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 often grouped together and 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 (meaning not continuously), typically during abnormal or emergency conditions.
This is important as the mode of operation is an input into various requirements throughout IEC 61511-1.
Key Points:
- Distinguished from continuous mode.
- Combination of low and high demand modes.
- Assumes non-continuous 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 3.2.39, table 5