In order to understand these two modes, it is necessary first of all to understand the division between a demand mode of operation and a continuous mode of operation.
A safety function operating in demand mode is only performed when required (i.e. on demand) in order to transfer the equipment under control (EUC) into a specified state. The E/E/PE safety-related system that performs the safety function has no influence on the EUC until there is a demand for the safety function to be performed. Examples include protection systems on chemical plants that respond to failures of the EUC or EUC control system and anti-lock braking systems on automotive vehicles.
A safety function operating in continuous mode operates to retain the EUC within its normal safe state. That is, the E/E/PE safety-related system continuously controls the EUC, and a dangerous failure of the E/E/PE safety-related system will lead to a hazard unless other safety-related systems or external risk reduction facilities intervene. Examples include speed control associated with machinery, burner control of furnaces or fly-by-wire operation of aircraft flight control surfaces.
IEC 61508 distinguishes between:
- low demand mode of operation, and
- high demand or continuous mode of operation.
This text contains extracts from the IEC Functional Safety Zone. All such extracts are copyright of International Electrotechnical Commission © 2005, IEC, Geneva, Switzerland. All rights reserved. IEC has no responsibility for the placement and context in which the extracts are reproduced. This notice takes precedence over any general copyright statement.
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