How to design safe machine control systems - A guideline to EN ISO 13849-1
The aim of this report is to give guidance when applying EN ISO 13849-1:2008 in projects, both for companies developing subsystems and for companies that are developing complete machines.
The report will give support in different areas in EN ISO 13849-1:2008 that are difficult to understand or parts that are described briefly.
The first part of the report gives some general information about the new EU machinery directive 2006/42/EC.
The following part of the report is focused on management of functional safety which means how to maintain a high degree of safety during the different steps of the safety lifecycle, all the way from risk assessment until modifications of the safety function is done.
The next part of the report describes shortly how to perform a risk assessment and define an appropriate PLr (Performance Level required) for each identified safety function.
A central part of EN ISO 13849-1:2008 is to choose an appropriate category for the identified safety functions. Categories were used also in the earlier machinery safety standard EN 954-1:1996.
The report describes in detail the meaning of each category and also gives an example of a category 2 safety function.
The next step after the identification of an appropriate category is to determine the hardware reliability for each safety function. The report gives both background information about reliability theory and how to perform these calculations in practice.
Diagnostic coverage is another important are in EN ISO 13849-1:2008 that together with the category and MTTFd determines which PL that is possible to reach.
The report gives a number of examples on how different diagnostic coverage techniques can look like.
The report also briefly discusses systematic failures, what it means and how to handle these during design and use of safety functions.
Software requirements are another area that is described in the report, where the report describes the difference between different kinds of programming languages and what it means to follow the V-model defined in EN ISO 13849-1:2008.
Finally the report describes a number of different methods to check that the PLr is reached.
This report shall be considered as an complement to the standard EN ISO 13849-1:2008 that gives examples on how different requirements can be interpreted.
Johan Hedberg Andreas Söderberg Jan Tegehall
SP Technical Research Institute of Sweden
SP Rapport 2011:81