The Simplified Approach

The numerical quantification of the probability of  failure of a subsystem can never be attained exactly, but only by approximation with the aid of statistical methods or other estimations are possible.

Any validated and recognized method can be used for this purpose. Such methods include reliability block diagrams (used in IEC 62061), fault tree analysis, Markov modelling (used in ISO 13849-1) or Petri nets.

However, in general, engineers lacking prior experience in quantification of the probability of failure of safety related control systems require some degree of support. This need was addressed, in ISO 13849-1, by developing a simplified approach, also called simplified method which, whilst being based upon sound scientific principles (Markov modelling), describes a simple method for quantification in successive steps.

The starting point, of the simplified method, is the observation that the majority of safety related control systems can be grouped in very small number of basic types, or to combinations of these basic types.

These types are:

• at one end of the spectrum, the single-channel untested system having components with different reliability level;
• in the middle of the spectrum, the same type, but enhanced by testing; and
• at the other end, the two-channel system featuring high quality testing.

Systems with more than two channels, three contactors in series on the same motor for example, are rare in machinery.

That was the starting point for the development of the probabilistic approach of ISO 13849-1. At the time, it was decided that the five categories of EN 954-1 could cover the majority of Safety Control Systems used in Machinery and, for that reason, continuity was intentionally assured with the previous standard.

EN 954-1 defined five structures as Categories, ISO 13849-1 supplements the former Category definition with quantitative requirements for the component reliability (MTTF_D), the diagnostic coverage of tests (DC_avg) and the resistance to common cause failures (CCF). In addition, it maps the Categories to five basic structural types, termed “designated architectures”.

Therefore, ISO 13849-1 provides a simplified approach, based upon the definition of five designated architectures, that fulfils specific design criteria and behaviour under a fault condition.