Page 270 - Handbook of Electrical Engineering

HAZARDOUS AREA CLASSIFICATION 255

Table 10.4. Enclosure codes for hazardous area equipment

Ex or EEx code Brief description
d Flameproof enclosure
e Increased safety
i Intrinsic safety. There are two types ia and ib
m Encapsulated enclosure
n Basically a UK concept that is similar to type ‘e’,
but only for use in Zone 2 areas
o Oil-immersed enclosure
p Pressurisation and continuous dilution by
non-hazardous air or inert gas such as nitrogen
q Sand-filled enclosure
s Special designs of enclosure or system of
components

digits for very low energy electronic equipment. The most frequently encountered codes are d, e, n,
p and i. The lesser used codes are o, m, s and q. Table 10.4 gives a brief description of each code.
The codes are usually embraced with double or single quotation marks, or less often single round
brackets ( ). The code is prefixed with the letters Ex or EEx. Occasionally two letters are combined
for special designs of equipment e.g. Ex ‘de’ for some types of motors.

Reference 3 gives a useful Table 1 therein, that relates various international standards to the
different types of protection ‘d’ to ‘s’.

10.5.1 Type of Protection ‘d’

This type of protection is also referred to as ‘flameproof’ in some literature. An enclosure that is
designed as type ‘d’ will be able to withstand an internal explosion of the gas-air mixture without
being damaged beyond repair. Furthermore the mating surfaces of joints e.g. terminal boxes, bearing
seals on shafts, will be so designed that the flame inside the enclosure will not pass to the outside
with sufficient energy to ignite the environmental gas-air mixture. In effect the design of the surfaces
is such as to act as a very slow pressure relief system for the internal explosion. (Care should be
exercised when dismantling such an enclosure after an internal explosion has occurred, because there
may be some residual pressure internally.)
By the form of the design these enclosures are usually robust, ‘heavy duty’ and often made
of thick cast iron, steel or bronze with many bolts to fasten the fabricated sections and lids. They
are therefore the most expensive enclosures when compared with the ‘e’ or ‘p’ types. It becomes
impractical to manufacture ‘d’ type enclosures for very large ratings of motors. The amount of metal
and machining required would not be economical and so the ‘p’ type would be an alternative.
This type of protection is mainly intended for Zone 1 areas.

In addition the electrical components inside the enclosure may be of the sparking type e.g.
commutators for DC motors, local control stations with push buttons, relay boxes.
When an internal explosion occurs or under normal running conditions, the outside surface of
the enclosure must not exceed the gas-air autoignition temperature i.e. Temperature Class.