Definition:
High Demand Mode describes a system that is expected to have greater than one demand per year on average.
High demand mode is one of three types of demand mode: low, high, and continuous. Confusingly, two of these are grouped together, low and high, to make up demand mode. These modes trigger various requirements throughout IEC 61511-1.
Key Points:
- Allows use of PFH rather than PFDavg for SIL calculations.
- Common for emergency shutdown systems.
Example:
An emergency flare system, with a demand once every 6 months, operates in Low Demand Mode.
See also: mode of operation, demand mode, low demand mode, continuous mode, PFH
Cited Source:
- IEC 61511-1:2016, Clause 3.2.46.
- Control Engineering – Measuring safety instrumented systems