Definition:
Demand Rate is the expected frequency at which a Safety Instrumented Function (SIF) will be called upon to act. This intertwines with the concept of mode of operation. The mode of operation then triggers various other requirements within the functional safety ecosystem.
This is a formal term defined by IEC 61511-1. A similar term is demand interval (DI) which is the inverse of demand rate. DI is used by some practitioners for specific pieces of guidance.
Key Points:
- Expressed as demands per year.
- Influences whether a function is low demand mode, high demand mode, or continuous mode.
- DI is the inverse of demand rate.
Example:
A process that sees a potential overpressure event once every five years has a Demand Rate of 0.2 demands/year. This would be a low demand mode. The DI would be 5 years.
See Also: mode of operation, low demand mode, demand mode, demand interval
Cited Source:
- IEC 61511-1:2016, Clause 3.2.47.