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Composition of Explosion-Proof Markings
According to the GB 3836 standard, the explosion-proof marking for electrical equipment must include:
Explosion-Protection Type + Equipment Category + (Gas Group) + Temperature Class
1. Explosion-Protection Types
Based on the protective measures employed, explosion-proof electrical equipment is classified into flameproof, increased safety, intrinsic safety, pressurized, oil-immersed, sand-filled, encapsulated, type "n", special protection, and dust-explosion-proof types. Their identifiers are shown in Table 1 below.
IEC Explosion-Proof Rating Format Example: Ex(ia)ⅡC T4
Ex: Common explosion-proof symbol (recognized under CENELEC standards)
ia: Protection type (Intrinsic Safety)
Ⅱ: Equipment group
C: Gas group
T4: Temperature class
Table 1: Basic Explosion-Protection Types
| Explosion-Protection Type |
Symbol |
Explosion-Protection Type |
Symbol |
| Flameproof |
Ex d |
Sand-filled |
Ex q |
| Increased Safety |
Ex e |
Encapsulated |
Ex m |
| Pressurized |
Ex p |
Type "n" |
Ex n |
| Intrinsic Safety (ia) |
Ex ia |
Dust Explosion-Proof (Type A) |
DIP A |
| Intrinsic Safety (ib) |
Ex ib |
Dust Explosion-Proof (Type B) |
DIP B |
| Oil-immersed |
Ex o |
Special Protection |
Ex s |
Explanation of Intrinsic Safety Levels:
Level ia: Electrical equipment that cannot ignite an explosive gas mixture during normal operation, with one fault, or with two faults.
Safety factor during normal operation: 2.0
Safety factor with one fault: 1.5
Safety factor with two faults: 1.0
Note: Contacts that generate sparks must be enclosed in a flameproof or hermetic enclosure, or the safety factor must be doubled.
Level ib: Electrical equipment that cannot ignite an explosive gas mixture during normal operation or with one fault.
Safety factor during normal operation: 2.0
Safety factor with one fault: 1.5
Note: During normal operation, contacts generating sparks must be protected by a flameproof or hermetic enclosure and include a fault self-indication mechanism. With one fault, the safety factor is 1.0.
2. Equipment Categories
Electrical equipment for explosive gas atmospheres is divided into:
Group I: Electrical equipment for coal mines.
Group II: Electrical equipment for all other explosive gas atmospheres (excluding coal mines).
Group II flameproof ("d") and intrinsic safety ("i") equipment is further subdivided into IIA, IIB, and IIC.
Electrical equipment for combustible dust atmospheres is divided into:
Type A Dust-Tight / Type B Dust-Tight
Type A Dust-Protected / Type B Dust-Protected
3. Gas Groups
The explosion hazard level of an explosive gas mixture is indicated by its ability to transmit explosions and its ease of ignition. The transmission capability is represented by the Maximum Experimental Safe Gap (MESG), while the ease of ignition is represented by the Minimum Ignition Current Ratio (MICR). Group II flameproof or intrinsic safety equipment is further classified into IIA, IIB, and IIC based on these parameters, as shown in Table 2.
Table 2: Relationship Between Gas Groups, MESG, and MICR
| Gas Group |
MESG (mm) |
MICR |
| IIA |
≥ 0.9 |
> 0.8 |
| IIB |
0.5 < MESG < 0.9 |
0.45 ≤ MICR ≤ 0.8 |
| IIC |
≤ 0.5 |
< 0.45 |
4. Temperature Classes
The ignition temperature of an explosive gas mixture is the critical temperature limit at which it can be ignited. Electrical equipment is classified into groups T1 through T6 based on maximum surface temperature. The surface temperature of the equipment must not exceed the allowable value for its corresponding temperature class. The relationship between temperature class, equipment surface temperature, and the ignition temperature of combustible gases/vapors is detailed in Table 3.
Table 3: Temperature Class vs. Equipment Surface Temperature & Ignition Temperature
| Temp. Class (IEC/EN/GB 3836) |
Max. Equipment Surface Temp. T [℃] |
Ignition Temp. of Combustible Substance [℃] |
| T1 |
450 |
T > 450 |
| T2 |
300 |
300 < T ≤ 450 |
| T3 |
200 |
200 < T ≤ 300 |
| T4 |
135 |
135 < T ≤ 200 |
| T5 |
100 |
100 < T ≤ 135 |
| T6 |
85 |
85 < T ≤ 100 |
IP Protection Rating
Definition of IEC IP Rating
IP stands for Ingress Protection. The IEC IP rating is a critical safety indicator for electrical equipment. This system classifies products based on their resistance to dust, water, and physical impact. Drafted by the International Electrotechnical Commission (IEC) and published in IEC 60529 (BS EN 60529:1992), this classification system is widely recognized across most European countries.
The rating consists of the letters "IP" followed by two digits that specify the protection level:
First Digit: Indicates the level of protection against solid foreign objects (e.g., dust). The highest level is 6.
Second Digit: Indicates the level of protection against water ingress. The highest level is 8.
Common examples include motor protection ratings such as IP65 and IP55.
