APPENDIX D — EN 13849 COMPLIANCE
pg. 137
Return to TOC Curtis Model 1351 – December 2018
It is important to note that the above analysis is based on the example wiring diagram provided. Every
system must be analyzed based on the I/O used, how it is applied and the VCL soware developed
custom for the application. e nal PL, MTTFd and DC% will vary with each system application
design. e designer applying the 1351 must refer to the calculations provided in the 1351 SRPCS
Calculation spreadsheet. ere, the designer can easily select the I/O used and nd the resulting
values for PL, MTTFd and DC%
Mean Time To Dangerous Failure (MTTFd) is related to the expected reliability of the
safety related parts used in the controller. Only failures that can result in a dangerous
situation are included in the calculation.
Diagnostic Coverage (DC) is a measure of the eectiveness of the control system’s self-test
and monitoring measures to detect failures and provide a safe shutdown.
Common Cause Faults (CCF) are so named because some faults within a controller can
aect several systems. EN13849 provides a checklist of design techniques that should be
followed to achieve sucient mitigation of CCFs. e CCF value is a pass/fail criterion.
Performance Level (PL) categorizes the quality or eectiveness of a safety channel to reduce
the potential risk caused by dangerous faults within the system with “a” being the lowest and
“e” being the highest achievable performance.
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