Rockwell Automation 2097-Vxxx Kinetix 350 Single-axis EtherNet/IP Servo Drive User Manual - Manual (Page 103)

Rockwell Automation Publication 2097-UM002C-EN-P - December 2013 103
Kinetix 350 Drive Safe Torque-off Feature Chapter 6
Troubleshooting the Safe Torque-off Function
PFD and PFH Definitions Safety-related systems can be classified as operating in either a Low Demand
mode, or in a High Demand/Continuous mode:
• Low Demand mode: where the frequency of demands for operation made
on a safety-related system is no greater than one per year or no greater than
twice the proof-test frequency.
• High Demand/Continuous mode: where the frequency of demands for
operation made on a safety-related system is greater than once per year or
greater than twice the proof test interval.
The SIL value for a low demand safety-related system is directly related to order-
of-magnitude ranges of its average probability of failure to satisfactorily perform
its safety function on demand or, simply, average probability of failure on demand
(PFD). 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).
PFD and PFH Data These PFD and PFH calculations are 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 39 - PFD and PFH for 20-year Proof Test Interval
ATTENTION: The safe torque-off fault is detected upon demand by the safe
torque-off function. After troubleshooting, a proof test safety function must be
performed to verify correct operation.
Attribute Value
PFH [1e-9] 5.9
PFD [1e-3] 1.0