Page 25 - Electrical Installation in Hazardous Area

4 Electrical installations in hazardous areas

low but identifiable. In this scenario catastrophe does not play a part and
although it is necessary to plan for catastrophe such plans are by and large
outside the scope of this technology. In addition this technology should not
be used in isolation but as part of an overall safety strategy for a location
where the problem occurs.
Having identified the possible presence of an Explosive Atmosphere it is
then the part of technology to identify those electrical installations which
really need to be present rather than those which convenience would make
desirable, and ensure that these are protected in a way which makes the
overall risk of an explosion sufficiently low.

1.3 History of development

The use of electrical equipment in explosive atmospheres was originally the
province of the mining industry and, although the technology was used
in surface industry, significant developments in this latter area are more
contemporary being to a large extent post war. While the present approach
is to minimize the chance of a release of flammable material, or where a
release occurs to minimize the build up of the material in the atmosphere, it
is probably somewhat surprising that in early coal production the method
used to deal with releases of methane (firedamp) was to deliberately burn
off the explosive atmosphere. This was done by a specifically designated
miner called the 'firelighter'. The method used took advantage of the fact
that methane is lighter than air (relative density is around 0.55) and thus
methane/air mixtures collected preferentially near the roof of the workings.
Warning was given by changes in colour of the flames of the lamps used by
miners and the workings were then cleared. A torch was inserted into the
methane air cloud, igniting it and burning off the methane. The technique
fell into disrepute for obvious reasons and was replaced by the introduction
of the use of ventilation to restrict the possibility of explosive atmospheres
forming and the employment of a safety lamp (Fig. 1.2) to minimize the
risk of ignition.
The introduction of electricity in the latter part of the nineteenth century
and the early part of the twentieth century led to significant other risks
being identified. Initially electricity was utilized for lighting and motive
force. The lighting was typically provided by incandescent filament lamps,
none of the more sophisticated lamps having been developed at the time,
and the motive force usually by either dc or wound rotor ac machines which
were initially typical of the motors available. Both lighting and machines
required control equipment (often as simple as a switch) but this equipment
also introduced risks associated with hot surfaces and sparks, together with
the possibility of the presence of both methane and coal dust.
The solutions to these problems in relation to gas, vapour and mist
releases were developed in both the UK and Germany along very similar
lines and in very similar time scales. In Germany the organization prin-
cipally involved was what is now known as the Berggewerkschaftlichen