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Temperature Classification

Another important consideration is the temperature classification of the electrical equipment. The surface temperature or any parts of the electrical equipment that may be exposed to the hazardous atmosphere should be tested that it does not exceed 80% of the auto-ignition temperature of the specific gas or vapor in the area where the equipment is intended to be used.

The temperature classification on the electrical equipment label will be one of the following (in degree Celsius):

USA °CInternational
(IEC) °C
Germany °C
Continuous – Short Time
T1 – 450T3A – 180T1 – 450G1: 360 – 400
T2 – 300T3B – 165T2 – 300G2: 240 – 270
T2A – 280T3C – 160T3 – 200G3: 160 – 180
T2B – 260T4 – 135T4 – 135G4: 110 – 125
T2C – 230T4A – 120T5 – 100G5: 80 – 90
T2D – 215T5 – 100T6 – 85
T3 – 200T6 – 85

The above table tells us that the surface temperature of a piece of electrical equipment with a temperature classification of T3 will not rise above 200 °C.

Auto-ignition temperatures

The auto-ignition temperature of a liquid, gas or vapor is the lowest temperature at atmospheric pressure at which the substance will ignite without any external heat source, such as a spark or flame. This is used for classification of temperature class for industry and technology applications. The exact temperature value determined depends on the laboratory test conditions and apparatus. Such temperatures for common substances are:

Methane580 °C
Hydrogen560 °C
Propane493 °C
Ethylene425 °C
Acetylene305 °C
Naphtha290 °C
Carbon disulfide102 °C

The surface of a high pressure steam pipe may be above the autoignition temperature of some fuel/air mixtures.

Auto-ignition temperatures (dust)

The auto-ignition temperature of a dust is usually higher than that of vapours & gases. Examples for common materials are:

Sugar460 °C
Wood340 °C
Flour340 °C
Grain300 °C
Tea300 °C