What is meant by maximum surface temperature and what can be the maximum surface temperature allowed for the correct operation of the equipment in presence of GAS, without causing an explosion just because the temperature is exceeded?

By surface temperature is meant the temperature detectable on the external surface of the equipment, a value that the system designer must carefully consider when choosing the temperature class. The maximum surface temperature of an equipment must never reach, even in fault conditions, the ignition temperature of the substance present in the dangerous atmosphere.

This value is normally reported on the certificate plate, where the temperature class "T ..." is reported for gases, while the temperature T ...° C is reported for dusts. 

For areas with the presence of gas, the equipment or materials are divided into temperature classes such as: 

• T6 for max. temperature of + 85° C (acceptable temperature class T6)
• T5 for max. temperature of + 100° C (acceptable temperature class T5-T6)
• T4 for max. temperature of + 135° C (acceptable temperature class T4-T5 - T6)
• T3 for max. temperature of + 200° C (acceptable temperature class T3-T4-T5-T6)
• T2 for max. temperature of + 300° C (acceptable temperature class T2-T3-T4-T5-T6)
• T1 for max. temperature of + 450° C (acceptable temperature class T1-T2-T3-T4-

All temperature classes with higher numbers are redundant on those with lower numbers.

For more information, read the "Guide to the Ex world".