Rockwell Automation 20D-P2-DG03 DriveGuard Safe Torque Off Option for PowerFlex 700S Phase II and 700L Drives - Manual (Page 12)

12 Rockwell Automation Publication 20D-UM007G-EN-P - March 2012
Chapter 1 General Description
Performance Level and Safety Integrity Level (SIL) CL2
For safety-related control systems, Performance Level (PL), according to
EN ISO 13849-1, and SIL levels, according to IEC 61508 and EN 62061,
include a rating of the system’s ability to perform its safety functions. All of
the safety-related components of the control system must be included in both
a risk assessment and the determination of the achieved levels.
Refer to the EN ISO 13849-1, IEC 61508, and EN 62061 standards for
complete information on requirements for PL and SIL determination.
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 Part 6 of
IEC 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.
PFD and PFH for 20-year Proof Test Interval
Attribute Test Result Frames 1…6 Test Result Frames 9…14
PFD
av
5.28 x 10 -5 2.14 x 10 -4
PFH 6.00 x 10 -10 1/h
(calculated according to IEC 61508)
2.64 x 10 -9 1/h
(calculated according to IEC 61508)
MTTF
D
High (4023 years) High (11415 years)
DC
av
low (69%) low (80%)