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.
Generally, electrical safety is ensured by the implementation of one of two considerations, i.e. that electrical apparatus be located where reasonably practicable outside hazardous areas, and that electrical apparatus be designed, installed and maintained in accordance with measures recommended for the area in which the apparatus is located.
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.
- 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 overcurrents, 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.
Several techniques are available for the explosion protection of electrical apparatus in hazardous areas. This standard describes the safety features of these types of explosion protection techniques and specifies the installation procedures to be adopted. It is most important that the correct selection and installation procedures be followed to ensure the safe use of electrical apparatus in hazardous areas.
BS EN 61241-0 specifies general requirements for the design, construction, testing and marking of electrical apparatus protected by any recognized safeguard technique for use in areas where combustible dust may be present in quantities that could lead to a fire or explosion hazard.
It includes the relevant text which interfaces the standard directly to the ATEX Directive without technical change other than in marking and amplification of the instructions in order to allow harmonization of the standard.
Part 0 of BS EN 61241 covers Category 1D, 2D and 3D apparatus. Only in cases specifically mentioned, the requirements differ for Categories 1D, 2D and 3D.
The application of electrical apparatus in atmospheres which may contain explosive gas as well as combustible dust, whether simultaneously or separately, requires additional protective measures.
This standard does not specify requirements for safety, other than those directly related to the explosion risk.
Where the apparatus has to meet other environmental conditions, for example, protection against ingress of water and resistance to corrosion, additional methods of protection may be necessary. The method used is not to adversely affect the integrity of the enclosure.
This standard does not apply to dusts of explosives that do not require atmospheric oxygen for combustion, or to pyrophoric substances.
This standard is not applicable to electrical apparatus intended for use in underground parts of mines as well as those parts of surface installations of such mines endangered by fire damp and/or combustible dust.
This standard does not take account of any risk due to an emission of flammable or toxic gas from the dust.
