Explanatory notes on the terms used in the zone definitions

Normal operation

Means situations when an installation is used within its design parameters. Start-up, shut-down, sampling and cleaning operations are also considered as normal operations. Malfunctions which cause a shut-down or require repairs are not part of normal operation.

Explosive atmosphere

An explosive atmosphere is a mixture with air 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. An explosive atmosphere only exists if the lower explo- sion limit (LEL) is exceeded and the upper explosion limit (UEL) is not reached. The LEL and UEL are substance-specific characteristics1), which are experimentally determined. For dusts the UEL is normally not determined.

Hazardous explosive atmosphere

An explosive atmosphere which may occur in such quantities as to require special precautions to protect the health and safety of the workers concerned is considered to be hazardous.
An explosive atmosphere which is not expected to occur in such quantities as to require special precautions is considered as non-hazardous and does not result in a zone.

Dust layers

Layers, deposits and heaps of combustible dust must be taken into account in the same manner as any other source which can form an explosive atmosphere (cloud ).
Good housekeeping, i.e. the removal of dust deposits, is an important safety measure in implementing explosion prevention measures and is advantageous when considering the area classification.

Likelihood or probability

Which zone occurs depends on the duration and frequency of occurrence of the explosive atmosphere. The explosive atmosphere occurs with a certain likelihood or probability which is expressed as follows:

A: frequently or for long periods: the occurrence related to the effective operating time (e.g. in Germany > 50 %)

B: not likely to occur in normal operation or for a short period only: few times per year for say half an hour

C: occasionally: the occurrence and frequency between A and B
The explanations of these terms are given as starting points only and should not be considered as absolute values.

The explanations of these terms are given as starting points only and should not beconsidered as absolute values.

Source: ISSA Prevention Series No. 2050 (E)

Note: in Germany > 50 % means in case of above 50% of total operational time.

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