How to easy calculate Safety Integrity Level by IEC61508, IEC61511, OLF070 Guideline


This article describes how to easy to calculate Safety Integrity Level (SIL) according IEC61508, IEC61511 and OLF070 Guideline standards by using MidTechnology approach (SIL Toolbox)

As you know there are a lot information how to determine Safety Integrity Level using IEC61508 and IEC61511, but this calculations is rather difficult and little bit confusing for engineers. Sometimes we need make small changes in loop and see how this changes influence on SIL level.
To solve this problem, company MidTechnology AS developed SIL toolbox application which allows easy to calculate Safety Integrity Level simply.

For example take loop from picture below and we need to calculate SIL for this loop:
SIL Loop Schema

Base approach

The core idea of SIL Toolbox is present (or transform) loop schema as a tree. Every loop can be represented as a hierarchical tree structure.
Let's do this with our loop. First of all, split loop into 3 subsytems:

  • Sensors
  • Logic
  • Final elements subsystem
Creating loop tree structure and specifying needed parameters (instruments, voting, test intervals) toolbox will calculate SIL level for this loop.

Sensor subsystem

Look at sensor subsystem: Sensor subsystem consists of 2 independent equal parts( transmitter chains) to provide redundancy. So sensor subsystem has voting 1oo2. SIL sensor structure IEC61508
Each of transmitter chain has voting 2oo2.
In the picture you can see the main idea. On the right side tree representation is shown from SIL toolbox application: SIL sensor subsystem as tree IEC61508

Logic subsystem

SIL logic subsystem as tree IEC61508

Final elements subsystem

SIL final elements subsystem as tree IEC61508

Summary loop structure

This is a summary loop tree structure which we created in SIL toolbox:
SIL loop tree IEC61508
Loop visualisation in application:
SIL loop schema IEC61508

Specifying element's parameters

For each element in the loop you have to specify primary values. For 22PT1428A you have to set on the following paramaters: Beta, Voting, Full Test Interval, Partial Test Coverage, MTTR, Instrument. Notice what selected instrument contains UnitType, Beta, BetaD, Ls, Ld, Ldu, Ldd, SFF: SIL tag primary parameters IEC61508

Safety Integrity Level Calculation

To calculate SIL you should press button Calculate and you will get determined Safety Integrity Level and detailed calculation report:
SIL tag primary parameters IEC61508


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