Page 268 - Handbook of Electrical Engineering

HAZARDOUS AREA CLASSIFICATION 253

The definitions in the USA differ slightly by the use of ‘Class’ rather than ‘Group’.

The grouping code influences the design of jointed and mating surfaces and shaft seals, because
the gases in different groups have different explosive and burning characteristics e.g. speed of flame
propagation, rate of rise of explosive pressure. Hence the grouping codes influence the physical design
of enclosures. The gases hydrogen and acetylene for example are notably difficult gases to cater for
in designs. Hydrogen is often encountered in the oil industry, and acetylene in the chemical industry.

10.4 HAZARDOUS AREA ZONES
In the European and UK standards the term Zone is used for hazardous areas, whereas the term
Division is used in the USA. In practice the end result of selecting appropriate equipment for a Zone
or Division is usually very similar. There are a few subtle differences, especially when selecting
electric motors. The zonal definitions vary in wording from one document to another but the essential
elements are as follows.

10.4.1 Non-hazardous Area

In the earlier period the term Safe Area tended to be used to mean an area that was deemed to be
completely free of potential hazards. As with many technologies their terms and definitions take on
slight changes as time passes, usually because of the feedback effect of experience. In this way the
term Non-hazardous Area seems to have superseded Safe Area. (Curiously the zonal numbering is
in opposite sense to the severity of hazard, zero is the worst and 2 is the least, as discussed below.)

10.4.2 Zone 2 Hazardous Area

The lowest non-zero risk of hazard is to be found in a Zone 2 area. In a properly designed and
maintained plant the occurrence of leakage of flammable gas, vapour or volatile liquid from within
the vessels, tanks, piping, valves, seals, pumps, compressors and the like is accepted as being unlikely
but possible. The possibility is deemed to exist when a fault develops in the equipment e.g. a flange
gasket fails, a pipe fractures. These occurrences come under the category of ‘wear and tear’. It
is possible that a leakage may result from some mal-operation e.g. a heavy object is accidentally
dropped onto equipment that contains a hazardous fluid e.g. a pipe, which either pierces the metal or
loosens a joint. These occurrences may be categorised as ‘accidental’ causes, and can be considered
as being statistically low, hence the risk of explosion is also low. They may be considered unlikely
to occur over a long period of time (months, years) or if they do then the time period will be short
(up to 10 hours per year, see Reference 1).

10.4.3 Zone 1 Hazardous Area

A higher risk of hazard than that applicable to a Zone 2 area, is found in a Zone 1 area. Again it
can be considered that the plant is properly designed and maintained. However, some parts of the
plant are more prone to leakage than others, some types of seals used in rotating shafts of pumps
and compressors, discharges from safety valves, some methods of venting gases and vapours, some
types of open drains for volatile liquids. Hence leakage may be considered likely to occur some time