The main precautionary measure for the prevention of fires and explosions from electrical equipment is the proper selection and operation of equipment in explosive and fire-hazardous areas. According to PUE, the premises are divided into explosive (B-I, B-Ia, B-Ib, B-Ig, B-P, B-Pa) and fire-hazardous (B-I, P-P, P-Pa, P- W) zones.
Explosive zone is a space in which explosive mixtures are or may be present
The fire zone is a space where combustible substances can be present during normal technological process and in case of possible violations thereof.
Class B-I - Areas of premises in which flammable gases and vapors are emitted in such quantity and with such properties that they can create explosive mixtures with air or other oxidants under normal short operating modes.
Class B-Ia - Areas of premises in which an explosive concentration of gases and vapors is possible only by accident or malfunction.
Class B-Ib are the same zones as Class B-Ia but have the following features:
- combustible gases have a high lower concentration limit of flame spread (15% or more) and a sharp odor;
- in the event of an accident in these areas, only local formation is possible! explosive concentration, which extends to a volume not exceeding 5% of the total volume of the room (zone);
- flammable gases and liquids are used in small quantities without the use of open flames, in exhaust hoods or under exhaust umbrellas.
According to PUE, if the volume of the explosive mixture exceeds 5% of the free volume of the room, then the whole room belongs to the corresponding class of explosive. If the volume of the explosive mixture is equal to or less than 5% of the free volume of the room, then the area of the room within 5 m horizontally and vertically from the technological apparatus from which the release of combustible gases or steam of LZR is possible is considered explosive. Premises outside the explosive zone shall be considered explosive unless there are other factors which create an explosion hazard.
Class B-Ig - External installations containing explosive gases, vapors, liquids, the explosive concentration of which can be formed only by accident or malfunction.
Class B-II - Areas of premises where the formation of explosive concentrations of dust or fibers with air or other oxidizing agents may occur under normal, short operating conditions.
Class B-IIa - Areas similar to those of Class B-IIa, where explosive concentrations of dust and fibers can be generated only by accident or malfunction.
Class P-I - Areas of premises in which flammable liquids with a flash point above 61 ° C are used or stored.
Class P-II - Areas of premises where combustible dust or fibers with a lower concentration limit of flame spread of more than 65 g / m3 of air volume or explosive dust are emitted, the contents of which do not reach explosive concentrations in the air of the premises.
Class P-IIa - Areas of rooms containing solid or fibrous combustibles. Combustible dust and fibers are not released.
Class P-III - External installations where flammable liquids with a flash point above 61 ° C are used or stored, as well as solids.
The zone class is determined by the technologist together with the electricians of the design or operational organization, based on the environmental characteristics.
According to the PUE, closed-loop electrical equipment is used in fire-hazardous areas, the interior of which is separated from the outside by a shell. Control and protection equipment, luminaires is recommended to be used in dustproof version. All wiring must have sound insulation. Explosion-proof equipment manufactured in accordance with GOST 12.2.020-76 shall be used in hazardous areas and in external installations. Starting equipment, magnetic actuators for Classes B-I and B-II must be carried out outside of explosive rooms with remote control. Wires in explosive areas must be laid in metal pipes. Armored cable may be used. Luminaires for Classes B-I, B-II, B-IIa must also be explosion-proof.