ISO 13849: Functional safety of machinery

Determining the achievable PL

A key component in determining the achievable PL is the assessment of the failure probability of the corresponding control components. This is done on the basis of the parameters (discussed in Chapter 6 of the standard):

• PFH (Probability of Dangerous Failure per Hour): average frequency of a dangerous failure per hour).
• Category: Classification of the subsystem of an SRP/CS with regard to its resistance to faults and its subsequent behavior in the event of a fault. 

In addition to the systematic requirements of the components used, their interaction, and a redundancy or test channel, each category requires compliance with certain metrics. These are:

• MTTFD (Mean Time to Dangerous Failure): average operating time until dangerous failure (measure of reliability).
• DC (Diagnostic Coverage): Key figure for the reliability with which a system detects dangerous faults.
• CCF (Common Cause Failure): Probability of faults based on a common cause in redundant systems.

The completely revised and restructured version of ISO 13849-1 was published in April 2023 and now includes more detailed specifications for the design of safety requirements. With Chapter 7, “Software safety requirements,” software now plays a much more important role. The standard addresses various programming languages and also considers different types of software (safety-related application software, safety-related embedded software, parameterization software). The protection of data in machine software is also taken into account.

With the new Chapter 10, Part 1 of the standard is expanded to include the aspect of validation, which was previously only covered in Part 2 (see above). In addition, the new version specifies a number of guidelines, in particular for determining the achievable performance level.

Manufacturers and designers will also find more detailed, concrete assistance for the safety design of a system in the appendix, for example with regard to electromagnetic immunity. For software developers, the verification steps and the results of the phases in the V-model are specified in more detail, covering the entire software safety lifecycle. For programming languages with limited language scope (LVL) that use validated function blocks and run on tested hardware, there is now a simplified, two-stage variant of the V-model.

ISO 13849 is compatible in key aspects with the functional safety standard for machine construction IEC 62061 and the basic functional safety standard IEC 61508. However, IEC 62061 is limited to electrical, electronic, and programmable systems (E/E/PES), while ISO 13849 includes mechanical, hydraulic, and pneumatic components in addition to electrical ones.

The generic standard IEC 61508 also focuses on E/E/PES, but also comprehensively covers quality management and product life cycle (ISO 13849, on the other hand, concentrates on the safety-related design and evaluation of control components). Both the basic standard and IEC 62061 use classifications based on SIL (Safety Integrity Level).

During development, the choice of standard should be made depending on the selected technology and safety requirements. However, since the standards are largely compatible, safety-related parts of machine controls developed in accordance with ISO 13849 may contain components that comply with IEC 62061 or IEC 61508. 

The Machinery Regulation (Regulation (EU) 2023/1230) is a binding set of rules issued by the European Union that will apply in all EU countries from 2027 and replace the previous Machinery Directive 2006/42/EC. It requires that the safety of machines, including safety components, software, and mechanical components, be implemented in accordance with the “state of the art.” In addition, all relevant risks associated with the use of the machines must be reduced to an acceptable level.

ISO 13849 is harmonized under the Machinery Directive 2006/42/EU. Harmonization with the Machinery Regulation 2023/1230 is considered very likely. When applying the standard, it is therefore assumed that ISO 13849-compliant safety-related parts of machine controls meet the basic safety and health requirements of the directive. By applying ISO 13849, manufacturers, operators, and integrators can thus demonstrate that the safety requirements of the binding Machinery Directive or Machinery Regulation are being implemented.