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Institute of Measurement and Control. Functional Safety 2016
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In many cases these features exist because they form part of a manufacturer’s safety concept which
adopts the aforementioned techniques and measures listed in IEC 61508. Often these features are
automatically invoked. In some cases there are options which must be selected or they may require
the end users active “participation”. For example the effectiveness of access protection by password
will depend on good discipline on behalf of the end user organisation. The system can enforce a
strong password but can’t stop an engineer from putting it on a post-it on top of the screen or on a
scrap of paper in the top drawer.
Figure 3 Techniques and Measures in Practise
In deciding which techniques and measures to apply it may well still be helpful to refer back to the
tables in IEC61508 for guidance and, once a selection is made, it should be planned and documented
and the selection rationale should be to be recorded.
Example of techniques and measures to be found in IEC 61508 part 2 Annex A & B which form part of
typical SIS and which can be leveraged by systems integrators and end users (reference to relevant
detailed description in 61508 is shown in brackets):-
1. Program Sequence monitoring (A9)
2. Modification protection (B4.8)
3. Input Acknowledgement (B4.9)
4. Computerised specification tools(B2.4)
5. Separation (B1.3)
6. Simulation (B3.6)
7. Modularisation (B3.4)
8. Use of well tried components (B3.3
9. Observance of guidelines & Standards (B3.1)
10. Checklists(B2.5)
11. Operator friendliness (B4.2)
12. Maintenance Friendliness (B4.3)
13. Protection against operator mistakes(B4.6)