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Operator Response within Safety Instrumented Systems in the Chemical, Oil & Gas, and Specialist Industries


This document provides guidance on the expected standard for instrumented safety functions that require an operator response to an alarm at hazardous installations.


Duty holders often identify instrumented alarm functions as providing prevention or mitigation risk reduction of major accidents.

When an alarm function provides significant safety risk reduction (i.e. reduces risk by a factor of more than 10), it is also classified as a safety instrumented system (SIS).  Good practice BS EN 61511 does allow an operator as part of a SIS, i.e. cases where the operator takes the appropriate action in the event of an alarm sounding. 

A safety instrumented alarm function (SIAF) is defined in this document as follows:

A SIAF is represented diagrammatically as follows:

SIAF flow chart

Reference should be made to BS EN 61511 for definitions of other acronyms and terms.

It should be noted that this guidance:


Inspectors should:


For the purpose of the Enforcement Management Model, this guidance is an interpretative standard.

Relevant good practice is provided by BS EN 61511 and EEMUA191.  



Onshore and offshore major hazard installations.




To be used by HID EC&I Specialist Inspectors during established intervention processes.

Recording & Reporting

No special requirements.

Health & Safety

No special requirements.


No special requirements.

Further References

Relevant Acts and Regulations

Relevant Good Practice

Related Documents

(Note – this is not an exhaustive list)


HID Chemicals, Explosives and Microbiological Hazards Division 2E Electrical, Control and Instrumentation Team.


Updated 2015-05-05