Temperature class controls a possible ignition source
Hot equipment surfaces can ignite some explosive atmospheres. A gas temperature class therefore identifies a maximum surface-temperature class for the equipment, not the temperature of the air and not the normal operating temperature of an internal component.
The public IECEx reference lists the six gas temperature classes as follows:
| Gas temperature class | Maximum equipment surface temperature shown by IECEx |
|---|---|
| T1 | 450 degrees C |
| T2 | 300 degrees C |
| T3 | 200 degrees C |
| T4 | 135 degrees C |
| T5 | 100 degrees C |
| T6 | 85 degrees C |
A higher T number has a lower maximum surface-temperature limit. If a project requires T4, equipment marked T4, T5 or T6 may satisfy that temperature-class field, but only when every other marking, ambient, certificate and application requirement also matches.
Do not describe T6 as universally "better" than T4. The correct selection is the equipment that satisfies the complete application and remains within its documented operating conditions.
Temperature class is mainly gas language
T1 to T6 are used for gas, vapour and mist atmospheres. Combustible-dust markings commonly state a specific maximum surface temperature in degrees Celsius instead of using the gas T-class table.
Dust selection also has to consider the relevant dust characteristics, deposits, layers and the project method used to determine the permitted equipment temperature. IEC 60079-31:2022 covers equipment dust ignition protection by enclosure "t" and surface-temperature limitation.
Do not carry a T4 gas selection directly into a dust project. Confirm the exact dust-zone assessment, dust group, temperature limit, ambient conditions and equipment documents.
Ambient temperature describes a different boundary
Ambient temperature is the temperature of the surrounding environment in which the equipment is intended to operate. It can affect component temperature, LED-driver life, seals, insulation, cable entries and the measured maximum surface temperature.
The current IEC 60079-0:2026 states standard atmospheric conditions, relating to explosion characteristics, of -20 degrees C to +60 degrees C. It also states that equipment operating outside the standard temperature range is subject to additional requirements in IEC 60079-0 and the applicable protection standards.
An official IECEx installation presentation for IEC 60079-14:2024 notes that IEC 60079-0 Edition 8 requires permitted ambient-temperature marking for all ranges, including the previously common -20 degrees C to +40 degrees C range. Buyers should therefore expect an explicit ambient range and compare it with the exact certificate and instructions.
This standard reference range is not permission to use every Ex product anywhere between -20 degrees C and +60 degrees C. The exact model marking, certificate schedule, instructions and manufacturer data define the conditions covered for that product. A narrower or otherwise specified ambient range must be respected.
Why T-class and Tamb must be checked together
An equipment temperature class is established for stated conditions. If the real ambient temperature exceeds the documented range, the product may run hotter than the assessed condition, and the published T-class or maximum surface-temperature claim cannot simply be assumed to remain valid.
The reverse problem also matters. Low ambient temperature can affect materials, seals, cable glands, batteries, emergency power components and mechanical impact performance. A product that meets the required gas T-class may still be unsuitable for the minimum site temperature.
Ask for both fields in every enquiry:
- Required gas temperature class or dust maximum surface temperature
- Minimum and maximum site ambient temperature
Then compare both with the exact equipment marking and documentation.
A worked selection example
Consider an illustrative project schedule that states Zone 1, EPL Gb, gas group IIB, T4 and an ambient range of -10 degrees C to +50 degrees C.
A candidate marked only "Gb, IIB, T4" is not yet fully checked. The reviewer still needs evidence that the exact model and configuration cover the required ambient range. The reviewer must also check the protection concept, certificate status, model suffix, electrical ratings, entries, mounting, environmental protection and any special conditions.
If the candidate is marked T5 but its documented ambient range ends at +40 degrees C, it may satisfy the temperature-class field but fail the site's +50 degrees C ambient requirement. One matching field cannot compensate for a mismatch in another.
This example is a reading exercise, not an approval of any product or project.
Temperature information for LED luminaires
Hazardous-area LED lighting combines the enclosure, LED modules, driver, terminals, seals and cable entries into a thermal system. Wattage alone does not determine the final surface temperature or the approved ambient range.
When comparing fixtures, request the exact configuration because options can affect thermal performance:
- Input voltage and driver configuration
- Nominal power and any output option
- Mounting orientation and bracket arrangement
- Emergency battery or backup-power option
- Cable-entry size, quantity and fitted glands or plugs
- Enclosure material, coating and environmental conditions
- Maximum ambient temperature and any temperature-class variation
The Explosion-Proof LED Lighting FAQ expands this into a complete buyer checklist.
What to verify on the nameplate and certificate
Use the nameplate to identify the exact equipment, then cross-check it against the current certificate and schedule. Verify:
- Legal manufacturer and exact model, suffix and variant
- Complete Ex marking and applicable protection concepts
- Gas or dust group and Equipment Protection Level
- T1 to T6 class for gas, or stated maximum surface temperature for dust
- Marked ambient range or linked conditions in the certificate schedule
- Certificate number, issue, status and special conditions
- Electrical ratings and option-dependent limits
- Instructions affecting mounting, entries, maintenance and inspection
Our guide How to Read Ex Markings explains the other fields that surround T4 or another temperature entry. The certificate verification guide explains how to compare the model with current documents.
Common temperature-selection mistakes
- Reading T4 as an ambient-temperature rating
- Treating the standard atmospheric reference range as the rating of every product
- Assuming T6 is automatically the best selection for every project
- Using a gas T-class as the dust maximum surface-temperature value
- Ignoring minimum ambient temperature
- Comparing a certificate for a related model instead of the exact suffix
- Assuming the same T-class applies to every voltage, power, mounting or emergency option
- Treating an IP rating as proof of temperature or Ex conformity
- Ignoring dust layers, blocked heat dissipation or process deposits
The article IP66 vs Explosion Proof explains why ingress protection and explosion protection must be checked separately.
Current MMAIATEX certification status
The public MMAIATEX About page states that ATEX, IECEx and CCC certification work is in progress. A T-class shown in a product title, photograph or category is not proof that a specific model has a completed certificate.
Request current documents for the exact legal manufacturer, model, suffix and configuration. Verify the temperature information, ambient range, certificate status, scope and conditions through the applicable official system before approval.
Safety disclaimer
This article is general educational information. It does not calculate a permitted surface temperature, assess an explosive material, interpret a certificate for a specific project or approve equipment. Temperature selection, hazardous-area equipment selection, installation and inspection must be completed by competent personnel using current material data, project documents, standards and exact product evidence.
Request a temperature-matched product review
Send MMAIATEX the destination country, approved Zone, required EPL or ATEX category, gas or dust group, T-class or maximum surface-temperature requirement, site ambient range, electrical duty, mounting and document list. We can then identify potentially relevant product information for the responsible team to review.