Principles, Standards and Implementation

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System Design According to IEC/EN 62061

System Design According to IEC/EN 62061 Subsystem Design: IEC/EN 62061 Transition Methodology for Categories IEC/EN 62061 Terminology Overview Diagnostic Coverage (DC)
Management of Functional Safety Probability of Dangerous Failure (PFHD) Proof Test Interval Safe Failure Fraction (SFF) Systematic Failure
IEC/EN 62061 Terminology Overview

Architectural Constraints

The safety integrity level that can be claimed for a system or subsystem is limited by the architectural characteristics. The two primary characteristics are hardware fault tolerance and safe failure fraction. Secondary characteristics include common-cause faults and fault exclusion.

When combining subsystems, the SIL achieved by the SRCS is constrained to be less than or equal to the lowest SIL Claim Limit of any of the subsystems involved in the safety related control function.

B10 and B10d

For electromechanical subsystems, the probability of failure should be estimated taking into account the number of operating cycles declared by the manufacturer, the load and the duty cycle. The probability of failure is expressed as the B10 value, which is the expected time at which 10% of the population will fail. B10d is the expected time at which 10% of the population will fail to danger.

Common Cause Failure (CCF)

CCF (common-cause failure) is when multiple faults resulting from a single cause produce a dangerous failure. Information on CCF will generally only be required by the subsystem designer, usually the manufacturer. It is used as part of the formulae given for estimation of the PFHD of a subsystem. It will not usually be required at the system design level.

Annex F of IEC/EN62061 provides a simple approach for the estimation of CCF. The table below shows a summary of the scoring process.

No. Measure Against CCF Score
1 Separation/Segregation 25
2 Diversity 38
3 Design/Application/
Experience 		2
4 Assessment/Analysis 18
5 Competence/Training 4
6 Environmental 18
 
Table 16: Scoring Process Summary

Points are awarded for employing specific measures against CCF. The score is added up to determine the common cause failure factor. The beta factor is used in the subsystem models to "adjust" the failure rate.

Overall Score Common Cause Failure Factor (ß)
<35 10% (0.1)
35…65 5% (0.05)
65…85 2% (0.02)
85…100 1% (0.01)
 
Table 17: Common-Cause Failure Factor