Specificities of safety PLC based implementation of the safety functions

There are many off-the-shelf programmable logic controllers (PLC) available on the market - not only standard, but so-called safety PLCs as well. These are primarily designed to be used in industrial safety relevant applications. Not so long ago, the use of PLCs was uncommon in the applications, which perform safety functions. An identification of required safety functions, together with their safety integrity level (SIL) is risk-based, which involves a risk analysis of the controlled process. Implementation of the safety functions with the help of the safety PLC followed by a safety evaluation is associated with some very specific problems. A few of these problems lie in the scope of this paper.