Page 218 - Electrical Installation in Hazardous Area

182 Electrical installations in hazardous areas

Clause 10.4 of BS/EN 50020 (1993). This test corresponds approximately to
a released energy of 40 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 less than 0.45 of that
necessary 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) using the calibration circuit specified in that Annex).
In the case of sub-group IIC, the statement may sound a little odd as
the apparatus or system is tested with hydrogen/air which has an MIC of
around 0.45, and more sensitive gas/air mixtures may be ignited by the
apparatus. The fact is that hydrogen/air is the most sensitive gas known
and more sensitive gas mixtures can only be produced by additional oxygen
in the mixture. Such mixtures are outside the scope of this Standard and
need to be treated specially.
Flameproof enclosure and similar concepts do not limit the release of elec-
trical energy within the apparatus but seek to prevent the progress of the
flame to any explosive atmosphere outside their enclosure. To do this they
have any apertures closely controlled to ensure that any internal conflagra-
tion is sufficiently cooled or otherwise treated such that no external ignition
will take place. Flameproof apparatus for a specific group (the groups being
IIA, W and IIC as before) is tested in specific gas/air mixtures to ens=
that the transmission cannot take place. These test mixtures are arranged
to give a safety factor in use and the situation is more complex than is the
case for intrinsic safety. Gases, vapours and mists are, in this case, associ-
ated with the groups on the basis of tests in a speaal test apparatus. This
has 25mm long flanged joints of adjustable aperture, as described in IEC
79 - 1A (1975), titled 'Electrical Apparatus for Explosive Gas Atmospheres,
Part 1: Construction and Test of Flameproof Enclosures of Electrical Appa-
ratus. First Supplement 'Appendix D Method of Test for Ascertainment of
Maximum Experimental Safe Gap' (see Fig. 8.1).The apertures are adjusted
until flame propagation from inside the test apparatus to the surrounding
explosive atmosphere does not take place. This aperture is termed the
maximum experimental safe gap (MESG). The mixtures, both inside the test
apparatus and outside, are mixtures of the same gas and air but that inside
is the stoichiometric mixture (the mixture where all the fuel gas and oxygen
are consumed) and outside is the most easily ignited mixture, which may be
slightly different. The reason for this is that the internal mixture produced
gives the worst conditions for flame transmission and the outer mixture the
worst condition for ignition if transmission occurs.
Sub-group ZZA: Apparatus will not ignite an external explosive atmosphere
when filled with and surrounded with a mixture of 55 per cent hydrogen
in air (equivalent to an MESG of 0.65mm) at atmospheric pressure. Gases,
vapours and mists in mixture with air will be associated with sub-group
IIA when their MESG is measured by the method described in IEC 79-1A
(see Fig. 8.1) at above 0.9mm. A safety factor is produced by using a more
sensitive test mixture for the apparatus.