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Selection of Electrical Apparatus in Hazardous Environment
Meaning of markings and temperature specifications according to IEC/EN 60079.

An example of marking according to IEC/EN 60079 is "Ex ib IIC T4". My question is about the temperature classes.

T1 450 C
T2 300 C
T3 200 C
T4 135 C
T5 100 C
T6 85 C

Suppose an transmitter is certified as T4. Now, what does this mean? I and my colleague have 2 opposing interpretations.

1) I argue that it specifies the maximum temperature that the surface of the transmitter could reach under normal/abnormal conditions. So, by this logic, T5 is better than T4 and T6 is better than T5.

2) My colleague argues that it specifies the maximum surface temperature from outside that the transmitter can withstand without going haywire. So, by this logic, T5 is better than T6 and T4 is better than T5.

So, which one is correct?

Surely equipment having temp. class T6 is better/superior to T5, T4 and so forth in that order. The subject instrument having Temp cl. T4 implies that instrument temp. shall not exceed T4 class i.e. 135 Deg c under normal/abnormal conditions and thereby rule out possibility of any ignition/explosion that could have caused due to surrounding flammable hydrocarbons/gases having auto ignition temp of more than 135 Deg C. which can come in contact with instrument surface.

In subject case if instrument selected/offered with Temp. Cl T5 or T6 it will be even better as the instruments thermal limits will be controlled more stringently (and thus more challenging to design) than that of T4.

Rule is Equipment/instrument Temp class (T1 to T6) must be be < auto ignition temp of surrounding flammable hydrocarbon/vapours/gases. e.g.

- T6 certified equipment can be installed in hazardous zone having Temp Cl. T1 to T6.

- T3 certified equipment cannot be installed in hazardous zone having temp. Class T4/T5/T6.