Understanding temperature class in flameproof lighting / equipment

The Ignition temperature of a flameable liquid or gas is the lowest temperature at which ingnition can happen. Therefore, the maximum surface temperature of the lighting product must be lower than the ingnition temperature of ambient temperature.

Temperature classes from T1 to T6 have been introduced for electrical equipments for group II hazardous area.

Class Maximum surface temperature
of equipment allowed
Ignition temperature of
combustible substances
T1 450°C >450°C
T2 300°C >300°C
T3 200°C >200°C
T4 135°C >135°C
T5 100°C >100°C
T6 85°C >85°C

The electrical equipment is certified by certifing agency to a temperature class, based on its measured maximum surface temperature. Electrical equipment correpoding to a higher temperature class can be used for applications requiring lower temperature class.

For example, a T6 certified electrical equipment which has maximum surface temperature of 85°C can be used in T5, T4, T3, T2 & T1 environment.

Similarly, a T3 certified equipment whihc has a maximum surface temperature of 200°C can NOT be used in T4, T5 or T6 environment, but can be used in T2 or T1 environments.

A T6 certified equipment is a best selection as it can be used in all T classes.