Ex-pert Electrical Technologies

Dynamic elaboration on the Art & Science of Hazardous Area Protection

Temperature Class

Temperature Class

The concept of Temperature Class is best described by IEC 60079.0 by combining definitions, tables and specification , thus  creating a road-map to clarity

IEC 60079-0 entrance

Let us follow that road-map.

IEC 60079-0 states that the standard atmospheric conditions, under which, it may be assumed that electrical equipment can be operated are:
• Temperature –20 °C to +60 °C;
• Pressure 80 kPa (0,8 bar) to 110 kPa (1,1 bar)
• Air with normal oxygen content, typically 21 % v/v.

(Ambient Temperature: The temperature of air or any other media in the immediate vicinity of the equipment under consideration)

Although Temperature Range is specified, as from –20 °C to +60 °C, but most of the electrical components carry the –20 °C to +40 °C temperature tag as Normal ambient temperature range for operation. IEC 60079-0 allows this for the following reasons:-
1. Ambient Temperature Range of –20 °C to +40 °C is appropriate for most of the equipment and their operation
2. Designing & manufacturing of all equipment for the –20 °C to +60 °C range would be expensive.

So… all Electrical equipment designed for use in a normal ambient temperature range of –20 °C +40 °C does not require marking of the ambient temperature range.
However, electrical equipment designed for use in other than this normal ambient temperature range is considered to be special…..
And so should have the special marking of either the:-
symbol : Ta or Tamb  together with both the upper and lower ambient temperatures


if this is impracticable, the symbol “X“.

Some important guide-line definitions:

Maximum Surface Temperature   (IEC 60079-0)
Highest temperature which is attained in service under the most adverse conditions (but within
the specified tolerances) by any part or surface of electrical equipment.

NOTE 1 For electrical equipment in an explosive gas atmosphere, this temperature may occur on an internal
component or on the external surface of the enclosure, depending upon the type of protection employed.
NOTE 2 For electrical equipment in an explosive dust atmosphere, this temperature occurs on the external
surface of the enclosure and may include a defined dust layer condition.

Ignition Temperature of an explosive gas atmosphere
Its the lowest temperature of a heated surface which, under specified conditions according to
IEC 60079-20-1, will ignite a flammable substance in the form of a gas or vapor ( mixture with Air)

Ignition Temperature of a dust layer
Its the lowest temperature of a hot surface at which ignition occurs in a dust layer of specified thickness on a hot surface.
(NOTE : The ignition temperature of a dust layer may be determined by the test method given in IEC 61241-2-1.)

Ignition Temperature of a dust cloud
Its the lowest temperature of the hot inner wall of a furnace at which ignition occurs in a dust cloud in air contained therein

Limiting Temperature
Maximum permissible temperature for equipment or parts of equipment equal to the lower of  the two temperatures determined by:
a) the danger of ignition of the explosive atmosphere;
b) the thermal stability of the materials used

All this is about the equipment which are to be used in the Hazardous Areas.

Now it is time to enter the hazardous area.

We will start…… by going below ground level into a COAL MINE .

Temperature Class Slide 2

Caution from IEC : We can only use electrical equipments which belong to GROUP I.
According to IEC 60079-0 the maximum surface temperature shall be specified in
relevant documentation (declaration by the manufacturer) .
This maximum surface temperature shall not exceed:-
– 150 °C on any surface where coal dust can form a layer,
– 450 °C where coal dust is not likely to form a layer (i.e., inside of a dust-protected enclosure)
NOTE from IEC : When choosing Group I electrical equipment, the user should take into account the influence and the Smouldering temperature of coal dusts if they are likely to be deposited in a layer on surfaces with temperatures
above 150 °C.

So the maximum temperature allowed inside the coal mine is  150 °C

Now ..on to the surface …into another Hazardous Area .. an OIL WELLS farm.

Temperature Class Slide 3

Group II electrical equipment are only allowed here .

The materials  , general used in this category of Hazardous Area are many and their number is increasing . ( Although the active use of many harmful & environment-damaging materials is also increasing). To accommodate this situation a Temperature Table is recommended to be followed.

Classification of maximum surface temperatures
for Group II electrical equipment
(As per IEC 60079-0)
Temperature Class Maximum Surface Temperature  °C
T1 450
T2 300
T3 200
T4 135
T5 100
T6 85

(As per IEC 60079-0) The maximum surface temperature determined  shall not exceed:

– the temperature class assigned as in the above table  or

– the maximum surface temperature assigned, or

– if appropriate, the ignition temperature of the specific gas for which it is intended.

The Safety Rule (hidden in these statements ) is as follows:

Although Group II equipment can attain Maximum Surface Temperatures as per the Table  ( From 85 °C to 450 °C ).(Flood lights of  1000 watts Halogen lamp ,belonging to Group II, will attain a temperature of   300 °C  ).

The ignition Temperature of the gas/vapors  being processed in that area should NOT be 300 °C or less, referring to our example of the Flood light.

The Ignition Temperature of the gas should be HIGHER than the maximum Surface Temperature attained by any electrical or other equipment during operation , so that the gas molecules does not get enough energy to react with air (oxygen) in form of an EXPLOSION .             (Common sense & simple logic)

Tempeature Class Example

For Combustible Dusts(Group III)
Electrical equipment of Group III is intended for use in places with an explosive dust atmosphere excluding mines .
Electrical equipment of Group III is subdivided according to the nature of the explosive dust atmosphere for which it is intended.
The three sub-divisions are :-
• IIIA: combustible flyings
• IIIB: non-conductive dust
• IIIC: conductive dust

Following Two scenarios exists for the combustible dust :-
1. Maximum surface temperature determined without a dust layer
The maximum surface temperature determined  shall not exceed the maximum surface temperature assigned.
2. Maximum surface temperature with respect to dust layers
In addition to the maximum surface temperature required in scenario 1,  the maximum surface temperature may also be determined for a given depth of layer, TL, of dust surrounding all sides of the equipment, unless otherwise specified in the documentation, and marked with the symbol “X” to indicate this specific condition of use in accordance with the following:-

the maximum surface temperature in degrees Celsius and the unit of measurement °C preceded with the letter “T”, (e.g. T 90 °C).
Where appropriate , the maximum surface temperature TL shall be shown as a temperature value in degrees Celsius and the unit of measurement °C, with the layer depth L indicated as a subscript in mm, (e.g. T500 320 °C) or marking shall include the symbol “X” to indicate this condition of use.
Where the Group III electrical equipment has multiple maximum surface temperatures, e.g. for multiple ambient temperature ranges, and it is impractical to include the complete information in the marking, or where there are external sources of heating / cooling then :
• the complete maximum surface temperature information shall be included in the certificate and the marking shall include the symbol “X” to indicate this specific condition of use.

• the maximum surface temperature range shall be shown in the marking with the lower and upper limits of the surface temperature separated by “…”, e.g. “T80 °C…T195 °C”. Ex cable glands, Ex blanking elements, and Ex thread adapters need not be marked with a maximum surface temperature.  (IEC 60079-0)

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About Asif Mirza

electrical engineer artist


This entry was posted on July 18, 2014 by in Uncategorized.
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