Safety-related parts of control systems

Status:

completed 09/2007

Aims:

The method for the assessment of safety-related control systems described in the generic machinery standard EN ISO 13849-1: "Safety-related parts of control systems - Part 1: General principles for design" has become substantially more complex as a result of the standard's revision. The definition of the "category" (structure, redundancies, tests), which was required prior to the revision, has now been supplemented by the assessment of component reliability (MTTF_d), diagnostic coverage (DC) and resistance to common-cause failure (CCF). Future users therefore have a major need for advice. In particular, the transfer of a technical control plan (schematic diagram, circuit diagram) into a logical block diagram requires explanation. As an aid to users, the new methods are therefore described with the aid of several model safety controls embodying a range of technologies. The models represent a comprehensive resource for consultancy, seminars and publications. At the same time, solutions already available on the market are examined in consideration of the new provisions of the standard.

Activities/Methods:

In co-operation with the industry and with expert committees, a search was first conducted for typical modern control architectures in machine construction. Of those found, some 30 to 40 representative control solutions were selected. Block diagrams of these solutions and descriptions of the functions were then produced. Based upon these, the individual steps to be followed during analysis of the performance level (PL) of EN ISO 13849-1 are then illustrated. The safety controls selected as models cover all safety levels (performance levels a to e) and all control technologies (mechanical, hydraulic, pneumatic, electrical, electronic and programmable electronic). The combination of control components from different categories or employing different technologies and the associated quantification of the overall performance level are also illustrated by means of the models.

Results:

Inspection of BGIA Report 6/97 revealed that 25 examples of controls are still relevant in the field. These were adopted and thoroughly revised. In addition, 12 new model circuits were selected in consultation with expert committees and manufacturers. The collection of examples covers a wide spectrum of safety-related machine control systems used in the field. The following information was prepared for each model circuit: a block diagram showing the essential safety-related components, the definition of the safety function, a function description, the chief design features, comments on use, and a bibliography. In addition, the quantification of the probability of failure, a new aspect, was described in detail. Quantification is based upon the following parameters: structure (category), component reliability (MTTF_d), diagnostic coverage (DC) and measures to prevent common-cause failure (CCF). This subject is further illustrated by the reproduction of a safety-related block diagram and a screenshot of SISTEMA, the BGIA's own software application, which assists in analysis. With these 37 cross-sector examples, which are all included in the new BGIA Report on safety-related machine controls, the BG Institute for Occupational Safety and Health (BGIA) and the BG expert committees have a comprehensive resource at their disposal for training and consultancy. The application in practice of all methods described in the standard is shown, for example with regard to the particular aspects of the various technologies. Some of the examples have already been used in various papers.

Further informations:

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