Rockwell Automation Publication 2198-UM001D-EN-P - May 2014 161
Kinetix 5500 Safe Torque-off - Hardwired Safety Chapter 9
Probability of Dangerous
Failure Per Hour (PFH)
Safety-related systems are classified as operating in a High-demand/continuous
mode where the frequency of demands for operation made on a safety-related
system is greater than once per year.
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 Data
These PFH calculation is based on the equations from EN 61508 and show
worst-case values.
This table provides data for a 20-year proof test interval and demonstrates the
worst-case effect of various configuration changes on the data.
Table 74 - PFH for 20-year Proof Test Interval
Determination of safety parameters is based on the assumption that the
system operates in High demand mode and that the safety function is
requested greater than once a year.
Attribute Value
PFH (1e-9) 0.35
Proof test (years) 20
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