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# My supplier has given me PFD values only (λD missing)

PFD (or unavailability, Un) is defined as:

Un ≅ λMDT

In the context of Safety Instrumented Functions (SIFs), Mean Down Time (MDT) is typically taken as the midpoint of the proof test interval, hence:

PFD ≅ Un ≅ λ * (Tp / 2)

A PFD value cannot therefore be assigned to an element without the proof test interval (Tp) and the appropriate failure rate, λ.

If you have a PFD value for an element that you wish to use in SIL Comp©, the following relationship provides an approximate and conservative λD value, which can then be assigned to an element:

λD ≅ PFD / (Tp / 2)