Performance Level Calculator - PLC

for safety related control systems

To easily determine the performance level (PL) of safety related control systems for machinery the PLC disc can be used. It has been developed by IFA with support of ZVEI (German Zentralverband Elektrotechnik- und Elektronikindustrie - Fachverband Automation) and VDMA (German Verband Deutscher Maschinen- und Anlagenbau e.V.).

How to use the disc
The principles laid down in EN ISO 13849-1 are visualised by means of two movable layers which are fixed in the centre. The performance level (PL) can be easily determined by first turning the upper disc until the desired Mean Time to Failure dangerous (MTTF_d) of one channel appears in the lower window. After that chose the desired category and diagnostic coverage (DC) besides the upper window and read the respective figures. The displayed value has then to be multiplied by a factor (order of magnitude) according to the legend; thus you will get the average probability of a dangerous failure per hour of the safety-related control system. The colour code indicates which order of magnitude has to be chosen and which PL is achieved.

Background

The revision of ISO 13849-1, formerly known as EN 954-1, gives guidelines to develop, validate and certify in a simple way safety-related control systems in the field of machinery. Deterministic and probabilistic requirements are combined in a practicable manner: The category of the control system (e.g. redundancy and testing) and its probabilistic properties (failure rates of components called MTTF_d, diagnostic coverage of tests called DC, common cause failure rates) underlie the determination of the so-called "Performance Level" (PL). Markov-models are used to determine the average probability of a dangerous failure per hour for designated architectures. The relation is shown in figure 5 and in table K.1 of EN ISO 13849-1.