Page 269 - Handbook of Electrical Engineering

254 HANDBOOK OF ELECTRICAL ENGINEERING

during a long period (tens of hours or longer in a year, see Reference 1). The risk of an explosion
in a given period of time is therefore higher than with a Zone 2 area.

10.4.4 Zone 0 Hazardous Area

The highest risk of hazard is to be found in a Zone 0 area. These areas are usually the gaseous
volume immediately above a volatile liquid contained in a vessel or tank. In some situations the
contents of a vessel or tank will normally be at atmospheric pressure e.g. crude oil storage tanks in
an exporting tank form, refinery feed stock tanks, temporary storage of liquid products in a chemical
plant. Some designs of vessels and tanks need a means of venting to prevent an increase in surface
pressure as the liquid rises, or a reduction in pressure as the same liquid falls. In a large tank these
small changes in surface pressure, and hence the enclosed vapour pressure, can cause serious damage
to the tank walls and roof if some form of two-way venting is not allowed. The surroundings close
to the atmospheric vent aperture would therefore be a Zone 0 area.

10.4.5 Adjacent Hazardous Zones

In many locations where a hazardous area is identified and numbered as 0, 1 or 2 the immediate sur-
roundings are given the next higher number, except for an original Zone 2 area, which automatically
is surrounded by a non-hazardous area. A Zone 0 area is surrounded by a Zone 1 area, which in turn
is, surrounded by a Zone 2 area. A Zone 2 area stands alone.
During the preparation of drawings that show the extent and shapes of the areas surrounding
a source, it is usually found that overlapping areas create complicated geometrical shapes, e.g. two
adjacent circular boundaries almost touching each other. In these situations the shapes should be
simplified by using tangent lines. Local pockets within the geometry should be absorbed into a
more uniform shape, especially non-hazardous pockets in Zone 2 geometry. Experience shows that
equipment located in a non-hazardous area that is ‘near’ to a Zone 2 area will usually be of the same
specification as that which is to be installed in the Zone 2 area. The same approach is sometimes
used for Zone 2 areas near to a Zone 1 area if the equipment are small items e.g. junction boxes,
lighting fittings, instrument casings, and local control stations.

10.5 TYPES OF PROTECTION FOR HAZARDOUS AREAS

Most electrical equipment consists of live or active static parts, and in some cases such as motors,
solenoid valves and relays moving mechanical parts, encased in an enclosure. The electrically live
conductors are kept out of touch to prevent electric shock hazards. The detrimental effects of the
atmosphere e.g. rain, sprayed water, fine dust and particles are kept out of contact with the con-
ductors, insulation, bearings and the like. For equipment that is to be used in hazardous areas there
is the additional requirement that gases and vapours should be restricted from entering into the
enclosure. There are various basic methods that attempt to ensure that this requirement is achieved
at a more or less degree, which generally is dependent upon the Zone of the intended area of
installation.
The design of the enclosure with regard to hazardous area applications is defined by several
lower case letter codes, mostly single digits for electrical power equipment but occasionally two