A gas detector destined for a hazardous area carries a marking like Ex ia IIC T4 Ga. Buyers routinely treat that string as a single box ticked, confirm the word “explosion-proof” appears somewhere on the datasheet, and move on.
The string is doing more work than that. It states which protection concept the device uses, which gases it covers, how hot its surface can get, and which zone it may be installed in. It also belongs to one of three separate certification systems, and a certificate from one system has no legal standing in the territory of another.
Three systems, three jurisdictions
| System | Where it applies | Legal basis | Who issues |
|---|---|---|---|
| IECEx | International, voluntary adoption | IEC 60079 series | IECEx approved certification bodies |
| ATEX | European Union, mandatory | Directive 2014/34/EU | EU notified bodies |
| China Ex | China, mandatory | GB 3836 series | CNEx and accredited Chinese bodies |
The technical standards underneath the three are close relatives: they all descend from IEC 60079. That similarity is exactly what causes confusion. A device built to one system usually meets the technical requirements of the others, but the certificate itself is territorial. Mutual recognition between IECEx and ATEX is partial and administrative, and China Ex sits outside both.
For a buyer, the practical consequence is short: a certificate issued in China does not permit installation in a European Union classified area, and an ATEX certificate does not satisfy a Chinese inspection.
Reading Ex ia IIC T4 Ga
Every segment answers a different question.
| Segment | Meaning | What it constrains |
|---|---|---|
Ex | Explosion protection marking | Signals that the rest of the string follows the standard notation |
ia | Intrinsic safety, level ia | The circuit cannot ignite the atmosphere even with two faults; permits Zone 0 use |
IIC | Gas group IIC | Covers the most easily ignited group, including hydrogen and acetylene |
T4 | Temperature class | Surface temperature stays at or below 135 °C in the worst case |
Ga | Equipment protection level | Suitable for Zone 0, the highest of the three gas zones |
Gas groups run from I (mining, firedamp) through IIA and IIB to IIC, and a higher group covers the gases of the lower groups. Temperature classes run from T1 at 450 °C down to T6 at 85 °C, so T6 is the strictest. Protection levels run Ga, Gb, Gc, mapping to Zones 0, 1 and 2.
Put together: a Ga device with IIC and T4 may be installed in Zone 0, in an atmosphere containing hydrogen, provided the ambient temperature does not push the surface above 135 °C.
Two constraints follow from that reading and catch people out. A device certified IIB cannot be installed where hydrogen is present, even if the gas it is there to detect is methane. And T4 is not the strictest class: where the process involves carbon disulphide, T6 may be required, and a T4 certificate will not satisfy the assessor.
Four assumptions that cause problems
“It has CE, so it must be ATEX.” CE marking covers a family of directives. ATEX is one of them, and equipment for explosive atmospheres requires assessment under 2014/34/EU specifically. A CE mark from the EMC directive alone says nothing about explosion protection.
“China Ex is accepted internationally.” It is not. IECEx acceptance depends on the destination country’s scheme, and the EU recognises only ATEX. A Chinese certificate can support a technical file, but it does not replace the local approval.
“The certificate covers the whole product range.” Certificates are issued against a specific construction, with defined gas groups, temperature class and ambient range. Change the housing material, the cable entry, or the internal barrier, and the certificate may no longer apply. When comparing suppliers, ask which exact model numbers the certificate lists.
“Certification is permanent.” Certificates have validity periods and require surveillance or re-assessment. A certificate that expired two years ago still looks convincing as a PDF.
How to verify before you specify
- Ask for the certificate PDF, not a datasheet claim. The number, issuing body, validity date and standards list should all be legible.
- Check the model numbers listed against what you are ordering.
- Check the ambient temperature range in the certificate against the site conditions. A device rated to +55 °C is not interchangeable with one rated to +40 °C.
- Confirm the gas group covers every gas present, including the ones the detector is not measuring.
- For EU installation, confirm ATEX specifically. For international projects, check whether the destination country accepts IECEx.
Where this leaves the specification
A manufacturer outside the EU commonly holds China Ex alone, because that is what its domestic market requires. ATEX and IECEx are separate investments: separate test samples, separate fees, separate lead times, and a separate technical file for each. The absence of ATEX does not mean a product is unsafe, and the presence of ATEX does not make a product suitable for a particular project.
Certification is a scope, not a badge. The question that decides an order is which certificate lists the exact model being bought, for the gases actually present, across the ambient range at that site, and whether it is currently valid.
That applies in both directions. A supplier who answers “explosion-proof, certified” without naming the system, the certificate number and the models covered has not answered the question, and a buyer who accepts that answer has bought a claim rather than an approval.
Certification details vary by model and by revision. Confirm against the certificate issued for the exact model you receive; this page describes the marking convention, not a specific product approval.