Page 220 - Electrical Installation in Hazardous Area

184 Electrical installations in hazardous areas

out either an MIC or an MESG test and not both. This ceases to be true,
however, at the upper limits and where both need to be carried out. In such
cases the following will be true: where MIC is between 0.8-0.9mm, MESG
will determine the sub-group; where MIC is between 0.45-0.5mm, MESG
will determine the sub-group; where MESG is between 0.5-0.55mm, the
MIC will determine the sub-group.
These criteria are necessary to ensure that the most sensitive parameter is
used to determine the sub-group.
To assist in sub-grouping gases, vapours and mists which have not been
tested, it is often possible to identify them as one of a range of materials
of similar structure, in which case it is highly likely that their sub-group
will be the same as other gases, vapours and mists within similar materials
which have a lower molecular weight. In all cases, however, care must be
taken with materials not already allocated to a sub-group to ensure that
no special feature of the material may make allocation unacceptable. Ethyl
nitrate, for instance, will produce an explosion pressure in excess of any
allocated material. There is no guarantee that flameproof apparatus will
withstand an internal explosion of this material and special precautions are
necessary.
The system for sub-grouping now used was preceded by different
systems intended to achieve the same objective in the TJK and other
countries. The approximate relationship between the current system and
these historic systems is given in Table 8.1.

Surface temperature classification

Any unprotected surface to which an explosive atmosphere has access may
cause ignition. This means that while for such protection concepts as flame-
proof enclosure or pressurization, only the external enclosure temperature
is important. When intrinsic safety and increased safety are considered, the
temperature of internal components becomes important as the explosive
atmosphere has access to them and there is no method of preventing flame
transmission. For all apparatus surfaces where any ignition caused would
produce uncontrolled burning, be they inside or merely on the outer enclo-
sure of the apparatus, it is necessary to identify the attained temperature
in the worst case of operation, which includes supply variation (which in
the case of mains-fed apparatus is normally plus or minus 10 per cent.)
This is done by temperature classifying apparatus into six temperature
classes on the basis of the maximum temperature it reaches in the extreme
of its designed operating conditions (with a safety factor) and associating
gases, vapours and mists with those classes on the basis of their ignition
temperatures, and giving apparatus a temperature classification . Because
of the greater difficulty in causing thermal ignition due to the effects of air
movement, etc., the safety factor in this case is smaller than that normally
used for grouping. The temperature classes are as follows.