Rockwell Automation Publication 2097-UM002D-EN-P - April 2017 103
Kinetix 350 Drive Safe Torque-off Feature Chapter 6
Troubleshoot 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 that
is 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 that is 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 perform its
safety function on demand satisfactorily 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 41 - PFD and PFH for 20-year Proof Test Interval
ATTENTION: Permanent magnet motors can, if there are of two
simultaneous faults in the IGBT circuit, result in a rotation of up to 180
electrical degrees.
ATTENTION: When the safe torque-off function is activated, the drive posts
a Start Inhibit (Sc05). 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
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