Question from the PROFIsafe Designer Workshop in October 2023:
We briefly heard about a "communication PFH" or "allocating the communication PFH values / residual error rates". What is this about? Do we have to calculate this "communication PFH"?
"Communication PFH": What is it, and how is it calculated?
The source for this concept is IEC 61784-3. Subclause 5.1 describes the decomposition of a safety function into input/sensor, logic and output/actuator subsystems. These subsystems are connected by a "logical connection" each, i.e., a functional safe communication protocol like PROFIsafe. It is recommended that each of these connections does not "consume" more than one percent of the maximum PFH of the whole safety function.
For SIL3 this means that the PFH for the whole safety function shall be lower than 10^-7 errors per hour. One percent means lower than 10^-9 errors per hour.
The good part is that there is no further calculation to be done, since a correct implementation of PROFIsafe is guaranteed to meet this target value. This means that PROFIsafe may be used for SIL3 applications, which is what it claims.
The same is true for the analogous PFD values for low demand safety functions.