Article | REF: SE4058 V3

Probability of failure of the safety function

Author: Olivier IDDIR

Publication date: September 10, 2024

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ABSTRACT

Today , IEC 61508 and IEC 61511 are the central standard for specification, design and operation of Safety Instrumented Systems (SIS). There are four levels ranging from SIL 1 the lowest level to SIL 4 the highest level of risk reduction. Safety Integrity Levels are order of magnitude bands of risk reduction. IEC 61508 and IEC 61511 contains a lot of useful information and guidance for safety improvement regarding the use of safety systems. This paper discusses the methodology to assess the probability of failure on demand with simplified formulas or fault tree.

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AUTHOR

  • Olivier IDDIR: Risk quantification engineer – Expertise and modelling department – Member of TECHNIP's network of experts - TECHNIP France, Paris-La Défense, France

 INTRODUCTION

In order to prevent dangerous phenomena such as fires, explosions or the release of hazardous materials from causing damage to people, the environment or property, manufacturers are required to implement risk control measures (RCMs) whose role is to prevent the occurrence of such phenomena or to limit their consequences.

Among these layers of protection are safety instrumented systems (SIS), which enable the implementation of safety instrumented functions (SIF).

Different layers of protection can be implemented to reduce risks and make them acceptable. These different layers are evaluated during quantitative or semi-quantitative analyses carried out to determine the required SIL level of RIS (SIL review carried out using methods such as LOPA or the risk graph, for example).

Once the required SIL level is known, the analyst must demonstrate that the probability of failure allows verification of the required SIL level allocated during the SIL review. To do this, IEC 61508 and IEC 61511 can be used to :

  • define the RIS architecture to meet a targeted SIL level;

  • estimate the probability of RIS failure.

It is important to stress that the calculation formulas presented in Annex B of IEC 61508-6 are for information purposes only, and that other methods can be used to assess the probability of failure of a RIS.

After some basic information on the IEC 61508 and IEC 61511 standards, this article first gives an overview of the methods used to estimate the probability of failure of a RIS, then reviews the architectural constraints introduced in the IEC 61508 and IEC 61511 standards.

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KEYWORDS

Safety Instrumented Systems (SIS)   |   Redundancy   |   Probability of Failure on Demand (PFD)   |   IEC 61508   |   IEC 61511   |   Common cause failure

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Probability of failure of a safety instrumented function under load
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