Ex-pert Electrical Technologies
Dynamic elaboration on the Art & Science of Hazardous Area Protection
Explosive Dust Atmosphere
Explosive Dust Atmosphere
It is a mixture with air, under atmospheric conditions, of flammable substances in the form of dust, or flyings in which, after ignition, self-sustaining flame propagation is possible (IEC 60079-10-2)
Types of Dusts
Combustible Dust
Finely divided solid particles, 500 μm or less in nominal size, which may be suspended in air ,may settle out of the atmosphere under their own weight, can burn or glow in air, and may form explosive mixtures with air at atmospheric pressure and normal temperatures
Conductive Dust
Combustible dust with electrical resistivity equal to or less than 103 Ωm
Non-Conductive Dust
Combustible dust with electrical resistivity greater than 103 Ωm
Any one or a combination of, these types of DUSTS, can generate a dust hazardous area.
Definition :
An area in which combustible dust, in the form of a cloud is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of equipment
Dust Hazard Area Classification:
Dust Hazardous Area is classified into ZONES.
Zone 20
Zone 21
Zone 22
The electrical equipment for use in the DUST HAZARDOUS AREA are designed with following PROTECTION LEVELS
Equipment Protection Level Da
Equipment for explosive dust atmospheres, having a “very high” level of protection, which is not a source of ignition in normal operation, during expected malfunctions, or during rare malfunctions
Equipment Protection Level Db
Equipment for explosive dust atmospheres, having a “high” level of protection, which is not a source of ignition in normal operation or during expected malfunctions
Equipment Protection Level Dc
Equipment for explosive dust atmospheres, having an “enhanced” level of protection, which is not a source of ignition in normal operation and which may have some additional protection to ensure that it remains inactive as an ignition source in the case of regular expected occurrences (for example failure of a lamp)
(IEC 60079-0)
The Traditional relationship between Zones and EPL
| Equipment protection level | Zone | |||
| Ga | 0 | |||
| Gb | 1 | |||
| Gc | 2 | |||
| Da | 20 | |||
| Db | 21 | |||
| Dc | 22 | |||
The EPL as stated by the manufacturers is expected to provide the corresponding protection
(IEC 60079-0 has tabulated this in form of following table)
|
Protection Afforded |
Equipment protection level |
Performance of protection |
Conditions of operation
|
|
GROUP |
|||
|
VERY HIGH |
Ma |
Two independent means of protection, or safe even when two faults occur independently of each other | Equipment remains functioning when explosive atmosphere is present |
|
GROUP I (MINES) |
|||
|
VERY HIGH |
Ga |
Two independent means of protection, or safe even when two faults occur independently of each other | Equipment remains functioning in zones 0, 1 and 2 |
|
GROUP II (GAS) |
|||
|
VERY HIGH |
Da |
Two independent means of protection, or safe even when two faults occur independently of each other | Equipment remains functioning in zones 20, 21 and 22 |
|
GROUP III (DUST) |
Collectively the equipment designed & manufactured for the Hazardous Dust Areas belong to Equipment Group III
For the sake of structured selection procedure of Electrical equipment , there is a further subdivision of Group III into following (sub-groups) according to the nature of the explosive dust atmosphere for which it is intended.
• IIIA: Combustible Flyings
• IIIB: Non-Conductive Dust
• IIIC: Conductive Dust
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