Temperature Classifications for Hazardous Area Installations

A few definitions related to the topic before discussing the Temp Classification.

Flammable Material:

A flammable material is a gas vapour, liquid and/or mist which can react continuously with atmospheric oxygen and which may therefore sustain fire or explosion when such reaction is initiated by a suitable spark, flame or hot surface.

Explosive Limits:

The extreme values for the concentration of a flammable gas or vapour in air under atmospheric conditions, which can be ignited by an electrical arc or spark. These limits are called the 'lower explosive limit' (LEL) and the 'upper explosive limit' (UEL)

Ignition Temperature:

The lowest temperature at which ignition occurs in a mixture of explosive gas and air.

Flash Point:

The minimum temperature at which the liquid gives so much vapour that this vapour, when mixed with air, forms an ignitable mixture and gives a momentary flash on application of a small pilot flame.

The temperature classification for electrical and other instruments is defined to limit the maximum surface temperature of the equipment during its conitnuous operation. This is the highest temp attained by the equipment.

Temperature Classification is done because besides the danger of explosion caused by an electric spark or arc, there is also a danger of ignition on a hot surface exposed to a flammable atmosphere. The maximum surface temperature of any unprotected surface of electrical equipment should not exceed the ignition temperature of the gas or vapour. To simplify the manufacturer of apparatus the permitted maximum surface temperatures have been classified as follows

T1 : 450 deg C
T2 : 300 deg C
T3 : 200 deg C
T4 : 135 deg C
T5 : 100 deg C
T6 : 85 deg C