Page 200 - Electrical Installation in Hazardous Area

7

Design philosophy for electrical
apparatus for explosive

atmospheres

General approach and applicable standards

The preceding three chapters discuss the various facets of the classification
of areas made hazardous by the possible presence of explosive atmospheres
of air mixed with gas, vapour, mist or dust. From these, it is possible to
identify the areas where the hazard exists, together with the severity of the
hazard. It now becomes necessary to address the construction of electrical
equipment which should be installed in such areas to minimize the risk of
fire or explosion. In doing this, it has to be recognized that however well
protected electrical equipment may be, there will always be residual risk if
it is placed in areas where explosive atmospheres may occur. To this end,
the first golden rule is: Only that electrical apparatus which is really necessary
should be placed in or communicate with explosive atmospheres.
The necessity of placing electrical equipment in explosive atmospheres
has to be determined not only on safety grounds but also on operational
and economic grounds. However, the yardstick which should always be
applied is that of real benefit and unless such benefit can be recognized the
equipment should not be installed.

7.1 History

The construction of electrical equipment specifically for use in areas where
explosion risks might occur due to clouds of gas, vapour, mist or dust
was a problem first addressed in the early part of the twentieth century
and initially was aimed at the coal-mining industry. This initial addressing
of the problem concentrated upon the problems associated with methane
(firedamp) and the problems of coal dust.

7.1.1 Gas, vapour and mist risks

The types of protection envisaged for coal mining were those associated
with protection against gas clouds and the levels of overall protection
envisaged were those which were suitable for underground areas where