© SBM Offshore 2012. All rights reserved.
www.sbmoffshore.com37
SRS for an inhibit SIF
• Use terminology “inhibit” not permissive. It causes great confusion to refer to a
permissive. The SIF is an inhibit function!
• Consider sensor failure state – typically failed sensor = inhibit.
• Consider software latch of the inhibit signal. Don’t want transmitter latching in
the “no inhibit” state. (PSHH can be the safe state)
• SIL3 requires HFT of 1. This means two final elements!