Page 217 - Electrical Installation in Hazardous Area

General requirements for explosion protected apparatus 1 8 1

8.1.2 Division of apparatus into sub-groups and surface temperature
classes

As earlier indicated, electrical apparatus is divided into groups and classes
according to its performance with regard to ignition capability. Initially,
the apparatus is divided into Group I, which is intended for use in gassy
mines (principally but not exclusively coal mines), and Group I1 which
covers apparatus for use in other industries (which in effect means surface
industry). This book is concerned with Group I1 Apparatus and the require-
ments for Group I will not be further explored.

Apparatus sub-grouping

As already stated, apparatus is divided into Group I and Group 11. Group I1
apparatus may be further divided into sub-groups to identify particular
factors appropriate to its use. At the moment, only one sub-grouping system
is in common use and that is related to the energy which may be released
in a spark within an explosive atmosphere, or with the ability of flame to
transmit through small gaps and ignite any explosive atmosphere down-
stream of the gap.
Intrinsic safety and similar protection concepts do not seek to prevent the
release of electrical energy, but only to limit its value to that which cannot
cause ignition. Apparatus and systems which are said to be intrinsically
safe will be sub-grouped as follows:-
Sub-group IIA: Apparatus and systems which will not ignite the most
easily ignitable mixture of propane/air when tested in accordance with
Clause 10.4 of BS/EN 50020 (1993). This test corresponds approximately to
an equivalent released energy of 160 microjoules from an inductive circuit
where energy release is very efficient. Gases, vapours and mists in mixture
with air are associated with this sub-group where the minimum current
required to cause their ignition (MIC) is more than 0.8 of that needed to
ignite the most easily ignitable mixture of laboratory methane and air (labo-
ratory methane is more than 95 per cent pure) in a spark test apparatus (see
Annex B of BS/EN 50020 (1993)' using the calibration circuit specified in
that Annex).
Sub-group IIB: Apparatus and systems which will not ignite the most
easily ignitable mixture of ethylene/air when tested in accordance with
Clause 10.4 of BS/EN 50020 (1993). This test corresponds approximately to
a released energy of 80 microjoules from an inductive circuit where energy
release is very efficient. Gases, vapours and mists in mixture with air are
associated with this sub-group when their MIC is between 0.45 and 0.8 of
that needed to ignite the most easily ignitable mixture of laboratory methane
and air when tested in a spark test apparatus (see Annex B of BS/EN 50020
(1993)l using the calibration circuit specified in that Annex).
Sub-group IIC: Apparatus or systems which will not ignite the most
easily ignitable mixture of hydrogen/air when tested in accordance with