Overview
ISO/TR 13849-3:2026 is a technical report from the International Organization for Standardization (ISO) that provides advanced methods for the calculation of the Probability of a Dangerous Failure per Hour (PFH) of safety-related parts of control systems in machinery. It enhances the approaches outlined in ISO 13849-1, specifically focusing on Markov model-based PFH calculations for both single-channel and two-channel architectures, with and without diagnostics.
This document serves as an alternative to the simplified PFH estimation procedures in ISO 13849-1 and offers greater flexibility and precision by using Markov models. These methods support improved risk assessment and reliable performance level (PL) determination for safety-related control systems.
Key Topics
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PFH Estimation Formulas
Provides comprehensive formulas to estimate the PFH value applicable to different architectures:
- Single-channel untested and tested systems
- Two-channel systems with and without diagnostics
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Markov Modelling
Utilizes Markov models to represent the behavior and failure modes of safety functions, enabling more accurate PFH calculations compared to traditional methods.
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Parameters and Variables
Explains the key variables and parameters (e.g., MTTF, diagnostic coverage, common cause factor, mission time) fundamental for PFH determination.
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Diagnostic Coverage (DC)
Addresses the role of diagnostics in reducing PFH and explains how diagnostics are modeled in both single and multi-channel systems.
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Basic Assumptions
Describes boundary conditions such as constant failure rates, series subsystems, and handling of repairs, providing a transparent basis for calculations.
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Architecture Types
Details relevant architectures in accordance with ISO 13849-1 categories (B, 1, 2, 3, 4), including:
- Single-channel (1oo1, 1oo1D)
- Two-channel (1oo2, 1oo2D) configurations
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Examples and Derivations
Includes informative annexes with examples of applying the Markov formula-based approach and explanations of the mathematical derivations.
Applications
ISO/TR 13849-3:2026 is valuable for professionals involved in machine safety, functional safety engineering, and the design or assessment of machinery control systems. Common use cases include:
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Design of Safety-Related Control Systems
Supports engineers in developing control circuits that achieve targeted performance levels, optimizing both safety and efficiency.
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Risk Assessment and Validation
Enables more precise evaluation of residual risks and validation of safety functions, particularly when:
- Using unconventional mission times
- Addressing asymmetric redundancy
- Customizing diagnostic intervals
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Documentation and Compliance
Assists manufacturers and safety assessors in generating clear documentation for regulatory compliance and third-party certification.
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Alternative to Table-Based Methods
Useful when the table-based approach in ISO 13849-1 does not provide sufficient flexibility or yields overly conservative or imprecise results.
Related Standards
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ISO 13849-1:2023
Safety of machinery - Safety-related parts of control systems - Part 1: General principles for design
(Provides the general requirements and simplified PFH procedures.)
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IEC 62061
Safety of machinery - Functional safety of safety-related electrical, electronic and programmable electronic control systems
(Presents equations for PFH calculation and focuses on quantitative assessments.)
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IEC 61508 Series
Functional safety of electrical/electronic/programmable electronic safety-related systems
(Defines PFH and broader requirements for functional safety, including use of Markov models.)
ISO/TR 13849-3:2026 can be utilized alongside these standards to provide enhanced, flexible, and precise PFH calculation, supporting sophisticated and robust machinery safety solutions.
Keywords: Markov model, PFH calculation, ISO 13849-1, safety of machinery, performance level, diagnostic coverage, single-channel, two-channel, safety-related control systems, risk assessment, functional safety, safety circuit design