The temperature class and the maximum surface temperature allow the equipment to be selected to operate in the explosion hazard zones up to the auto-ignition temperature of vapours and gases and the ignition temperature of the cloud or layer of dust. The manufacturers of equipment designed for use in potentially explosive atmospheres of:
- gas and/or vapours should mark the equipment nameplate with the temperature class;
- dust should mark the equipment nameplate with the maximum surface temperature;
- of gas and/or vapours and/or dust atmospheres should mark the equipment nameplate with the temperature class AND the maximum surface temperature.
For Group I equipment (equipment and protective systems intended for use in mines with methane gas explosion hazards or coal dust explosion hazards), the maximum surface temperature should not exceed:
- 150°C on any surface on which a layer of coal dust may be deposited;
- 450°C where the deposition of a layer of dust is excluded (for example by sealing or ventilation), provided that the actual maximum surface temperature is shown in the equipment marking.
Group II equipment (equipment and protective systems intended for use in locations with potentially explosive atmospheres other than those listed for Group I) should be marked with the temperature class and/or maximum surface temperature. When determining these parameters, the most unfavourable operating conditions for the equipment is considered (but within the determined tolerance).
| Temperature class | Maximum enclosure temperature [°C] | Gas/vapour flash point [°C] |
| T1 | 450 | >450 |
| T2 | 300 | 300÷450 |
| T3 | 200 | 200÷300 |
| T4 | 135 | 135÷200 |
| T5 | 100 | 100÷135 |
| T6 | 85 | 85÷100 |
When selecting the equipment with regard to the flash point of the dust cloud and/or dust layer, the operator of the plant shall determine the maximum permissible surface temperature (Tmax) of the equipment, considering the relevant temperature margin (according to EN 60079-14). Key information in this regard is presented in the table below.
| MAXIMUM PERMISSIBLE SURFACE TEMPERATURE FOR DUST | ||
| DUST CLOUD | DUST LAYER |
|
| Tmax = 2/3TCLgdzie TCLis the flash point of the foreseen dust cloud | The following methods are equivalent and provide the same safety level. The choice on the method used to determine the maximum surface temperature of the equipment: | |
| Dust-free method | Dust layer method | |
| Tmax = T5mm – 75oC where T5mm is the flash point of a 5 mm layer of dust NOTE: as the thickness of the dust layer increases, the minimum flash point of the dust layer decreases and the thermal insulation increases. Where the thickness of the dust layer exceeds 5mm, an appropriate correlation showing the relationship between the maximum permissible surface temperature and the thickness of the dust layer shall be used or tested. |
Tmax = T12,5mm – 25oC where T12.5mm is the flash point of a 12.5 mm layer of dust. NOTE: the formula only applies where the thickness of the dust layer does not exceed 12.5mm |
|
For explosive atmospheres caused by gas and/or liquid vapours, the temperature margin shall be specified by the manufacturer during the conformity assessment of the product and considered in the temperature class / maximum surface temperature of the equipment concerned.
Depending on the type of explosion protection applied, the information about the temperature class / maximum surface temperature may apply to the interior or exterior of the identified device.
In cases where the maximum surface temperature does not depend primarily on the equipment itself, but on special conditions (e.g. the mounting method or temperature of the medium inside of the equipment), the temperature class / maximum surface temperature marking should feature an additional “X”. It specifies to the user that special conditions must be met to use the equipment safely.
The temperature class and the maximum surface temperature are determined at within an ambient temperature range of -20 to +40oC; otherwise the marking of the equipment shall specify this special situation. In practice, the symbol Ta or Tamb with the corresponding ambient temperature range or an “X” is shown on the nameplate.
Note:
- For potentially explosive atmospheres caused by the presence of vapours and/or gases, the temperature class (e.g. T6) is shown on the equipment nameplate;
- For potentially explosive atmospheres caused by the presence of dust, the maximum surface temperature (e.g. T85°C) is shown on the equipment nameplate
