1. How to Read the Ex Marking (Breaking Down Ex d IIB T4)
Ex marking Ex d IIB T4 Gb in 5 segments:
| Segment | Meaning | Example value | What it should show |
|---|---|---|---|
| Ex | Explosion-proof electrical | Ex | Identifier |
| d | Protection type | d/e/p/i/n | Flameproof / increased safety / pressurized / intrinsically safe / encapsulated |
| IIB | Gas group | IIA/IIB/IIC | C is the most stringent, covers hydrogen |
| T4 | Temperature class | T1–T6 | T6 is the most stringent, ≤85°C |
| Gb | Equipment protection level | Ga/Gb/Gc | Gb applies to Zone 1 |
2. Comparison of 3 Main Protection Types (d vs e vs p)
| Format | Principle | Advantages | Drawbacks | Zone |
|---|---|---|---|---|
| Ex d flameproof | Flameproof joint quenches the flame | Wide application range | Heavy, high cost | Zone 1 / Zone 2 |
| Ex e increased safety | Raised safety level | Lightweight, low cost | No spark or ignition source permitted | Zone 2 |
| Ex p pressurized | Protective gas purging | Suitable for large cabinets | Requires continuous purging | Zone 1 / Zone 2 |
3. How do you select the gas group? (IIA / IIB / IIC)
Gas groups are classified bymaximum experimental safe gap (MESG) as follows:
- IIA: typical gas propane, MESG ≥ 0.9 mm, the widest applicability.
- IIB: typical gas ethylene, MESG 0.5–0.9 mm, covers most chemical plants.
- IIC: typical gases hydrogen / acetylene, MESG ≤ 0.5 mm, the most demanding requirement.
4. How do you determine the temperature class? (T1–T6)
Temperature class = maximum surface temperature of the equipment and it must stay belowthe ignition temperature of the flammable gas present. T6 (≤85°C) is the strictest; T1 (≤450°C) is the most permissive. When selecting, first measure the ignition temperature of the ambient gas, then set the T class. Common examples: methane T1, ethylene T2, gasoline T3, acetaldehyde T4.
5. Five-step method for selecting an explosion-proof distribution box (from zone to marking)
Selection in 5 steps:determine zone → determine gas → determine protection type → determine level → verify certificate. ① Determine the hazardous zone (Zone 0 / 1 / 2); ② check the ambient gas to establish the group (IIA / IIB / IIC) and ignition temperature (T1–T6); ③ select the protection type (d / e / p / i); ④ determine the equipment protection level (Ga / Gb / Gc); ⑤ verify the explosion-proof certificate number and the 3C compulsory certification.
FAQ FAQ
Does an explosion-proof distribution box require 3C certification?
Yes, it is required. Explosion-proof electrical equipment falls under the CCC compulsory certification catalog and must hold a valid certificate. The certificate number can be verified on the national certification and accreditation public service platform.
Can Ex d and Ex e be used interchangeably?
They cannot simply be interchanged. Ex d can be used in Zone 1; Ex e can only be used in Zone 2. Downgrading the application requires a fresh risk assessment.
What is the service life of an explosion-proof distribution box?
Generally 5–10 years, and it depends mainly on whether the flameproof joint surfaces have corroded and whether the sealing rings have aged. The flameproof gap and sealing performance must be inspected every year.
Can IIC-rated equipment be used in place of IIA?
Yes. IIC is the most stringent rating and is downward compatible with IIB and IIA. However, it costs more, so it is normally selected only where required.
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