Page 222 - Electrical Installation in Hazardous Area

186 Electrical installations in hazardous areas

T3

For T3 the maximum apparatus temperature must not exceed 195°C. (As
temperature classification is normally done at a 40 "C ambient temperature,
this usually means an elevation due to self heating of 155 "C.) Gases, vapours
and mists associated with this temperature class will have ignition temper-
atures of between 200 "C and 300 "C

T4

For T4 the maximum apparatus temperature must not exceed 130°C. (As
temperature classification is normally done at a 40°C ambient tempera-
ture, this usually means an elevation due to self heating of 90°C.) Gases,
vapours and mists associated with this temperature class will have ignition
temperatures of between 135°C and 200°C.

T5

For T5 the maximum apparatus temperature must not exceed 95°C. (As
temperature classification is normally done at a 40°C ambient tempera-
ture, this usually means an elevation due to self heating of 55°C.) Gases,
vapours and mists associated with this temperature class will have ignition
temperatures of between 100 "C and 135 "C.

T6

For T6 the maximum apparatus temperature must not exceed 80°C. (As
temperature classification is normally done at a 40°C ambient tempera-
ture, this usually means an elevation due to self heating of 40°C.) Gases,
vapours and mists associated with this temperature class will have ignition
temperatures of between 85 "C and 100 "C.
As indicated, the self elevation of the apparatus permitted in all cases
depends upon the ambient temperature at which temperature classification
is carried out. If an item of apparatus is temperature classified as T3 at an
ambient temperature of 100 "C its permitted self heating will be reduced to
95 "C, as the overall maximum temperature must remain the same.
It is also recognized that small components can exceed the ignition
temperature of a particular gas, vapour or mist without causing ignition,
and this has been demonstrated as the case. In general, this difference
depends upon factors such as the convection performance of the particular
gas, vapour or mist and the configuration of the hot surface. For this reason,
there is no general relaxation (except in the case of intrinsic safety, see
Chapter 13) and each type of small component must be treated individually.
To do this it is necessary to determine the temperature at which the surface
in question actually ignites a gas representative of the most sensitive in the