Functional safety - Safety instrumented systems for the process industry sector - Guidance for the determination of the required safety integrity levels
This part of IEC 61511 provides information on
– the underlying concepts of risk, the relationship of risk to safety integrity, see Clause 3;
– the determination of tolerable risk, see Annex A;
– a number of different methods that enable the safety integrity levels for the safety instrumented
functions to be determined, see Annexes B, C, D, E, and F.
In particular, this part
a) applies when functional safety is achieved using one or more safety instrumented
functions for the protection of either personnel, the general public, or the environment;
b) may be applied in non-safety applications such as asset protection;
c) illustrates typical hazard and risk assessment methods that may be carried out to define
the safety functional requirements and safety integrity levels of each safety instrumented
function;
d) illustrates techniques/measures available for determining the required safety integrity
levels;
e) provides a framework for establishing safety integrity levels but does not specify the safety
integrity levels required for specific applications;
f) does not give examples of determining the requirements for other methods of risk
reduction.
Annexes B, C, D, E, and F illustrate quantitative and qualitative approaches and have been
simplified in order to illustrate the underlying principles. These annexes have been included to
illustrate the general principles of a number of methods but do not provide a definitive
account.
NOTE Those intending to apply the methods indicated in these annexes should consult the source material
referenced in each annex.
Figure 1 shows the overall framework for IEC 61511-1, IEC 61511-2 and IEC 61511-3 and
indicates the role that this standard plays in the achievement of functional safety for safety
instrumented systems.
Figure 2 gives an overview of risk reduction methods.