Definition:
The mode of operation, at times called the operating mode, is a core concept heavily integrated in the rules of functional safety which is about how often a SIF operates. Within this, there are three modes: low demand, high demand, and continuous demand. Low demand and high demand are together referred to as demand mode. Because of the layers, this can be a confusing topic for new and even experienced engineers.
This is important as the mode of operation is key to various requirements throughout IEC 61511-1.
Key Points:
- Three types: low, high, and continuous.
- Confusingly, two of the three are grouped as demand mode, so some sources state there are only two modes of operation.
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: low demand mode, high demand mode, demand mode, continuous mode
Cited Source:
- IEC 61511-1:2016, Clause 3.2.39