Operator Response within Safety Instrumented Systems in the Chemical, Oil & Gas, and Specialist Industries
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- Fully Open
- Publication date
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- Guidance owner
HID Chemicals, Explosives and Microbiological Hazards Division 2E Electrical, Control and Instrumentation Team
- Target Audience
HID Electrical, Control & Instrumentation (EC&I) Specialist Inspectors
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:
- Reduces the risk of a major accident hazard by prevention or mitigation
- Reduces the risk of a major accident hazard by a factor of more than 10 (i.e. PFD<0.1)
- Includes process sensing components and an alarm annunciator and all necessary interconnecting equipment
- Includes an operator response by a human
- Includes output elements (operated directly or indirectly by the operator) and all necessary interconnecting equipment
- Operates in demand mode (as defined in BS EN 61511)
A SIAF is represented diagrammatically as follows:
Reference should be made to BS EN 61511 for definitions of other acronyms and terms.
It should be noted that this guidance:
- Does not cover instrumented alarm functions which reduce the risk of a major accident hazard but have been assessed as requiring to reduce the risk by a factor of 10 or less (i.e. PFD≥0.1). However it should be noted that such functions and the alarm systems in which they are implemented should meet general good practice requirements (e.g. EEMUA191)
- Does not cover what reliability should be assigned to an operator’s ability to perform a task. Note the use of human reliability assessment (e.g. ‘Human Error Assessment and Reduction Technique’ (HEART)) requires expert application and is subject to user error and therefore is outside the scope of this guidance.
- Does not cover reliability of operators carrying out task, which, if they fail, would be the initiating event for a major accident hazard.
- Does not cover reliability of personnel carrying out other tasks such as maintenance, proof test, design etc.
- Does not take precedence where application specific guidance is available (e.g. Buncefield type storage tanks)
- use the guidance described in Appendix 1 to verify, or otherwise, that relevant good practice has been applied;
- refer Duty Holders to the guidance described in Appendix 1 so that it is clear how to apply good practice.
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.
Relevant Acts and Regulations
- Health and Safety at Work Act 1974 (as amended)
- Offshore Installations (Safety Case) Regulations 2005
- Offshore Installations (Prevention of Fire and Explosion, and Emergency Response) Regulations 1995
- Control of Major Accident Hazards (COMAH) Regulations 1999 (as amended)
- Management of Health and Safety at Work Regulations 1999 (as amended)
- Provision and Use of Work Equipment Regulations 1998 (as amended)
Relevant Good Practice
- BS EN 61511:2004 Parts 1-3 Functional safety – Safety Instrumented Systems for the Process Industry Sector
- Engineering Equipment and Materials Users’ Association (EEMUA) Publication ‘191’:
- Alarm Systems – A Guide to Design, Management and Procurement. (ISBN 0 85931 076 0) (Edition 3)
(Note – this is not an exhaustive list)
- Engineering Equipment Materials Users’ Association (EEMUA) Publication ‘222’: A Guide to the Application of IEC 61511 to Safety Instrumented Systems in the UK Process Industries.
- IEC 62508: Guidance on Human Aspects of Dependability (currently draft).
BS EN 61508 Parts 1-8 – Functional safety of electrical/electronic/programmable electronic safety related systems
- Process Safety Leadership Group Final (PSLG) report (HSE) – Safety and Environmental Standards for Fuel Storage Sites
- Contract Research Report 373/2001 – Proposed Framework for Addressing Human Factors in IEC 61508
HID Chemicals, Explosives and Microbiological Hazards Division 2E Electrical, Control and Instrumentation Team.