High and Low Demand operation mode

IEC 61508-1 clarifies which target failure measure, or unreliability function F(t), should be used, depending upon the mode of operation:

• PFD_avg should be used for safety systems in low demand mode.
• PFH_D should be used for safety systems in high demand or in continuous mode.

[IEC 61508-1] 7.6 Overall safety requirements allocation

[…] 7.6.2.9 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, (PFD_avg), 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.