Safety Terminology

Health and Safety Glossary

Quick reference guide to definitions of commonly used general safety terms.

Reynolds Training’s glossary of commonly-used health and safety terms and definitions has been devised for health and safety committee members, representatives, managers and anyone who is responsible for health and safety in the workplace.


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As Low As Reasonably Practicable (ALARP). Reasonably practicable involves weighing a risk against the trouble, time and money needed to control it. ALARP describes the level to which regulators expect to see workplace risks controlled.

Area Classification

Area classification is a method of analysing and classifying the environment where explosive gas atmospheres may occur. The main purpose is to facilitate the proper selection and installation of apparatus to be used safely in that environment, taking into account the properties of the flammable materials that will be present. DSEAR specifically extends the original scope of this analysis, to take into account non-electrical sources of ignition, and mobile equipment that creates an ignition risk.

Auto Ignition Temperature

The auto ignition temperature of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition.


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Bonding is the connection of two or more conductive objects to one another by means of a conductor such as a wire.


Buncefield Standards Task Group (BSTG) was formed consisting of representatives from the Control of Major Accident Hazards (COMAH) Competent Authority and industry, with the aim of translating the lessons from Buncefield into effective and practical guidance that industry would implement as rapidly as possible. This also facilitated a joined-up approach to managing risk across the sector by providing an authoritative benchmark for standards and practices.


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The Control Of Major Accident Hazard (COMAH) Regulations implement the Seveso II Directive except for the land-use planning requirements which are implemented by changes to planning legislation. They replaced the Control of Industrial Major Accident Hazards Regulations 1984 (CIMAH) and came into force on 1st April 1999.

Competent Authority

The Competent Authority’s role is to oversee and coordinate the regulation of major hazards in the UK and ensure that the regime operates effectively. It comprises in England and Wales of the HSE and EA whilst in Scotland it comprises the HSEand SEPA.


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The Dangerous Substances and Explosive Atmosphere Regulations (DSEAR) places duties on employers and the self-employed to protect people from risks to their safety from fires, explosions and similar events in the workplace, this includes members of the public who may be put at risk by work activity.


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The Environment Agency (EA) is an Executive Non-departmental Public Body responsible to the Secretary of State for Environment, aims are to protect and improve the environment, and to promote sustainable development.


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Flame Arrester

Flame arresters are used as secondary protection against explosions by preventing the transmission of flame and explosion transfer in machines, equipment and plant, containing inflammable gas or steam-air mixtures of inflammable liquids. These autonomous safety systems limit the effects of the explosions, rendering them harmless, they are intended to allow flow but prevent flame transmission.


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In relation to static charges Grounding, also referred to as ‘earthing’, is a specific form of bonding wherein one or more conductive objects are connected to the ground by means of a conductor such as a wire or rod. Thus, proper ‘grounding’ of objects (conductors) in the field will normally incorporate both bonds between objects and a specific grounding to the earth.


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HASWA 1974

The Health and Safety at Work Act 1974 is an Act of Parliament defining the fundamental structure and authority for the encouragement, regulation and enforcement of workplace health, safety and welfare within the United Kingdom.

Hazardous Area

Hazardous areas are defined in DSEAR as “any place in which an explosive atmosphere may occur in quantities such as to require special precautions to protect the safety of workers”. Also See – ‘Area Classification’


Health and Safety Executive (HSE) is an independent regulator that acts in the public interest to reduce work-related death and serious injury across Great Britain’s workplaces.


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The Process Safety Leadership Group was a joint industry and regulators group, set up in September 2007 to drive forward high standards in process safety leadership and to complete the implementation of the Buncefield Major Incident Investigation Board’s recommendaations.


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Relaxation Time

In relation to static electrical charges ‘Relaxation Time’ can be defined as the period of time that a material retains this is related to its conductivity; the lower the conductivity of the material, the greater the relaxation time, in other words … Time taken for electrostatic charge to relax or dissipate from a liquid.


A Remotely Operated Shut Off Valve (ROSOV)is a valve designed, installed and maintained for the primary purpose of achieving rapid isolation of plant items containing hazardous substances in the event of a failure of the primary containment system (including, but not limited to, leaks from pipework, flanges, and pump seals). Closure of the valve can be initiated from a point remote from the valve itself. The valve should be capable of closing and maintaining tight shutoff under foreseeable conditions following such a failure (which may include fire).


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Safety Critical Equipment

Safety Critical Equipment could be defined as any structure, plant, equipment, system (including computer software) or component part whose failure could cause or contribute substantially to a major accident is safety critical, as is any which is intended to prevent or limit the effect of a major accident. Identifying an item as safety critical should follow from identifying major accident hazards.

Safety Critical Task

Safety critical tasks can be defined as those where sub-standard performance could contribute to a major accident hazard.

Settling Time

In relation to static electrical charges ‘Settling Time’ relates to the settling of a solid or an immiscible liquid through a liquid (e.g. water, rust or other particles through the product). This process may continue for up to 30 minutes after completion of transfer into a vessel.


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Hazardous areas are classified into zones based on an assessment of the frequency of the occurrence and duration of an explosive gas atmosphere, as follows:
  • Zone 0: An area in which an explosive gas atmosphere is present continuously or for long periods;
  • Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal operation;
  • Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it occurs, will only exist for a short time.
See also – ‘Area Classification’ and ‘Hazardous Area’