Classification of the maximum Surface Temperature of apparatus has been established internationally to create a uniform reference table. The following Temperature Classifications are those defined in the IEC Standards for Group II Electrical Apparatus :-
|Temperature Classification||Maximum Surface Temperature of Apparatus (ºC)||Ignition temperature of Gas or Vapour (ºC)|
(Unless otherwise specified on apparatus selected, the maximum Ambient Temperature is taken as being 40ºC, in line with IEC Standards).
The purpose of the Temperature Classification is to place apparatus into an appropriate category according to its' specific thermal properties when exposed to the worst case conditions. Often this takes into account such things as maximum Ambient Temperature and maximum Operating Voltage with a +10% over voltage or an overload condition applied. In some cases the external surface temperature measurement is used to determine the T rating. This is not always the case, however, since certain forms of equipment protection have their temperature measurement taken internally as is the case with most Ex e apparatus.
With the Ignition Temperature of the flammable mixture known by the user, he must ensure that the maximum Surface Temperature (T) Rating of the apparatus selected is lower than the limiting temperature of the flammable atmosphere identified as potentially being present.
The Ambient Temperature is the Surrounding Temperature of the environment in which the equipment is installed, whether indoors or outdoors. A given piece of Electrical Equipment will be approved for a stated maximum ambient temperature in which it is safe to operate.
For example under the IEC 60079 series the normal ambient temperature range for Ex d equipment would be -20ºC to +40ºC, unless otherwise stated on the product certification. In some cases the maximum (or minimum) permitted ambient temperature will be adjusted taking into account any temperature rise under normal operation and the surface temperature rating of the equipment. The maximum (or minimum) ambient temperature will be stated on the hazardous area certificate, described as T amb, if it differs from the upper or lower limits of the IEC standards.
AUTO IGNITION TEMPERATURE
Ignition temperature is defined as the lowest temperature determined by a standardised method, at which the most explosive mixture of the given substance and air will automatically ignite when in contact with, or in the presence of, a heated surface. The Ignition Temperature of flammable materials (gases, vapours and liquids) is defined in EN 60079, along with their corresponding gas group. Examples of ignition temperatures for commonly occurring flammable mixtures are town gas (surface methane)/air, which ignites at over 600ºC, and petrol/air which ignites at approximately 250ºC.
SURFACE TEMPERATURE RATING
Any flammable mixture can be classified for explosion protection under two main characteristics; temperature of ignition by hot surfaces and the spark energy to ignite the mixture. Other important characteristics are the specific gravity and flash point, which are used to determine the Zonal classification of the explosive mixture.
Classification of Maximum Surface Temperatures has been established internationally to create a uniform reference table, but the European and North American numbering systems have not been fully harmonised to date. The following Temperature Classifications are those defined in the European (EN50014) and IEC Standards for Group II Electrical Apparatus :-