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The EN ISO 13849 series of standards describes simple methods for evaluating safety-related control systems for machines. The established PLC disc supports you in estimating the probability of dangerous failure as described in the standard – now also available in digital form as a web application.
Figure 1: After setting the appropriate MTTFD value on the front of the calculator, you can rotate the discs and use the legend at the top to read the resulting PFH value and PL.
Source: IFA
Figure 1 shows the two steps of use: You can easily determine the average frequency of a dangerous failure per hour (PFH) and the corresponding Performance Level (PL) by (1.) rotating the upper disc against the lower disc until the desired mean time to dangerous failure (MTTFD) of a single channel appears in the lower window. In the upper window, you can then (2.) read off the corresponding PFH value depending on the desired category and the desired average diagnostic coverage (DC) for fault detection. To do this, the displayed value must be multiplied by a factor (order of magnitude) corresponding to the colour code in the legend; the colour also indicates which PL the safety-related control system can achieve. In Category 4, for MTTFD values greater than 100 years, you can use the scale printed on the back to determine the PFH.
Figure 2: By adjusting the input values using the two sliders in the web application, you can view the results and check their position in the bar chart.
Source: IFA
The web application is operated using two sliders (see Figure 2): The red one (1.) is used to select the MTTFD, and the blue one (2.) is used to select the appropriate bar based on a combination of category and diagnostic coverage (DC). The selected MTTFD and the resulting PFH value (3.) are displayed in the upper section of the bar chart and marked by an orange dot (4.) on the corresponding bar. This illustrates where you are on the PL scale and what impact variations in the selected parameters might have. The corresponding performance level is marked on the vertical axis of the bar chart (5.). Using the “To the PLC back side” button (6.), you can select MTTFD values greater than 100 years for Category 4.
EN ISO 13849-1 formulates requirements that facilitate the development, validation, and certification of safety-related machine control systems. The standard combines deterministic and probabilistic requirements in a manner suitable for practical application: the category, representing the structure of a control system (e.g., with redundancy and testing), and its probabilistic properties (time to dangerous failure of components: MTTFD, diagnostic coverage of fault detection: DC) are incorporated into the determination of the Performance Level (PL). Markov models have been used as a mathematical method to determine the average frequency of a dangerous failure per hour (PFH) from the numerous input variables. The resulting relationships between the input variables – category, DC, and MTTFD – are presented in the standard in simplified form as a bar chart and as table values. The paper disc and the web application simplify use of these values and make them comparable. They serve to quickly assess which combinations of input variables are suitable for a specific required PL.
IFA Deputy
Accident Prevention: Digitalisation - Technologies
Tel: +49 30 13001-3500
