Requirements for electrical equipment intended for use in potentially explosive atmospheres

Electrical equipment installed in hazardous areas and outdoor installations must have a design that ensures its safe use in a wide variety of categories and groups of explosive mixtures. However, it would be irrational to produce electrical equipment in one design for all categories and groups of explosive mixtures, since explosion-proof electrical equipment can have a different design that ensures its safe use in explosive premises and outdoor installations.

Depending on the type of execution, as well as the highest category of explosive mixture and its self-ignition group, for which this electrical equipment is recognized as explosion-proof, the following symbols are established: Classification and marking of explosion-proof electrical equipment

The main requirements for electrical equipment intended for work in explosive areas of different classes are divided into:

• requirements that define the scope depending on the version;

• requirements for installation of equipment and installation parts;

• requirements for the design of explosion-proof electrical equipment.

The above basic requirements are not the same for different types of electrical equipment.

Consider the general requirements for electrical equipment in explosive areas and outdoor installations designed to ensure its normal continuous operation under operational conditions.

The main condition for the reliable operation of electrical equipment is the correct selection of it, high-quality production and the mandatory performance of preventive tests and scheduled maintenance under operating conditions. Whenever possible, the use of portable energy consumers should be limited.

If this does not cause particular difficulties, it is recommended to remove electrical equipment, especially with parts that spark during normal operation, outside of potentially explosive areas.

The flange gaps of the housings of electrical machines and devices of explosion-proof design must not adjoin any surface, but must be at a distance of at least 100 mm from it.

Electrical equipment must be protected from possible mechanical and chemical impacts, as well as from prolonged exposure to moisture (it is recommended to maintain air humidity of at least 75%).

Ventilation devices must create an excessive pressure of frequent air in the chambers or housings of the machines and apparatus that are vented. In Class B-Ia rooms, it is allowed to use a closed cooling cycle with pre-purge when starting with fresh air or inert gas.

When the pressure in the air or the chamber (enclosure) falls below the safe limit, the electrical equipment of rooms of classes BI and B-II must be automatically disconnected from all sources of electricity, and in rooms of classes B-Ia and B-IIa, the alarm for danger should be activated automatically.

Purging chambers or shells, as well as air ducts, must be mechanically sound and provide tight closure of machinery or apparatus, and their design must exclude the formation of "pockets" of gases or vapors (i.e. local accumulations of explosive concentrations ).

Air ducts must be made of non-combustible material. The connection of individual sections must be done by welding or in another way that guarantees the strength and tightness of the joints. Doors or covers of ventilation chambers opening in explosive areas must have a lock to prevent them from being opened when the electric motor or apparatus is switched on.

The switching on of electric motors and electrical devices must be carried out with a delay in relation to the start-up time of the ventilation devices for the time necessary to remove an explosive atmosphere that may penetrate the chamber or enclosure.

Movable parts of structures of explosion-proof electrical equipment that open access to live parts must be arranged so that they can be opened or removed only with the help of special devices (spanners).

In rooms of class B-I and B-II, doors and removable covers of electrical devices must have a lock that allows them to be opened only when the voltage is removed.Moving parts of electrical equipment must have a sealing device.

To prevent sparking that can be caused by static electricity, only wedge-type transmissions from electric motors to mechanisms should be used. In exceptional cases, when conventional belt drives are used, measures must be taken to safely remove the static charge with belts (lubricated with special pastes).

Both low-voltage and high-voltage (up to 10 kV) electric motors can be used in hazardous areas and outdoor installations. In this case, electric motors with a voltage of 10 kV are allowed only in the version blown by excess pressure.

Oil-filled electric devices are usually installed in stationary installations, oil-filled electric motors can also be used in crane installations, taking precautions against oil splashing.

In an explosion-proof (explosion-proof) design, the electric motor has a casing, which is an element of its structure capable of containing the highest explosive pressure (inside this casing) and not transmitting an explosion to the surrounding explosive environment.

The fulfillment of the above condition is ensured by the fact that all connections between individual structural elements of electric motors, which make up the fireproof housing, are carried out in accordance with the norms for the minimum permissible width and length of the safe gap for a given environment.

The engine is designed in such a way that during continuous operation the heating temperature of its external surfaces is not dangerous from the point of view of ignition of the surrounding explosive atmosphere.The dimensions of the gaps and temperatures are standardized by the rules for the production of explosion-proof electrical machines and devices.

Electric motors are manufactured with rolling bearings only. The use of journal bearings requires an increase in the clearance between the rotor and the stator by 10%.

Electric motors in the overpressure blown version differ from conventional electric motors in a hermetically sealed shell capable of maintaining an increased pressure inside it relative to ambient pressure. Excessive pressure is required to prevent gas from entering the shell and forming explosive mixtures there. The excessive pressure (pure air or inert gas) during continuous exchange of air or inert gas is carried out by a ventilation device.

The design requirements for various types of explosion-proof equipment and devices are similar to those listed for electrical machinery.

Electrical devices and devices can be explosion-proof, overpressure blown, intrinsically safe (class B-I only) and special versions.

When placing electrical devices and devices in hazardous areas, it is necessary to take into account that clamps, plug connections in a normal design must be removed outside the premises. When installing the bracket in Class B-I and B-II explosive areas, they must be fireproof or oil-filled.

Plug connections in Class B-Ia premises are also permitted in a dustproof design where contacts are made and broken only inside closed receptacles.

The installation of plug connections is allowed only for the inclusion of intermittently operating electrical receivers (portable).The number of plug connections should be as limited as possible and should be located where explosive mixtures are least likely to form.

The connection of wires to devices and devices installed permanently must be done especially reliably: by soldering, welding, screwing or in another equivalent way. Screw terminals must have means to prevent self-loosening.

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