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With the publication of Technical Report ISO/TR 13849-3:2026, machine manufacturers now have, for the first time, a standard-based method for calculating the probability of hazardous failure (PFH) of safety functions using Markov models. The report complements EN ISO 13849-1 and opens up new possibilities for a more realistic and flexible assessment of safety functions, particularly in the case of more complex architectures.
In our short information, we provide an overview of the key questions regarding the document.
General information on the document
The ISO/TR was published on 20 March 2026 by ISO, the International Organisation for Standardisation, and has since been available for purchase directly from ISO or via the national standards organisations. There are no plans to adopt it as a European document (in which case it would be a CEN/TR), so the technical report will not be published in the Official Journal of the EU.
The document is currently available in English only; a German-language edition could only be produced if it were adopted at national level by DIN or Austrian Standards. However, there are no plans for this at present.
Why was ISO/TR 13849-3 developed?
EN ISO 13849-1 is currently the most widely used standard for the assessment of safety-related parts of control systems (SRP/CS). To determine the Performance Level (PL), it uses a simplified method based on architectural specifications, MTTFD, diagnostic coverage (DC), common cause failures (CCF) and tabulated PFH values.
This method has proven its worth in practice, but is based on several fixed constraints. For example, assumptions regarding mission time, the CCF factor or the treatment of redundant systems are structurally predetermined. Whilst this makes the method easy to apply, it offers only limited flexibility.
This is precisely where ISO/TR 13849-3 comes in. The Technical Report expands the possibilities for PFH calculation through mathematical formulae based on Markov models. The aim is to combine the advantages of more precise modelling with the architectural philosophy of ISO 13849. The authors explicitly regard the report as a supplement to and further development of the methods described in ISO 13849-1 and IEC 62061.
What are Markov models?
Markov models are stochastic models used to describe systems that can transition between different states. For the safety analysis of a control system, these states might, for example, be ‘fault-free’, ‘dangerous fault detected’ or ‘dangerous fault undetected’. The transitions between these states are described using mathematically defined failure and test rates.
The key advantage of these models is that they can realistically represent even complex interactions between channels, diagnostics and tests. Markov models have been used in functional safety for many years and are a suitable method for reliability analysis in accordance with IEC 61508.
What benefits does the new approach offer to machine manufacturers?
Of particular interest to machine manufacturers is the greater flexibility compared with the simplified procedure set out in ISO 13849-1.
Among other things, the Technical Report allows for the consideration of:
As a result, the PFH calculation can be carried out much more closely in line with the actual system characteristics. Particularly in the case of innovative control architectures or electronic diagnostic systems, this can lead to a more robust assessment of the safety level.
What styles of architecture are being considered?
The Technical Report describes formulas for calculating the MTBF of:
In addition, the document contains application examples and the mathematical derivation of the equations used. This provides clarity and makes it easier to understand the underlying relationships.
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It is important for users to note that ISO/TR 13849-3 does not replace EN ISO 13849-1. The requirements relating to categories, architectural principles, diagnostics, measures against systematic errors and software continue to be set out in EN ISO 13849-1.
The Technical Report deals exclusively with the quantitative demonstration of the PFH and thus with one aspect of the PL determination. All other requirements of EN ISO 13849-1 must continue to be met without change.
The new method can therefore be regarded as an alternative procedure for determining the PFH, whilst the fundamental system assessment continues to be carried out in accordance with EN ISO 13849-1.
Implications for machine manufacturers
For many standard applications, the tried-and-tested procedure in accordance with EN ISO 13849-1 remains a cost-effective and practical solution. However, the new Technical Report extends this approach to include additional options, particularly where the simplified assumptions of the standard do not adequately reflect the actual design of more complex safety-related control systems.
The extended calculation methods enable developers of safety-related control systems to model complex safety architectures more realistically, improve the traceability and assessment of diagnostic concepts, provide greater flexibility in defining mission times and test strategies, and bring the calculation methodology closer to the procedures set out in IEC 62061 – whilst retaining the tried-and-tested architectural philosophy of ISO 13849.
Conclusion
ISO/TR 13849-3:2026 provides the mechanical engineering industry with a powerful tool for PFH calculation. The report supplements the tried-and-tested EN ISO 13849-1 with a mathematically sound approach based on Markov models. This bridges the gap between the table-based method of ISO 13849-1 and the formula-based methods of IEC 62061.
This opens up the possibility for manufacturers developing modern safety architectures – or those reaching the limits of simplified methods – to assess safety functions more precisely and flexibly, without departing from the normative framework of EN ISO 13849.
Posted on: 2026-07-09
Several years of experience as a CE coordinator and technical editor in the field of food machinery. Responsible for coordinating the conformity assessment procedure for machinery, assemblies of machinery and equipment. Management of risk and hygiene assessments. Contact person for material compliance and technical documentation in an international company.
E-Mail: robert.boenisch@ibf-solutions.com
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