Demand Mode of Operation Safety Systems
safety system designed to achieve its safety action only when receiving a specific request from its
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