Rockwell Automation 2094-EN02D-M01-S1 Kinetix 6200 and Kinetix 6500 Safe Speed Monitoring Safety Reference Manual - Manual (Page 17)

Rockwell Automation Publication 2094-RM001C-EN-P - May 2013 17
Safety Concept Chapter 1
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 IEC 61508
and show worst-case values.
This table provides test data for a 20-year proof test interval and demonstrates
the worst-case effect of various configuration changes on the data.
Table 1 - PFD and PFH for 20-year Proof Test Interval
Safe State The Safe State encompasses all operation that occurs outside of the other
monitoring and stopping behavior defined as part of the drive. In addition,
configuration takes place in the Safe State. While the drive is in the Safe State, all
safety control outputs, except the Door Control (DC_Out) output, are in their
safe state (de-energized). The Door Control (DC_Out) output is in either the
locked state or in the de-energized state depending upon the condition that
resulted in the safe state.
When you cycle power, the drive enters the Safe State for self-testing. If the self-
tests pass and there is a valid configuration, the drive remains in the Safe State
until a successful request for safe speed monitoring occurs.
If a Safe State fault is detected, the drive goes to the Safe State. This includes
faults related to integrity of hardware or firmware.
For more information on faults, refer to Chapter 12 .
Attribute Single Encoder Dual Encoder
PFH [1e-9] 5.88 2.37
PFD [1e-4] 10.3 4.15
SFF % 99.4% 99.5%