic tests are generally

implemented in order to reveal the corresponding latent faults.

Note 3 to entry: When the demand frequency increases, an on-demand mode safety system may be assimilated to

a continuous mode of operation systems.

Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global Standards

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