Frequently Asked Question
I have a question about the 1oo2D voting, and how it effects the PFD Average and subsequent Proof test guidelines for sensor groups.
Last Updated 3 years ago
A 1oo2D voting indicates that there is a 1oo2 voting with an additional, independent, diagnostic output switch that can transition the voting arrangement to a safe state. Typically for sensors, there are no additional diagnostic outputs in addition to the 4-20 mA signal (a digital relay output that shows trip or no trip is not a diagnostic output).
PFDavg tends to go down with a 1oo2D voting as a result of faults being identified and the system transitioning to the safe state or flagging dangerous failures as detected. The voting arrangement is highly depending on good diagnostics. Proof tests should be independent of the actual voting arrangement, i.e. the proof test of a transmitter does not change between a 1oo1 and 1oo2 voting.