Explosion-proof principle for electrical equipment in explosive atmospheres

Increased safety electrical equipment is a specialized explosion-proof type of explosion-proof electrical equipment, indicated by the symbol “e”. It is a widely used type of explosion-proof electrical equipment in industrial enterprises where flammable gases are present.

It is generally accepted that when the rated voltage of the power supply does not exceed 11kV (AC RMS or DC value), under normal operating conditions or recognized abnormal conditions will not produce sparks, arcs and (or) dangerous humidity of the electrical equipment, is allowed to design and manufacture of safety-enhancing electrical equipment. Obviously, electrical equipment that does not meet these conditions is not allowed to be made into an explosion-proof type of increased safety.

Under the above qualifying conditions, safety-enhanced electrical equipment is not “explosion-proof” by using the so-called “explosion-proof enclosure” used in explosion-proof electrical equipment, but rather, according to the sufficient conditions necessary for combustion and explosion, the various parts of the electrical equipment are mechanically and/or electrically enhanced to further increase their degree of safety and reliability, thereby avoiding the possibility of their generating sparks, arcs and/or dangerous temperatures. Mechanical and (or) electrical enhancement measures to further improve their degree of safety and reliability, so as to avoid them to produce sparks, arcs and (or) the possibility of dangerous temperatures, that is to say, the electrical equipment to take and put forward a number of structural measures and safety requirements so that it is unlikely to become a source of ignition of flammable gases.

This is the principle of explosion protection for increased safety electrical equipment.

According to the explosion-proof principle of the increased safety electrical equipment, you should know that the designers must put forward necessary and sufficient special requirements on the mechanical structure of the electrical equipment, shell protection, electrical insulation, wire connections, electrical problems and creepage distances, limit temperatures, etc. to ensure that the increased safety electrical equipment has reliable explosion-proof safety performance.

Generally speaking, this type of explosion-proof type of live electrical equipment types are: AC motors (including rotating motors, transformers and solenoids, etc.); lamps and lanterns (including lamps and lanterns bad inductive ballasts); resistance to heaters; battery type; connection (sub) wire box (box) type; instrumentation current transformers and non-instrumentation current transformer type. In addition to other electrical equipment to be made of this type of explosion-proof explosion-proof type, explosion-proof electrical designers and producers should comply with the general requirements of the explosion-proof explosion-proof type in addition to the general requirements, but also must be considered to put forward a number of additional technical measures and safety requirements.

These additional technical measures and safety requirements should, in principle, include the following:

① The use parameters of the electrical components used under the installation conditions shall not exceed 2/3 of their nominal ratings.

② The heat generating component shall not generate dangerous temperatures exceeding the limit temperature and shall not adversely affect the circuit unit (structure, parameters, insulation) around it. ③ The resistive element should be a film type or wire-wound resistor.

④ The inductive element should have measures to prevent the generation of a reverse electromotive force at the moment of circuit disconnection.

⑤ Capacitor elements should be solid insulating dielectric capacitors, electrolytic capacitors and tantalum capacitors shall not be used.

(vi) Switching elements are protected by explosion-proof enclosures, etc.

Generally, such explosion-proof types are not classified as explosion-proof. If necessary, one can determine the required explosion-proof level of a certain type of electrical equipment through testing, for example, high-voltage or large-capacity safety-enhancing AC motors are such that they can also be classified as Class ⅡA, Class ⅡB, or Class ⅡC through testing. However, the increased safety explosion-proof type should have the corresponding equipment protection level, such as class b or class C; in practice, it can also be expressed as class Gb or class Gc. The enclosure of the increased safety electrical equipment is often made of sheet metal (e.g., some steel profiles and aluminum alloy profiles), cast iron, cast aluminum and engineering plastics.

Market sales of explosion-proof electrical products still do not meet the structural requirements of the relevant standards. The following is only from the structure of the level of analysis of how to screen the authenticity of explosion-proof compound explosion-proof electrical equipment.

Explosion-proof enclosure “d” for electrical equipment, a type of explosion-proof, the shell can withstand through the shell of any joints or structural gaps into the shell of the internal explosion of explosive mixtures in the internal explosion without damage, and will not cause the external by one or more gases or vapors formed by the formation of an explosive gas environment of the ignition.

Increased safety type “e” is an explosion-proof type of electrical equipment, i.e., additional measures are taken to increase the safety of electrical equipment to prevent the possibility of generating dangerous temperatures, arcs and sparks in normal operation or under specified abnormal conditions.

When different parts of an electrical device or Ex components using different explosion-proof type, explosion-proof signs should include all the explosion-proof type symbols used, explosion-proof type symbols should be arranged in alphabetical order, there should be a small interval between them. In practice, explosion-proof compound (Ex d e) explosion-proof electrical products generally have two structures are most widely used:

1. Explosion-proof electrical products, such as explosion-proof lighting (power) distribution box, explosion-proof magnetic starter, etc., are formed by the combination of explosion-proof cavity and safety-enhancing cavity. This structure for the different parts of the electrical equipment, respectively, the use of explosion-proof and increased safety of the explosion-proof type, as shown in the figure below. 2. Increased safety cavity installed inside the explosion-proof or explosion-proof and increased safety composite Ex components of the explosion-proof electrical products, such as explosion-proof lighting switches, explosion-proof corrosion-resistant columns and other electrical products. This structure is the main body of the electrical equipment for the increased safety, with Ex components using explosion-proof or explosion-proof increased safety explosion-proof type, as shown in the figure below.
     However, it should be noted that the structure of the introduction of the device through the head and the equipment shell installed between the rubber gasket, and through the screw from the equipment shell internal fastening, as shown below, does not meet the requirements of the explosion-proof shell, can be used for the increased safety shell.

As a result, if the equipment nameplate explosion-proof marking of the explosion-proof type for the explosion-proof Zengan composite type, and the existence of the following structure, it can be determined that the product does not meet the explosion-proof requirements: 1. single-cavity electrical equipment, the introduction of the device there is a structure shown in Figure 3, and the internal presence of circuit breakers, power supply modules, household plugs, non-intrinsically safe circuit boards and other ignition components. Ans: The enclosure with the above structure cannot be used as an explosion-proof enclosure, and the ignition elements cannot be installed inside the safety-enhancing enclosure, as shown in the figure below.   

A single-chamber equipment introduction device exists in the configuration shown in Figure 4 and does not contain an internal element with an explosion-proof structure.
    Explanation: The device does not have an explosion-proof structure and does not correspond to the explosion-proof type in the explosion-proof marking, as shown in the figure below.