COMBUSTIBLE DUST HAZARDS
Many dusts that are generated, processed, handled and stored are combustible. When ignited they can burn rapidly and with considerable explosive force if mixed with air in the appropriate proportions. When electrical apparatus is used in locations where combustible materials are present, it is important that suitable precautions are taken. In electrical apparatus, potential ignition sources include electrical arcs and sparks, hot surfaces and frictional sparks. To reduce the likelihood of ignition of the external explosive atmosphere, all such electrical apparatus should be adequately protected.
Areas where dust, flyings and fibres in air occur in dangerous quantities are classified as hazardous and are divided into three zones according to the level of risk.
Zone 20 An area in which combustible dust, as a cloud, is present continuously or frequently, during normal operation, in sufficient quantity to be capable of producing an explosive concentration of combustible dust in a mixture with air.
Zone 21 An area, in which combustible dust, as a cloud, is occasionally present during normal operation, in a sufficient quantity to be capable of producing an explosive concentration of combustible dust in a mixture with air.
Zone 22 An area, in which combustible dust, as a cloud, may occur infrequently and persist for only a short period, or in which accumulations of layers of combustible dust may give rise to an explosive concentration of combustible dust in a mixture with air.
The standards and corresponding forms of protection that have been established for dust hazard protection are detailed as follows :-
|IEC 61241-0||Electrical apparatus for use in the presence of combustible dust – General requirements|
|IEC 61241-1||Electrical apparatus for use in the presence of combustible dust – Protection by enclosures 'tD'|
|IEC 61241-4||Electrical apparatus for use in the presence of combustible dust – Type of Protection 'pD'|
|IEC 6124-11||Electrical apparatus for use in the presence of combustible dust – Protect by intrinsic safety 'ID'|
|IEC 61241-18||Electrical apparatus for use in the presence of combustible dust – Protection by encapsulation 'mD'|
The standard for classification of Explosive Atmospheres where dust are or may be present was previously IEC 61241-10, but is now recoded as follows :-
|IEC 61241-0||Electrical apparatus for use in the presence of combustible dust – Classification of areas where combustible dusts are, or maybe present.|
The standard test methods for determining the characteristics of the dust hazard are shown as follows :-
|IEC 61241-2-1||Electrical apparatus for use in the presence of combustible dust – Part 2: Test methods – Section 1: Methods for determining the minimum ignition temperatures of dust.|
|IEC 61241-2-2||Electrical apparatus for use in the presence of combustible dust – Part 2: Test methods – Section 2: Methods for determining the electrical resistivity of dust in lawyers.|
|IEC 61241-2-3||Electrical apparatus for use in the presence of combustible dust – Part 2: Test methods – Section 3: Methods for determining minimum ignition energy of dust/air mixtures.|
Combustible dust can be ignited by electrical apparatus in several ways :-
- By surfaces of the apparatus that are above the minimum ignition temperature of the dust concerned. The temperature at which a type of dust ignites is a function of the properties of the dust, whether the dust is in a cloud or layer, the thickness of the layer and the geometry of the heat source.
- By arcing or sparking of electrical parts such as switches, contacts, commutators, brushes, or the like.
- By discharge of an accumulated electrostatic charge.
- By radiated energy (e.g. electromagnetic radiation).
- By mechanical sparking or frictional sparking or heating associated with the apparatus.
Selection of Equipment for Dust Hazards :-
When selecting equipment intended for use in areas where potentially explosive dust hazards may be present, and accessible to dust, the selection process would entail the review of certified equipment rated for dust hazards to a standard set of procedures. This would involve selection of the form of equipment protection, and ATEX dust group when applicable, and also identifying the permissible Ignition Temperature of the apparatus.
Dust in its deposited form has a different ignition temperature than in its stirred form. IEC 61241-2-1 specifies the method of determining the ignition temperatures identified as procedure A for deposits, or layers, and procedure B for clouds.
The permissible Ignition Temperature of the apparatus would be determined by firstly identifying the ignition temperature of the dust as a deposit, and then as a cloud :-
Smouldering temperature based upon 'Deposit' - Method A according to IEC 61241-2-1 Ignition temperature based upon 'Cloud' - Method B according to IEC 61241-2-1
Secondly by subtracting 75 K from the value determined using method A and by multiplying by 2/3 the value determined using method B, the two ignition temperature values for the combustible dust will be known. The lower of the two values determined in this way corresponds to the lowest permissible surface temperature of the equipment.
For example the following results may be drawn for brown coal :-
T max = Smouldering temperature -75 K e. g. brown coal 225°C -75°C = 150°C or,
T max = 2/3 of the ignition temperature e. g. brown coal 2/3 * 380°C = 254 °C,
In which case equipment should be selected that is limited to a Tmax 150°C rating.
It should be noted that compliance with this standard will only provide the required level of safety if the electrical apparatus is operated within its rating and is installed and maintained according to the relevant codes of practice or requirements, for example in respect of protection against over currents, internal short-circuits, and other electrical faults. In particular, it is essential that the severity and duration of an internal or external fault be limited to values that can be sustained by the electrical apparatus without damage.