Page 202 - Electrical Installation in Hazardous Area

166 Electrical installations in hazardous areas

7.2 Protection of electrical apparatus for gas, vapour
and mist risks

As previously noted, released gas, vapour or mist clouds disperse relatively
quickly after release and do not form layers on equipment as dust does. If
equipment is protected against condensation, particularly in the case of hot
vapours or mists, access of these to the inside of the equipment is not neces-
sarily a problem, provided that the electrical components and circuits within
the enclosure do not spark incendively or become excessively hot. If they do
spark incendively or become excessively hot then their entry does, of course,
have to be prevented unless the results of the resultant ignition can be effec-
tively dealt with. There are thus four basic approaches which can be adopted
to prevent uncontrolled ignitions of gas, vapour or mist and air clouds.
1. The gas, vapour or mist/air cloud can be excluded by mechanical barriers
or other equivalent means.
2. The components of the electrical circuits can be prevented from sparking
or becoming sufficiently hot to ignite the gas, vapour or mist/air cloud.
3. The gas,vapour or mist/air cloud can be permitted to enter the equip-
ment enclosure and ignited, but the ensuing propagation of the explosion
outside the enclosure can be prevented by some form of quenching.
4. The electrical components within the enclosure can be permitted to spark,
but the energy fed to the enclosure will be limited to a level which is not
capable of igniting the gas, vapour or mist/air cloud.

The above are the four main approaches to the limitation of explosion
risk to an acceptable level. Clearly, they will, in many cases, have significant
effect upon the design of the equipment and it should never be forgotten
that there will be certain basic requirements for the equipment in a partic-
ular location, even if the explosion risk is not taken into account (e.g.,
equipment sited out of doors will normally need to be weather proof).
The operational reliability of the equipment relies upon these basic require-
ments, which protect it from its general environment and the explosion
protection techniques applied to the equipment are normally based upon
its operational reliability. This means that, in general, the explosion protec-
tion features of equipment will be additional to, and not instead of, the
basic operational requirements, such as protection from the environment.
Using the four criteria described above, specific types of protection have
been developed over the years and are mostly referred to in detailed
construction Standards.

7.2.1 Exclusion of the explosive atmosphere (criterion a)

Three methods of satisfying this requirement in the construction of
explosion protected apparatus have been developed and are in general
current use. These are immersion of the electrical circuits in oil,