Ex-pert Electrical Technologies

Dynamic elaboration on the Art & Science of Hazardous Area Protection

Explosive Dust Atmosphere

Explosive Dust Atmosphere

dust cloud 2

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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