Page 38 - Electrical Installation in Hazardous Area

Introduction 17

'N' indicating it is to a UK national Standard and not a Euro-
pean Standard)
IIC - indicates the explosive atmosphere in which the apparatus was
tested for spark ignition
IIA - equipment is tested in an ideal mixture of propane and air
IIB - equipment is tested in an ideal mixture of ethylene and air
IIC - equipment is tested in an ideal mixture of hydrogen and air (where
the equipment is to a type of protection where sparking is prevented
the letter will be omitted and only 11 will appear as sub-grouping
is not necessary)
T4 - indicates the maximum temperature achieved by any ignition
capable part of the equipment and indicates that no part of the
equipment will exceed
T1 450°C
T2 300°C
T3 200°C
T4 135°C
T5 100°C
T6 85°C
An ambient temperature of 40 "C is assumed unless otherwise stated
on the label and certificate
Historically, there have been other methods of coding in Europe and remain
different methods in the USA. These are shown in Tables. 1.1 and 1.2. It must
however be noted that because of slightly differing information forming the
basis for these that the relationship is only approximate.

Table 1.1 Relationship of grouping and classification systems
~~~
Current Historic GB German USA
European

Flame proof Intrinsic Safety
sub-group IIA Group I1 class 2c class 1 Group D
sub-group IIB Group I11 class 2d class 2 Group C
sub-group IIC Group IV class 2e and 2f class 3a and 3n Group Band A

Notes :
1 UK Flameproof Group IV excludes acetylene which was treated separately.
2 UK intrinsic safety class 2e did not include acetylene, but 2f did.
3 German class 3a excludes acetylene as does USA Group B and UK Class 2e. Acetylene is
included in German class 3n, USA Group A and UK Class 2.
4 In the USA the following Groups are also present: E - Conducting dusts; F - nonCon-
ducting dusts; G - Flour and Grain dust.
5 To add further complexity, USA apparatus is classed as to its intended use. Class A is for
gases and vapous, Class B is for dusts and class C is for fibres and flyings.