Gas groups, temperature ratings and dust
Gas and Dust Groups
Electrical apparatus for use in areas with flammable gases and vapours are divided into groups:
| Group I | Underground coal mining applications |
| Group II | Industries other than underground coal mining |
| Group III | Industries having combustible dust or fibres |
Group II is subdivided into 3 group groups based on their explosive properties (see table below).
| Groups | Example Types |
| Group I | Methane |
| Group IIA | Propane |
| Group IIB | Ethylene |
| Group IIC | Hydrogen, Acetylene, Carbon Disulphide |
Temperature or 'T' Rating
Hot surfaces are a potential ignition source in hazardous applications. Hazardous materials will have different minimum temperatures at which ignition becomes a danger.
Electrical equipment for hazardous areas is tested and allocated a ‘T Rating’ based on the maximum temperature of the equipment.
The ‘T Rating’ must always have a lower maximum temperature than the ignition temperature of the gas group present. Temperatures are either measured internally or externally depending on protection.
| T Rating | Maximum Temperature |
| T1 | 450C |
| T2 | 300C |
| T3 | 200C |
| T4 | 135C |
| T5 | 100C |
| T6 | 85C |
| Example Gas | Ignition Temperature | T Rating |
| Methane | 580C | T1 |
| Hydrogen | 500C | T1 |
| Kerosene | 295C | T3 |
| Petroleum | 400C | T2 |
| Carbon Disulphide | 95C | T6 |
| Diethyl ether | 160C | T4 |
| Ethyl Nitrite | 95C | T6 |
| Hydogen Sulphide | 270C | T3 |
Combustible Dust
Dust fibres or flyings, which are combustible or ignitable when mixed with air are known as combustible dusts. Dust explosions may be initiated by ignition of a cloud of dust. It is not unusual for an explosion, which starts from the ignition of a dust layer, to cause the dislodging of dust accumulated on various surfaces, which then leads to a chain reaction dust cloud explosion.
Characteristics of some common combustible dusts are:
| Material | Ignition temperture | |
| Cloud, C | Layer, C | |
| Coal Dust | 380 | 225 |
| Polythene | 420 | (melts) |
| Methyl Cellulose | 420 | 320 |
| Starch | 460 | 435 |
| Flour | 490 | 340 |
| Sugar | 490 | 460 |
| Grain Dust | 510 | 300 |
| Phenolic Resin | 530 | >450 |
| Aluminium | 590 | >450 |
| PVC | 700 | >450 |
| Soot | 810 | 570 |
NOTE: The data given in this table is derived from AS 2430.2:1986 (superseded by AS/NZS 61241.3 which is superseded by AS/NZS 61241.10)
