Since its release as an Australian standard in July of 2004, AS61511 is rapidly being accepted and applied on Safety Instrumented Systems throughout the process industry. Principles such as independence between control and protective instruments have existed for many years, however they continue to often be overlooked even with the introduction of this standard.
Older prescriptive standards particularly in burner / boiler applications recognize the need for independence between control & safety. With older technology it was more difficult to share signals. Separate switches were often used to provide the shutdown function and alarming function.
Today’s improved technology has made it easier to share signals. Signal isolators and splitting devices are often used to share transmitter signals between control and safety instrumented systems (SIS). Splitters are also available with certain SIL ratings thereby giving the impression that signals can be safely shared.
AS61511 requires an assessment to be made that considers the independence of protection layers. This is particularly important when a failure in the control function can cause a demand on the safety function. This is the critical aspect often overlooked; a further examination of the problem will clarify the issue.