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PFD, PFH, and MTTFd
Definitions
PFD, PFH, and MTTFd Data
Wiring Your Safe Torque-off
Circuit
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.
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).
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.
Determination of safety parameters is based on the assumption that the system
operates in high demand mode and that the safety function will be requested at
least once a year.
Table 23 - PFD and PFH for 20-year Proof Test Interval
Attribute
PFH (1e-9)
PFD (1e-3)
Proof test (years)
Refer to
Safe Torque-off Connector
The National Electrical Code and local electrical codes take precedence over the
IMPORTANT
values and methods provided.
To be sure of system performance, run wires and cables in the wireways as
IMPORTANT
established in the user manual.
Pins 8 and 9 (24V+) are used only by the motion-allowed jumper. When
IMPORTANT
wiring to the wiring-plug header, the 24V supply (for an external safety
device that triggers the safe torque-off request) must come from an
external source, otherwise system performance will be jeopardized.
Rockwell Automation Publication 2094-UM003A-EN-P - May 2012
Using the Safe Torque-off Feature with the Kinetix 6000M System
Value
0.35
0.062
20
on
page 40
for wiring details.
Appendix A
107
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