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System Design According to IEC/EN 62061

Probability of Dangerous Failure (PFH_D)

Part of the requirements needed to achieve any given SIL capability for a system or subsystem is data on PFH_d (probability of a dangerous failure per hour) due to random hardware failure. Table 12 gives the probability ranges for each SIL.

This data will be provided by the manufacturer. Data for recent Rockwell Automation safety components and systems (e.g. GuardLogix, GuardPLC, SmartGuard, Kinetix with GuardMotion) is already available. Data for other Rockwell Automation safety components and systems will become available during 2007.

IEC/EN 62061 also makes it clear that reliability data handbooks can be used if and where applicable.

For low-complexity electromechanical devices, the failure mechanism is usually linked to the number and frequency of operations, rather than just time. Therefore, for these components, the data will be derived from some form of lifetime testing; e.g. B10 testing. Application-based information such as the anticipated number or operations per year, is then required in order to convert the B10_d or similar data to MTTF_d (Mean-Time-To-Dangerous Failure). This, in turn, is then converted to PFH_d.

In general, the following can be assumed:

PFH_d = 1/MTTF_d

And for electromechanical devices:

MTTF_d = B10_d/(0.1 x mean number of operations per year)