Rockwell Automation Publication 2198-UM004A-EN-P - October 2019 381
Kinetix 5100 Safe Torque Off (STO) Feature Chapter 13
Average Frequency of a
Dangerous Failure per Hour
Safety-related systems are classified as operating in a High-demand/continuous
mode. The SIL value for a High-demand/continuous mode safety-related system
is directly related to the probability of a dangerous failure occurring per hour
(PFH).
PFH calculation is based on the equations from IEC 61508 and show worst-case
values. Table 112
provides data for a 20-year proof test interval and demonstrates
the worst-case effect of various configuration changes on the data.
Table 112 - PFH for 20-year Proof Test Interval
IMPORTANT Determination of safety parameters is based on the assumptions that the
system operates in High-demand mode and that the safety function is
requested at least once every three months.
Attribute Value
PFH (1e-9) 0.96
Proof test (years) 20
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