Area classification in potentially explosive atmospheres

Area classification defines hazardous areas on the basis of the probability of a flammable atmosphere being present. It is a legal requirement under DSEAR for any employer whose business involves flammable materials in any form, and it results in the division of the workplace into one or more zones. The levels of probability used in area classification are high, medium and low. Historically, dust and gas or vapour were dealt with separately which is reflected in different nomenclature for their zoning. Zones 0, 1 and 2 are for gas, vapour and mist;  zones 20, 21 and 22 are for dust.

The definitions for gas or vapour zones and dust zones are very similar and are paraphrased here for simplicity:

• Zone 0, Zone 20 – High probability (A flammable atmosphere is present continuously, for long periods or frequently)
• Zone 1, Zone 21 – Medium probability (A flammable atmosphere is likely in normal operation occasionally)
• Zone 2, Zone 22 – Low probability (A flammable atmosphere is unlikely in normal operation and, if it occurs, will exist only for a short time)

Hazardous areas or flammable atmospheres

The zones do not reflect the ignition properties of the flammable materials; this is dealt with separately by their ignition characteristics. It should also be noted that the zones describe ‘atmospheres’, which are mixtures of flammable material with air under atmospheric conditions. Any other situations are not part of area classification but require consideration as part of the overall safety case for the workplace.

‘Normal operation’ is considered to be the actual, day-to-day operation of the workplace. It is not the idealized operation of the workplace as it was designed, and the area classification has to be reviewed regularly to accommodate any changes to the workplace and the work therein. ‘Not likely in normal operation’ raises the question of how unlikely an event does area classification consider. This is best illustrated by considering a pipeline with flanged joints carrying a flammable gas.

For a properly designed, constructed and maintained pipeline, fracture of the pipeline would not be considered; this is considered as a catastrophic failure and dealt with by the overall risk assessment. Leakage at a flanged piping joint would, however, be considered. There are circumstances where area classification is not appropriate and these will be revealed by the overall risk assessment.

For example, small quantities of flammable materials in the workplace may represent such a small risk to workers that other control measures may suffice.