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General
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The selection of electrical
apparatus for explosive gas atmosphere is based on the general principle
that the likelihood of the simultaneous present of a hazardous atmosphere
and a source of ignition is reduced to an acceptable low level.
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When the electrical
apparatus does not have an internal release of flammable material, the classification
of the hazardous area surrounding the electrical equipment decide the selection
of an adequate type protection.
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When the electrical
apparatus has an internal release of flammable material, the actual hazard
is a combination of the external and internal hazards and both must be taken
into account when selecting an adequate type of protection.
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The selected electrical
apparatus shall be adequately protected against corrosive and solvent agents,
and against water ingress and thermal and mechanical stresses as determined
by the environmental conditions. These construction requirements should
ensure that the protection against explosion is not reduced when the apparatus
is used in the specified conditions of service.
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Particular consideration
shall be given to the location of apparatus which incorporates aluminium
or light alloys in construction of its enclosure. These have been outlined
in IS: 5571-1979* reference to which should be made (see also B of IS:2148-1981)
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As electrical apparatus
for flammable gas atmospheres require special safety maintenance after they
are installed, the selection of electrical apparatus shall be made with
full considerations to the facility and frequency of inspection and maintenance,
preparation for spares and repairing materials and the extent of allowable
interruption of power supply during maintenance work.
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It is advisable that
the selection of electrical apparatus be made after full considerations
have been given not only to the initial cost of installation but also to
the expected life and expenses for operation and maintenance of the apparatus.
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Unless otherwise specified
for a particular equipment, the guidance provided in Table 2 shall be followed
in the selection of equipment for hazardous areas. However, any equipment
which in the opinion of the authorised inspector affords a degree of safety
not less than that afforded by the equipment specified may be accepted as
an alternative.
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The necessity for equipment
with flameproof enclosures any sometimes be eliminated by the adoption of
special design, such as pressurised equipment. |