A mixture with air, under atmospheric conditions, of flammable substances in the form of gases, vapours, mists or dusts in which, after ignition has occurred, combustion spreads to the entire unburned mixture.
The Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations and The Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations (Northern Ireland) provide a definition of a ‘potentially explosive atmosphere’ as:
An atmosphere that could become explosive due to local and operational conditions.
Special precautions need to be taken in an explosive atmosphere to prevent equipment from being a source of ignition, especially in situations where an explosive atmosphere has a high likelihood of occurring. Identifying potential explosive atmospheres, where personnel will be expected to attend and operate, will assist in identifying suitable equipment.
In most circumstances, it is impossible to classify the atmosphere present, unless there is signage and available risk information. Entry points to explosive atmospheres must be marked with a specified ‘EX’ sign.
Zones and corresponding equipment
Information on zones and corresponding equipment categories for explosive atmospheres is provided by the Health and Safety Executive (HSE) in their publication Explosive atmospheres – classification of zoning and selection of equipment.
Explosive atmospheres are classified into zones based on the frequency of the occurrence and duration of an explosive atmosphere.
For gases, vapours and mists the zone classifications are:
- Zone 0 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is present continuously or for long periods or frequently.
- Zone 1 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally.
- Zone 2 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
For dusts the zone classifications are:
- Zone 20 – A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently.
- Zone 21 – A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.
- Zone 22 – A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only
The corresponding equipment categories are:
- Zone 0 or zone 20 – category 1 equipment
- Zone 1 or zone 21 – category 2 equipment
- Zone 2 or zone 22 – category 3 equipment
Pre-planning
For fire and rescue service operations, a risk assessment must be completed to identify the foreseeable explosive atmospheres where personnel will be expected to attend and operate. This will identify the necessary intrinsically safe equipment to provide for personnel to use.
It may be beneficial to identify the potential hazards of explosive atmospheres through joint on-site training and exercises.
Industrial sites
If an explosive atmosphere is at an industrial site, such as petrochemical refinery, the responsible person or on-site staff should be able to provide:
- Details about any present or potentially explosive atmospheres
- Specialist equipment needed by personnel
- Instructions about prohibited equipment such as:
- Mobile phones
- Smart watches
- Radios
Intrinsically safe equipment
‘Intrinsically safe’ is a term widely used for equipment and tools that are considered safe to use in explosive atmospheres. Any equipment that is not intrinsically safe can provide an ignition source for flammable and explosive atmospheres, which may cause combustion or explosion.
The use of intrinsically safe equipment can mitigate the risk of there being a source of ignition. There is a variety of intrinsically safe equipment available, including:
Non-sparking equipment
Non-sparking, spark reduced, spark-resistant or spark-proof tools are terms given to specialist tools made of metals such as:
- Brass
- Bronze
- Monel® metal (copper-nickel alloy)
- Copper-aluminium alloys (aluminium bronze)
- Copper-beryllium alloys (beryllium bronze)
Where there is a concern about sparks igniting flammable solvents, vapours, liquids, dusts or residues, non-sparking tools provide protection against fires and explosions.
ATEX equipment
When working in explosive or potentially explosive atmospheres, any equipment in use should conform to the ATEX Equipment Directive 94/9/EC.
The requirements of the Directive were put into effect through:
- The Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations
- The Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations (Northern Ireland)
The regulations apply to all equipment intended for use in explosive atmospheres, whether electrical or mechanical, and also to protective systems. The Department for Business, Energy & Industrial Strategy has policy responsibility for the regulations, although the HSE and Health and Safety Executive for Northern Ireland (HSENI) enforces them.
If equipment has official ATEX certification, it has been fully tested and approved to be safe to use in explosive atmospheres. ATEX approved equipment must show the official ‘EX’ logo.
It should be noted that some personal items and equipment with rechargeable batteries, such as mobile phones, smart watches and radios, do not comply with the ATEX Equipment Directive. Therefore, they should not be taken into explosive atmospheres or potentially explosive atmospheres.
For more information on ATEX and equipment for explosive atmospheres refer to: