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Tempearature Classification

What is Temperature Classification?
Temperature classification (also known as temperature class, or T class) defines the maximum surface temperature that a product destined for use in a potentially hazardous atmosphere is allowed to operate at, relative to an ambient temperature of -20°C to +40°C.
According to the type of protection used on the product e.g. Exd, Exe etc, the temperature corresponds either to the maximum temperature of the external surface of the product, or to the maximum temperature of the internal surface of the product. Generally, Temp-class is based on fault conditions or, at the very least, worst case normal operating conditions.
Why is Temperature Class Important?
All flammable gases have an auto-ignition temperature. If a flammable mixture of the gas is exposed to a component above the auto-ignition temperature then the mixture will ignite. Therefore, when selecting equipment, the Temperature class must be below the auto-ignition temperature of the potentially explosive atmosphere where it will be installed.
If several different flammable materials may be present within a particular area, the material that gives the lowest auto ignition temperature dictates the overall area classification.
T-Class and Equipment Marking
The ATEX directives typically require all products certified suitable for use in a hazardous area, e.g. instruments, enclosures, luminaries etc to be marked with their temperature class. Look out for a T number at the end of the protection concept marking on the productís label e.g. EEx de IIB T3 indicates that this explosion proof apparatus has a temperature classification of T3 which corresponds to a maximum surface temperature of 200°C.
Temperature Class for Group I Applications
"T" classes do not apply to Group I applications. Equipment for use in the mining industry has either a rigid 150°C or 450°C limit.
Temperature Class for Group II Applications
Temperature Classification
Max. Surface
Temp °C
North AmericanEuropean

The Atlantic Divide
North American and Canadian standards differ from European standards with smaller temperature ranges allowed in Europe. See table above.

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Further Reading

Practical Electrical Equipment and Installations in Hazardous Areas
Electrical Installation in Hazardous Areas
C & I reources and instrument utilities

C&I Resources

Cable Gland Selector
CE Marking
Flange Dimensions
Ingress Protection
Pipe Schedules
Relief Valve Orifice Size
Thermocouple Types
Temperature Conversion
Valve Leakage
Vortex Meters
Wires and Cables

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