Last edit: 03/03/2023
IEC 61508-1 clarifies which target failure measure, or unreliability function F(t), should be used, depending upon the mode of operation:
- PFDavg should be used for safety systems in low demand mode.
- PFHD should be used for safety systems in high demand or in continuous mode.
[IEC 61508-1] 7.6 Overall safety requirements allocation
[…] 188.8.131.52 When the allocation has sufficiently progressed, the safety integrity requirements, for each safety function allocated to the E/E/PE safety-related system(s), shall be specified in terms of the safety integrity level in accordance with Table 2 or Table 3 and shall indicate whether the target failure measure is, either:
- the average probability of dangerous failure on demand of the safety function, (PFDavg), for a low demand mode of operation (Table 2), or
- the average frequency of a dangerous failure of the safety function [h-1], (PFH), for a high demand mode of operation (Table 3), or
- the average frequency of a dangerous failure of the safety function [h-1], (PFH), for a continuous mode of operation (Table 3).
The content of Table 2 of IEC 61508-1 is indicated hereafter, as well as the content of Table 3.