SYTCOM - Assessment of ISO 13849-1 parameters

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RISK ASSESSMENT IN ACCORDANCE WITH ISO 14121


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SYTCOM - Assessment of ISO 13849-1 parameters

Estimates for Common Cause Failures (CCF)

F.1 CCF requirements
A complete procedure for CCF prevention measures, covering sensors / actuators on the one hand, and the control logic on the other hand, is given for example in IEC 61508-6: 2000, Annex D. The measures provided in this regard are not all applicable to the
NOTE: In this part of ISO 13849 and in compliance with IEC 61508-6: 2000, Annex D, factor ß for redundant systems should be generally less than or equal to 2%.
F.2 CCF impact estimation
This quantitative method should be applied to the overall system. Each part of the elements that relate to the safety of the control system should be considered.
Table F.1 lists the measures and contains the associated values, based on an expert assessment, which represent the contribution of each measure to reducing the common cause failures.
For each listed measure, only all-or-nothing results are to be declared. If a measure is only partially completed, the resulting score is zero.
Table F.2 shows a CCF quantification.

Table F.1 - Scoring process for CCF countermeasures

No. CCF countermeasure Score
1 Separation / Isolation
Physical separation between signal channels:
     separation in wiring, piping,
     sufficient clearance and creepage distances on the printed circuit boards.		 (sur 15)
2 Diversity
Different design principles / technologies or physical principles are used, for example:
     first electronic channel is programmable and second channel is wired
     sort of initiation
     pressure and temperature
Measuring the distance and pressure, for example:
     digital and analogue,
Components from different manufacturers		 (sur 20)
3 Design / application / experimentation
3.1 Protection against overvoltage (surge protection), pressure (pressure switch), overcurrent (fuse, circuit breaker), etc.. (sur 15)
3.2 Using proven components (sur 5)
4 Assessment / analysis
Are the results of an analysis of failure modes and their effects taken into account in order to prevent common cause failures at the design stage? (sur 5)
5 Skills / training
Are the maintenance specialist designers trained to understand the causes and consequences of common cause failures? (sur 5)
6 Environment
6.1 Prevention of contamination and electromagnetic compatibility (EMC) CCF countermeasures in accordance with the relevant standards
Fluid systems: filtration of the medium under pressure, prevention of absorption of impurities, drainage of compressed air, for example in accordance with the component manufacturer's requirements regarding the purity of the medium under pressure,
Electrical systems: has the electromagnetic immunity of the system been verified, for example as specified in the applicable product standards?
For combined fluids and electrical systems, both aspects should be taken into consideration.		 (sur 25)
6.2 Other influences
Have the requirements relating to immunity against all relevant environmental influences such as temperature, shock, vibration, humidity been taken into account, for example as specified in the applicable standards?		 (sur 10)
TOTAL

Table F.2 - CCF quantification

Total score CCF countermeasuresat
65 or better Satisfies the requirements
Less than 65 Method fails => choose additional measures
a  Where technical provisions are not relevant, the points detailed in the right hand column can be considered as a complete calculation.

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SYTCOM - Assessment of ISO 13849-1 parameters

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MTTFd Moy DC Moy PL calculated PFHd according to EN62061 SIL CL for information purposes only
         
         
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