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

Preface

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:

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

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.
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:

Logic subsystem

Final elements subsystem

Summary loop structure

This is a summary loop tree structure which we created in SIL toolbox:

Loop visualisation in application:

Specifying element's parameters

Safety Integrity Level Calculation