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Explosion-proof electrical equipment is in accordance with the requirements of GB 3836, and the explosion-proof markings of explosion-proof electrical equipment include:
Explosion-proof type + equipment category + (gas group) + temperature group
1 Explosion-proof type According to the explosion-proof measures taken, the explosion-proof electrical equipment can be divided into explosion-proof type, increased safety type, intrinsically safe type, positive pressure type,
Oil immersion type, sand filling type, encapsulation type, n type, special type, dust explosion-proof type, etc. Their identification is shown in Table 1. 
Table 1 Basic types of explosion protection

Explosion-proof type

Explosion-proof type mark

Explosion-proof type

Explosion-proof type mark

Flameproof

Ex d

Sand filling type

Ex q

Increased safety

Ex e

Molded type

Ex m

Positive pressure type

Ex p

N type

Ex n

Intrinsically safe

E x ia
Ex ib

Special type

Ex s

Oil immersion

Ex o

Dust explosion-proof type

DIP A
DIP B

2 Equipment category
Electrical equipment for explosive gas environment is divided into: 
Class I: Electrical equipment used in underground coal mines;
Class II: Electrical equipment for other explosive gas environments other than coal mines. 
Class II flameproof "d" and intrinsically safe "i" electrical equipment are further classified into IIA, IIB, and IIC. 
Electrical equipment for flammable dust environment is divided into: 
Type A dusttight equipment; Type B dust tight equipment;
Type A dustproof equipment; Type B dustproof equipment. 
3 The explosive capacity of explosive gas mixtures in gas groups indicates the degree of explosion hazard. The greater the explosive capacity of explosive mixtures, the higher the risk. The explosive capacity of an explosive mixture can be expressed by the maximum test safety clearance. At the same time, the difficulty of igniting explosive gases, liquid vapors and mists also indicates the degree of explosion hazard, which is expressed by the minimum ignition current ratio. Class II explosion-proof electrical equipment or intrinsically safe electrical equipment are further classified into categories IIA, IIB and IIC according to their maximum test safety clearance or minimum ignition current ratio applicable to explosive gas mixtures.
As shown in table 2. 
Table 2 Relationship between the group of explosive gas mixtures and the maximum test safety clearance or minimum ignition current ratio

Gas group

Maximum test safety clearance MESG (mm)

Minimum ignition current ratio MICR

IIA

MESG≥0.9

MICR>0.8

IIB

0.9>MESG>0.5

0.8≥MICR≥0.45

IIC

0.5≥MESG

0.45>MICR

4 Temperature group The ignition temperature of an explosive gas mixture is the temperature limit at which it can be ignited. 
The electrical equipment is divided into T1~T6 groups according to its highest surface temperature, so that the highest surface temperature of the corresponding T1~T6 group electrical equipment cannot exceed the allowable value of the corresponding temperature group. The relationship between the temperature group, the surface temperature of the equipment, and the ignition temperature of the flammable gas or vapor is shown in Table 3. 
Table 3 Relationship between temperature group, equipment surface temperature and ignition temperature of flammable gas or vapor

Temperature class IEC/EN /GB 3836

The highest surface temperature of the device T[°C]

Ignition temperature of flammable substances [ °C]

T1

450

T>450

T2

300

450≥T>300

T3

200

300≥T>200

T4

135

200≥T>135

T5

100

135≥T>100

T6

85

100≥T>8

5 Examples of explosion-proof signs
In order to further clarify the indication method of the explosion-proof mark, the gas explosion-proof electrical equipment is exemplified as follows:
For example, electrical equipment is Class I explosion-proof type: the explosion-proof mark is ExdI
If the electrical equipment is type II flameproof, the gas group is group B, and the temperature group is T3, the explosion-proof mark is: ExdIIBT3.
If the electrical equipment is type II intrinsically safe ia, the gas group is group A, and the temperature group is T5, the explosion-proof mark is: ExiaIIA T5. 
For type I special type: ExsI. 
For the explosion-proof electrical equipment used in the mine except for biogas, under normal circumstances, the Class II gas group is Group B, and the temperature group is T3 flammable gas, the explosion-proof mark is: ExdI/IIBT3. 
In addition, the contents of the explosion-proof mark can be adjusted as appropriate for the following special circumstances:
(1) If the electrical equipment adopts more than one composite type, the main explosion-proof type shall be marked first, followed by other explosion-proof types. Such as: Class II Group B main body explosion-proof type and increased safety type junction box T4 group of motors, the explosion-proof mark is: ExdeIIBT4. 〖JP3〗
(2) If only electrical equipment in a flammable gas or vapour environment is permitted, the marking may be expressed by the chemical formula or name of the gas or vapour. In this case, it is not necessary to indicate the gas group and temperature group. . Such as: Class II explosion-proof electrical equipment used in ammonia environment, the explosion-proof mark is: ExdII (NH3) or ExdII (ammonia). 
Conversely, using Table 2, the manufacturer can determine the temperature group of the product according to the environment in which the explosion-proof electrical product is used, and design the surface temperature or internal temperature of the casing of the electrical device according to the temperature group. Users of explosion-proof electrical equipment can conveniently select the temperature group of explosion-proof electrical products according to the type of explosive gas or vapor that may appear in the site. For example, if isobutane is present in the environment (ignition temperature 460 °C), explosion-proof electrical products in the T1 group can be selected; if butane (ignition temperature 160 °C) is present in the environment, the explosion protection of the T4 group must be selected. Electrical products. 
For dust explosion-proof electrical equipment: 
For type A equipment that can be used in Zone 21, the maximum surface temperature TA is 170 °C, and its explosion-proof mark is: DIP A21 TA170 °C or DIP A21TA, T3;
For type B equipment that can be used in Zone 21, the maximum surface temperature TB is 200 °C, and its explosion-proof mark is: DIP B21 TB200 °C or DIP B21TB, T3
6 Requirements for setting the logo
(1) Marks should be placed in obvious places on the main part of the electrical equipment;
(2) The mark must be considered to be clear and durable in the presence of possible chemical corrosion. If the mark Ex, explosion-proof type, category, temperature group can be marked with convex or concave marks on the obvious part of the outer casing,
The material of the signboard should be chemically resistant, such as bronze, brass or stainless steel. 
7 Examples of explosion-proof electrical equipment signs commonly used in the world
Table 4

International Electrotechnical Commission (IEC)
Ex d [ia] IIC T5
Where:
Ex - Explosion proof
d — protection method (explosion-proof type)
[ia] — Intrinsically safe output associated equipment
II — Equipment category
C — gas group
T5 — temperature group 〖〗

CENELEC (European Electrotechnical Commission)
EEx d [ia] IIC T5
Where:
EEx - European Flag
d — protection method (explosion-proof type)
[ia] — Intrinsically safe output associated equipment
II — Equipment category
C — gas group
T5 - temperature group

US (NEC 505) (United States)*
Class I, Zone 1, A Ex d [ia] IIC T5
Where:
Class I - allowed categories
Zone 1 — Locations allowed
A — American National Standards
Ex - Explosion proof
d — protection mode (explosion-proof type)
[ia] — intrinsic safety
II — Equipment category
C — gas group
T5 - temperature group

US (NEC 500) (United States)
Explosionproof with I. S. Outputs, Clɑss I , Division 1, Group A, B, C, D, T5
Where:
Explosionproof
With I. S. Outputs — protection mode (optional except IS)
Clɑss I — allowed categories
Division 1 — Level of permission (optional except level 2)
Groups A, B, C, D - Allowable gas groups
T5 - temperature group

* NEC is the American Electrical Code. In 1996, Chapter 505 was added to the US Electrical Code to harmonize with the hazardous area division method of the I EC standard system commonly used internationally. NEC 500 is indicated as suitable for the North American hazardous area division system.