Page 201 - Electrical Installation in Hazardous Area

Design philosophy for electrical apparatus for explosive atmospheres 1 65

men normally worked. They also presumed the equipment to be switched
off if any gas was detected. Although it was relatively rare for significant
outbursts of firedamp to occur (i.e., those which would engulf the men
and the equipment before the latter could be switched off) the difficulty of
escape from a mine was significant and a relatively high level of protection
was adopted. This level later became the norm for Zone 1 above ground.
Therefore the oldest types of protection currently used for electrical equip-
ment in surface industry are those appropriate to Zone 1.
Surface industry did not, in the early days, consider the installation
of electrical equipment in Zone 0, a situation contemporarily familiar to
those dealing with dust hazards, and it was only with the rapid devel-
opment of instrumentation that any such demand became significant. It
must be remembered that Zone 0 generally occurs inside such things as
process vessels and so, until the demand for automatic process monitoring
increased with the development of sophisticated instruments at a much
later date, there was no demand for protection suitable for Zone 0. Even
now it is generally only necessary to site instrument sensors in Zone 0 and
only the type of protection suitable for such devices has been developed
for Zone 0. The Zone 2 scenario did not exist underground and was very
much a surface industry phenomenon. Initially, it was presumed that good
quality industrial equipment, which did not spark or get hot in normal
operation, was suitable for Zone 2 and little work was done on Standards
for Zone 2 equipment until relatively recently; industry being content with
a Guide to Selection of Electrical Equipment for Zone 2'.
The approach to Zone 2 has become less and less acceptable over the
years and although it is still possible to utilize it there are now construction
Standards for Zone 2 equipment and EU Directives2 are making the old
'Selected Industrial Apparatus' approach less and less acceptable.
The above explains why, in respect of gas and vapour risks, the Standards
for Zone 1 equipment are much more developed than those for Zone 0 or
Zone 2 equipment at present, although much is being done to correct this.

7.1.2 Dust risks

A similar situation exists in respect of equipment for dusts as, histori-
cally it has not been considered as acceptable to put equipment inside
dust-processing vessels, etc. and the Standards for such equipment did not
recognize the interior Zone3. There is now a three-Zone system for dusts and
it is expected that the equipment Standards in this area also will develop
in the future. (It is worthy of note that one Code used a three-Zone system
in the early 1970'~~ and this gives some idea of the inertia in developing
ideas in the explosive atmospheres field.)
Fortunately, there is now an international three-Zone system for dusts,
which is similar to that used for many years for gases, vapours and mists,
with similar definitions and it is hoped that some parallelism will now
develop in addressing the problems of explosive atmospheres.