​Common Cause Failures Analysis
IEC 61508- IEC 61511
-EN 13849 -EN 62061

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What are Common Cause Failures? (Common Cause Failures)

A common cause failure occurs when more than one failure has the same source. Common cause failures are either common event failures, where the cause is a single external event, or common mode failures, where two systems fail in the same way for the same reason.

Within the state-of-the-art technology of RAMS Engineering, there are different techniques for modeling common cause failures as well as specialized software for failure models. For example, the beta factor model is defined in the IEC 61508 standard. The IEC 61508 standard also proposes a kind of checklist based on 37 questions whose analysis and answers help to reduce common cause failures.

Common mode failures can occur at different times due to a design fault or a repeated external event. Common event failures reduce the reliability of systems with on-line redundancy but do not reduce the reliability of systems using off-line redundancy. Common mode failures reduce the reliability of systems using off-line redundancy and on-line redundancy.

A common cause failure (CCF) is a single failure event that affects more than one component or function of a system. Common cause failures are an important part of any reliability or hazard model because they attempt to negate the improvements offered by redundancy. They are often the largest contributors to risk levels and should therefore always be carefully considered.

There are many system analysis methods that offer ways to consider common cause failures. However, these methods tend to be simplistic, such as taking the percentage of component failures and attributing them to common causes. Certain assumptions are often made, such as that components will all share the same common causes or have the same failure rate and distribution. The beta factor model, for example, is a very widely used method found in standards such as IEC 61508. To calculate the failure rate due to common causes, the beta factor is multiplied by the component failure rate.

The Importance of Considering Common Cause in Redundant Systems

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Within the scope of BEKRA regulation, CCF analysis is mandatory when creating Major Industrial Accident Scenarios.

For redundant systems, the resulting PFDavg and Risk Reduction (in Low Demand mode) or PFH (in high or continuous mode) can have a significant impact. Common cause failures represent events where multiple failures occur within a short period of time due to a common cause. This is often called a fundamental effect. Its operation and importance are particularly relevant to redundant systems, as its impact can completely negate the advantages of such complex architecture. Today, "all" functional safety standards require that common-cause failures be taken into account, regardless of the field of industry or field of application.

It is therefore important to include a Beta Factor when calculating PFDavg or PFH for a redundant set of equipment, otherwise we may be overestimating the amount of risk mitigation provided by the Safety Instrumented Function (SIF) and hence lead to overly optimistic results.

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Major Accident Scenario Document Communiqué According to the Regulation on Preventing Major Industrial Accidents and Reducing Their Effects
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Obligations:

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The operator of a lower or higher level organization prepares or has a major accident scenario document prepared by complying with the specified format and general rules and fulfilling the minimum requirements. The major accident scenario document is not sent to the Ministry of Family, Labor and Social Services, but is kept in the organization.

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NOTIFICATION OF MAJOR ACCIDENT SCENARIO DOCUMENT TO BE PREPARED REGARDING MAJOR INDUSTRIAL ACCIDENTS

Annex-1 FORMAT AND GENERAL RULES OF MAJOR ACCIDENT SCENARIO DOCUMENT

8.4. Principle of independence and effectiveness
In calculating the frequency of occurrence of scenarios, the principle of independence, the effect of common cause errors and the principle of effectiveness are also taken into account.

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MINIMUM REQUIREMENTS FOR MAJOR ACCIDENT SCENARIO DOCUMENT:

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• Major accident scenario document team: A team should be formed to prepare the scenario document.

• Steps for preparing scenarios: Steps such as identification and classification of hazardous substances, identification of hazardous equipment, identification of internal hazards, identification of external hazards, creation, analysis and evaluation of major accident scenarios are followed.

• Identification and classification of hazardous substances: Hazardous substances are identified and classified. The SEA Regulation is taken as basis.

• Identification of hazardous equipment: Hazardous equipment is identified and listed.

• Identification of internal hazards: Internal hazards related to hazardous equipment are identified.

• Identification of external hazards: External hazards in the organization's environment are identified.

• Creation, analysis and evaluation of major accident scenarios: Scenarios are created, frequencies are calculated, compared with limit values and necessary measures are decided.

• Other issues: Other important issues such as critical equipment, frequency calculation methods, operator errors, independence and efficiency principles are stated.

The steps to be followed to prepare the major accident scenario document are explained in detail below:

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Step 1: Identification and Classification of Hazardous Substances

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Step 2: Identifying Hazardous Equipment

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Step 3: Identifying Internal Hazards

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Step 4: Identifying External Hazards

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Step 5: Creation, Analysis and Evaluation of Major Accident Scenarios

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This document specifies how the major accident scenario document should be prepared in accordance with the requirements of the "Regulation on the Prevention of Major Industrial Accidents and Reducing Their Effects" and the limit value of the major industrial accident frequency.

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Limit Value of Major Industrial Accident Frequency:

For hazardous equipment that may cause major accidents, the operator reduces the frequency of occurrence of each major industrial accident scenario to 1x10-4/year or less.

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This document explains how an enterprise should prepare its major accident scenario document in accordance with these regulations and what kind of values are targeted in order to prevent major industrial accidents and reduce their effects.

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