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When an electric motor is used in an atmosphere with explosive gas, the motor surface might become hot enough to cause auto-ignition. Auto-ignition temperature is the temperature at which a gas will ignite spontaneously without any other source of ignition. When hot surfaces are in contact with an explosive atmosphere, auto-ignition is likely to occur.
The table below gives the classification, which is used to indicate the maximum surface temperature that a given piece of an electric motor (the table is for motors designated as EExd) can reach when it is running normally. In general applications, the maximum surface temperature is based on a surrounding temperature of 40°C. The motor’s T-classification can be compared with the auto-ignition temperature for gases. The T-classification helps in making the right decisions regarding the AC motor’s use in areas with explosive atmospheres:
Temperature Class
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Maximum Surface Temperature
|
Groups of Gases
|
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IIA
|
IIB
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IIC
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T1
|
450°C
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Methane, Ammonia
|
Hydrogen
|
|
T2
|
300°C
|
Butane
|
Ethylene
|
Acetylene
|
T3
|
200°C
|
Kerosene, Cyclohexane
|
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T4
|
135°C
|
Acetaldehyde
|
Diethyl
Ether
|
|
T5
|
100°C
|
|||
T6
|
85°C
|
Carbon
Disulphide
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Groups of Gases and Vapors
Gases are divided into the following two explosion groups depending on which kind of industry the motor or equipment is to operate in: Explosion group I and II.
| Explosion Group | Industry Specified |
| Group I | Mines and other underground industries |
| Group II | Off-shore industries and industries above ground |
Explosion Group II Gases
Explosion group II is divided into three subgroups, IIA, IIB and IIC. Some typical gases in these groups are shown in the table below:
| Typical Gas Hazard | Gas Group |
| Acetylene | IIC |
| Hydrogen | IIC |
| Ethylene | IIB |
| Propane | IIA |
| Methane | I (firedamp) mining, IIA industrial |
As shown above, the danger of gas explosion increases from group IIA to group IIC. IIC is considered the most dangerous explosion group. The higher the dangers of explosion, the stricter are the requirements to the AC motor required to be used in these explosive environment. Therefore, it is mandatory that electrical equipment required for use in hazardous environment carries a clear marking of what explosion group it belongs to
