Last edit: 07/08/2023
Functional safety was born having in mind the Reliability aspects of Safety-related Control Systems, designed to be activated upon hazardous process deviations; the latter is a process demand generating a Demand Rate of the safety system that protects people, the environment and material assets.
The parameter used to indicate the Reliability of a Safety-related Control System is the Unreliability function F(t). More precisely, there are two F(t) used, depending if the safety system is working in Low or in High demand mode. Just to give an example, the car airbag safety system is operating in low demand mode since it may remain inactive for years, until a demand occurs (due to a car crash).
In low demand mode safety systems the F(t) is defined as PFDavg:
[IEC 61508-4] 3.6 Fault, failure and error
3.6.18 Average probability of dangerous failure on demand (PFDavg). Mean unavailability (see IEC 60050-191) of an E/E/PE safety-related system to perform the specified safety function when a demand occurs from the EUC or EUC control system
In high demand mode safety Systems, the parameter used is PFH(t):
[IEC 61508-4] 3.6 Fault, failure and error
3.6.19 Average frequency of a dangerous failure per hour (PFH). Average frequency of a dangerous failure of an E/E/PE safety related system to perform the specified safety function over a given period of time
