Page 219 - Electrical Installation in Hazardous Area

General requirements for explosion protected apparatus 1 83

sphere is the
most easily ignitable
mixture of the test
gas and air

Fig. 8.1 Method of determination of maximum experimental safe gap. (For full details
see IEC 79-1A (19754, Appendix D)

Sub-group IIB: Apparatus will not ignite a surrounding explosive
atmosphere when filled with, and surrounded by a mixture of 37 per cent
hydrogen in air (equivalent to an MESG of 0.35mm) at atmospheric
pressure. Gases, vapours and mists in mixture with air will be associated
with sub-group IIB when their MESG is measured by the method described
in IEC 79-1A (see Fig. 8.1) ia between 0.5mm and 0.9mm. A safety factor
is produced by using a more sensitive test gas for the apparatus.
Sub-group IIC: Apparatus testing is very different here as achieving a
safety factor on the test gas is more difficult. The safety factor is achieved
by increasing the gaps specified by the manufacturer and then testing with
the most sensitive mixtures of both hydrogen and acetylene with air. (These
are 28 per cent hydrogen in air and 7.5 per cent acetylene in air). Gases,
vapours and mists in mixture with air will be associated with sub-group
IIC when their MESG is measured in accordance with the method described
in IEC 79-1A (see Fig. 8.1) at less than 0.5 mm. In this case, the safety factor
normally achieved by enlargement of gaps is rather more sensitive than test
mixtures.
Once again, as in the case of intrinsic safety the IIC statement is based
upon the fact that hydrogen and acetylene are the two most sensitive gases
known and more sensitive mixtures can only be achieved by adding further
oxygen, which is outside the scope of this Standard and requires special
treatment.
Fortunately there is a relationship between MIC and MESG and to allocate
a gas, vapour or mist to a particular sub-group it is only necessary to carry