The 9 general requirements for explosion-proof product design are as follows:

1、 Fasteners

1. Design principles

(1) The size and material of fasteners must meet the structural requirements of explosion-proof types, such as the tensile strength of explosion-proof equipment fasteners that must withstand explosive pressure; The fasteners of increased safety and other equipment should ensure that the casing is fully compressed to achieve the specified protection level.

(2) If the fasteners of aluminum alloy and plastic shells are made of light metal or plastic bolts, the material and thread shape of the bolts must meet the fastening requirements before they can be used.

(3) The fastening of fasteners should ensure a structure that can only be opened with specialized tools.

2. Special fasteners shall comply with Article 9.1 of GB3836.1-2021.

——For fasteners that are necessary to ensure special explosion-proof types or to prevent contact with exposed live parts, only tools (such as keys, screwdrivers, or wrenches) should be used to loosen or remove them.

——If the fastener material is suitable for the casing material, the fastening screws for metal casings containing aluminum, magnesium, titanium, or zirconium can be made of aluminum, magnesium, titanium, zirconium, or non-metallic materials.

The cover or fastening screw hole required to be opened for adjustment, inspection, or other operations during operation can only be tapped on the shell material when the thread shape is suitable for the shell material.

2、 Adhesive materials

When resin composites are required for bonding between explosion-proof electrical components to achieve the specified bonding strength, the formula and process of the composites should be considered, and the ultimate temperature of the bonding material should be at least 20K higher than the surface temperature of the equipment. This is one of the 9 general requirements for explosion-proof product design.

3、 Requirements for explosion-proof shell materials

1. Commonly used metal materials include cast steel, cast iron, welded steel plates, cast aluminum alloys, stainless steel, and other materials. When using cast aluminum alloy, for Class I electrical equipment casings, the total content of aluminum, titanium, and magnesium is not allowed to exceed 15% (mass ratio), and the total content of titanium and magnesium is not allowed to exceed 6%; For Class II electrical equipment casings, the magnesium content is not allowed to exceed 6% (mass ratio). The thickness of the metal shell: for explosion-proof shells, they should be able to withstand the assessment of internal explosion pressure and external impact energy; For other types of explosion-proof enclosures, they should be able to withstand external impact energy assessment.

2. Plastic materials and plastic casings are commonly used in increased safety and intrinsic safety electrical equipment. Mainly considering the characteristics of lightweight structure and excellent resistance to environmental chemical corrosion. But the aging and deformation of materials are key defects in plastic products. Some plastics can overcome the above drawbacks, such as DMC and SMC plastic products, which are widely used in explosion-proof electrical product casings. When selecting a plastic grade, the thermal stability of the material should be considered to be at least 20K higher than the surface temperature generated by the equipment; Under the condition that the low-temperature characteristics are at least 5-10K lower than the lower limit of the ambient temperature for equipment use, it can withstand the specified impact or drop test without damage.

The influence of static charges should be considered for mobile electrical equipment and plastic surfaces that may be rubbed or wiped, which can be designed according to relevant requirements.

4、 Cable and conduit entry devices

1. Classification of common cable entry devices

The use of cable routing usually requires the use of cable entry devices, commonly including sealing ring and stuffing box types.

(1) The sealing ring type cable entry device reliably compresses the cable through the sealing ring inside the device to achieve sealing with the interior of the chamber, suitable for rubber sheathed cables entering the equipment chamber through the sealing ring.

Figure 1 Sealing Ring Cable Entry Device

(2) The stuffing box type cable entry device achieves isolation and sealing with the inside of the cavity by filling the stuffing box. When installing the incoming line, the cable sheath needs to be peeled off to allow the core wire to pass through the stuffing box. At the same time, the filling material needs to be filled around the core wire to ensure sufficient filling between the core wires and the filling length is not less than 20mm.

Figure 2 Filler Box Cable Entry Device

2. Cable introduction method

The cable introduction methods are divided into direct introduction and indirect introduction. The cable introduction device of the direct introduction method is directly installed on the main chamber shell of the equipment, and the cable is introduced into the equipment through the introduction device.

Figure 3 Direct introduction method

The indirect introduction method is to have two chambers, the wiring chamber and the main chamber. The cables are connected through the wiring chamber, and the connection between the wiring chamber and the main chamber is sealed with fillers or connected by wiring terminals. The common indirect introduction method is mostly a two chamber structure, with the upper chamber being the main chamber of the equipment, which is an explosion-proof structure used for installing electrical components; The lower chamber is a wiring chamber, which can be explosion-proof or increased safety. Indirect introduction methods are often used in high-power equipment where sparks, arcs, and high temperatures are generated during normal operation of the main chamber.

Figure 4 shows the indirect introduction method of Ex d in the wiring cavity

Figure 5 shows the indirect introduction method of Ex e in the wiring cavity

3. Scope of application of sealing ring type cable entry device and stuffing box type cable entry device

The installation process of the sealing ring type cable entry device is relatively simple, and it can meet the sealing requirements without ensuring that the cable sheath is not peeled off in advance. However, it is prone to affect the explosion-proof performance of the equipment due to the matching and aging problems of the sealing ring. It is mostly used for direct entry without internal sparks, arcs, or abnormal high temperatures, or indirect entry with wiring cavities.

The installation process of the stuffing box cable introduction device involves cable sheath peeling, branching filler, etc. The cured sealing filler is integrated with the wire core, which is relatively cumbersome to operate during equipment replacement and maintenance. However, it can be used as part of the explosion-proof structure and is often used for direct introduction, especially for explosion-proof electrical equipment with internal sparks, arcs, or abnormally high temperature components.

How to choose the cable entry device for explosion-proof electrical equipment

GB/T 3836.15-2017 clause 10.4.2b (specifies that if the cable meets the density requirements of GB/T 3836.15-2017 clause 9.3.2a), an explosion-proof cable entry device that meets the requirements of GB3836.2 standard can be used.

Whether to use a sealing ring type cable entry device or a filler type cable entry device for explosion-proof electrical equipment should be selected according to Figure 6.

Figure 6 Selection diagram of explosion-proof enclosure cable entry device for cables in accordance with 10.4.2b

Choose direct introduction method: According to the requirements of Figure 6 in GB/T 3836.15-2017, when selecting cable introduction devices,

1) Firstly, it is necessary to determine whether there is an ignition source inside the equipment casing (including sparks or the surface temperature of the equipment that can cause ignition under normal operating conditions). Equipment without an ignition source can use a sealed ring cable entry device. Equipment with an ignition source must determine whether its installation area requires IIC equipment (such as in a hydrogen environment);

2) If the installation area requires the use of IIC equipment, a sealing ring type cable entry device can be used when installed in Zone 2 or when the shell volume is less than 2dm3.

Select indirect introduction method: According to 10.4.1 of GB/T 3836.15-2017, "when the wiring cavity is Ex'd ', the cable system should meet the requirements of 10.4.2." In this case, "the indirect introduction of the shell is considered not to be an internal ignition source." The wiring cavity can be equipped with a sealed ring type cable introduction device; When the wiring cavity is Ex 'e', cable entry devices that meet the requirements of increased safety equipment should be used.

5. Precautions

If the explosion-proof cable entry device meets the requirements of GB3836.2 and is tested with a specific cable sample, and the combustible gas is ignited multiple times inside the casing without any ignition outside the casing, it is not necessary to meet the requirements of Figure 6 above.

5、 Logo

There should be a general sign indicating "Ex" or "Ex+explosion-proof level+temperature group" on the shell of explosion-proof electrical equipment. Explosion proof electrical equipment should have nameplates, which are made of brass or stainless steel materials with no specific thickness requirements. Plastic nameplates for foreign instruments have been widely used, and their adhesive material quality is quite good. The nameplate should at least include the following content:

1. Manufacturer's name or registered trademark;

2. Product name and model specifications;

3. Explosion proof signs;

4. Explosion proof certificate number;

5. Manufacturing date or factory number

6. Environmental temperature: -20 ℃ Ta 60 ℃ * When the environmental temperature is between -20 ℃ and 40 ℃, there is no requirement for this item.

6、 Connection piece

Internal and external grounding connectors should be installed on the metal casing of explosion-proof electrical equipment. The external grounding connector should be as close as possible to the cable entry device, and the internal grounding connector should be inside the wiring compartment. The size of the connector should be able to reliably connect with a protective wire of at least 4mm2 or more, and there should be anti loosening measures to ensure reliable compression. A grounding symbol should be set at the grounding connector to indicate correct connection. Electrical equipment with double insulation and reinforced insulation; Electrical equipment with metal conduit connections does not require the installation of grounding connectors.

7、 Electrical connectors and wiring cavities

The introduction of external cables or conduits for explosion-proof electrical equipment, except for the use of * cable method, is mostly carried out in the wiring chamber. When designing the wiring cavity, sufficient size should be ensured to facilitate reliable connection of wires. The explosion-proof type of the shell should comply with the explosive hazardous environment used. The specifications of the conductive bolts for the wiring terminals installed in the wiring chamber should have margin, which is important in the design of explosion-proof products.

8、 Ex components

Explosion proof housing, wiring terminals, ammeters, small switches, small buttons, indicator lights, instrument display entry devices, and other components, if made into Ex components, can be easily installed in increased safety housing, achieving the goal of lightweight structure and convenient installation and maintenance.

9、 Interlocking devices and warning signs

To prevent the live opening of the explosion-proof shell, an interlocking device should be installed, which should ensure that the mechanism cannot be opened by non specialized tools. If there is no interlocking device, a warning sign "Do not open the cover with electricity" should be set up.

In industrial environments, there are many electrical equipment with special environments, but such electrical equipment is prone to explosion with certain flammable and explosive gases in the environment. Therefore, it is necessary to design explosion-proof products for specific environments to meet the usage needs of special environments.