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Operator Response within Instrumented Safety Functions 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 against initiation / progression of major accident scenarios. Such alarms require more administration and documentation than other, general alarms, and are described as, “highly managed alarms,” (HMA) within relevant good practice, e.g. BS EN 62682.

BS EN 62682 requires that all alarms are classified. Alarm classification is used to set common requirements for managing groups of alarms, e.g. alarm classes which are HMA’s should have additional requirements above other general alarms (e.g. documentation, training, maintenance, testing and management).

Examples of alarm classification groups could be:

No. Alarm Class Type


Equipment reliability / availability

General alarm


Product quality

General alarm (unless GMP is applied, for example, at a pharmaceutical plant)


Low integrity safety instrumented alarm functions, where PFD ≥ 0.1 (RRF 10)

Highly managed alarm (LISIAF)


Safety instrumented alarm functions, where PFD < 0.1 (RRF >10)

Highly managed alarm (SIAF)

Note – only items 3 and 4 are included within the scope of this guidance.
An instrumented alarm function identified as providing risk reduction against a major accident hazard (MAH) should:

An instrumented alarm function is represented diagrammatically as follows:

SIAF flow chart

Low Integrity Safety Instrumented Alarm Functions (LISIAF)

A low integrity safety instrumented alarm function (LISIAF) provides safety risk reduction (i.e. reduces risk by a factor of up to 10) and is also classified as a low integrity instrumented safety function, where the operator takes the appropriate action in the event of an alarm activating. HSE Operational Guidance – OG46 provides guidance on the management of low integrity instrumented safety functions and would be applicable to LISIAF’s. BS EN 62682 is recognised as relevant good practice for alarm systems in process industries. LISIAF’s could be considered to be one class of highly managed alarm.

Safety Instrumented Alarm Functions (SIAF)

A safety instrumented alarm function (SIAF) provides significant safety risk reduction (i.e. reduces risk by a factor of more than 10), and is also classified as a safety instrumented system (SIS).  Relevant 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 activating. 

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 BS EN 62682.  



Onshore and offshore major hazard installations.




To be used by CEMHD E, C & 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)


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


Updated 2018-05-16