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, and hence T-class.
As can be seen from the Temperature Classes table below; a T6 certification allows a maximum permissible surface temperature of 85°C therefore a T6 instrument can be used in T5, T4, T3, T2 and T1 environments.
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 explosion protection concept marking on the product's 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. See our page on the ATEX Directive for further background on ATEX.
Temperature Class for Group I Applications
Temperature classes do not apply to Group I applications, i.e. equipment for use in the mining industry. Mining equipment has either a rigid 150°C or 450°C limit.
Temperature Class for Group II Applications
The temperatures corresponding to each temp class are shown below. It is worth noting that North American and Canadian standards differ from European standards by having sub groups e.g. T3A
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For those who want to delve further into Hazardous Area Classification and the methods employed in minimising fire and explosion risk, then the following books from Amazon will be of interest: