Definition:
The Functional Safety ecosystem has a concept of mode of operation. Within this there are three modes: low demand, high demand, and continuous demand. Low demand and high demand are both referred to as “demand mode”. Thus, broadly speaking, demand mode refers to a situation where a Safety Instrumented Function (SIF) is required to act only upon demand, typically during abnormal or emergency conditions. Demand mode is a combination of high demand mode and low demand mode.
This is important as the mode of operation is key to various requirements through IEC 61511-1.
Key Points:
- Distinguished from continuous mode.
- Assumes infrequent activation and can be low demand mode or high demand mode.
Example:
A high pressure trip function designed to operate only when vessel pressure exceeds a safe limit is in Demand Mode. Depending on the frequency it could be low demand mode or high demand mode.
See also: mode of operation, low demand mode, high demand mode, continuous mode
Cited Source:
- IEC 61511-1:2016, Clause 3.2.45.