Why does an Ex component certificate often have no temperature class?

Because an Ex component, the part with a "U" certificate, is not complete equipment. How hot its surfaces get depends on the enclosure it is built into, the other parts beside it and the heat dissipated inside, so a temperature class can only be fixed for the final equipment. IEC 60079-0 therefore does not apply temperature classification to Ex components; their certificate states operating temperature limits instead.

Short answer

• "U" marks an Ex component: part of equipment that is not intended to be used alone and needs further assessment when it is built into equipment (IEC 60079-0).
• No T-class by design. The Ex component annex of IEC 60079-0 (in the public copy of the 2007 edition) states that temperature classification is not applied to Ex components, but their operating temperature limits must be specified.
• The EU says the same. The Commission's ATEX Guidelines (6th edition, January 2026) note that in most cases a component is not marked with a temperature class because it is not itself a source of heat, or because its temperature can only be determined when fitted in the final equipment.
• The T-class belongs to the assembly. It is determined by testing or assessing the finished equipment at its maximum rated ambient temperature, and it appears on the equipment certificate and marking.
• A panel builder who combines U components becomes the manufacturer of the resulting Ex equipment and needs it assessed as equipment.

Why the temperature belongs to the finished equipment

A temperature class (T1 to T6) limits the hottest surface that the explosive atmosphere can touch. IEC 60079-0 requires that maximum surface temperature to be determined with the equipment at its maximum ambient temperature. For an assembly, that surface temperature is the result of the whole design:

• the power dissipated by every part inside (terminals carrying current, relays, power supplies, resistors);
• the size, material and surface of the enclosure, which decide how well heat escapes;
• the mounting position, the number and spacing of parts, and any external heat source.

The same certified terminal block can stay cool in a large enclosure at low current and run much hotter in a small, densely packed box at full load. An empty Ex enclosure has no heat source of its own at all. Neither component's certifier can know which case will occur, so no honest T-class can be put on the component.

The IECEx system applied the same logic in a 2019 draft decision on modules for Ex ec housings: when such modules are evaluated individually as "U" components, both service and surface temperature rise are to be determined after installation in a suitable end-product enclosure.

What a component certificate gives instead

A "U" certificate comes with a schedule of limitations: the information the equipment manufacturer and the certifier of the final equipment need. Depending on the component it typically covers:

• the operating (service) temperature range the component itself can withstand, for example of its gaskets, insulation or encapsulation;
• electrical ratings and the conditions under which they were assessed;
• mounting rules, clearances and creepage distances to surrounding parts, and which tests remain to be done in the final equipment;
• in some cases a maximum temperature where one was specified and verified for the component.

Under IEC 60079-0 an Ex component carries the symbol "U" and not "X"; its limitations are conditions for the next manufacturer, not conditions of use for the plant operator.

How the final equipment gets its T-class

1. The manufacturer (often a panel builder) designs the assembly within the limits of every component certificate.
2. The assembly is assessed as equipment: temperature rise is measured or assessed at maximum load and maximum rated ambient, together with the other checks the components' schedules call for. IEC 60079-0 limits this to what was not already assessed on the component, and names surface temperature, creepage distances and clearances to surrounding parts as typical examples for a component mounted inside an enclosure; a component that passes through the enclosure wall also needs its interface with the enclosure tested.
3. The equipment certificate states the T-class, often as a table linked to ambient temperature or load, and the equipment is marked with it.
4. Under ATEX the assembly is a product in its own right: for Category 2 electrical equipment this means EU-type examination followed by either conformity to type based on internal production control plus supervised product testing, or product quality assurance (Directive 2014/34/EU, Article 13), with unit verification as an alternative for a single item. Under IECEx it needs its own certificate of conformity.

Common mistakes

• Reading "no T-class" as "suits every T-class". It means the T-class has not been determined yet; a component is not installed on its own.
• Using the service temperature range as a surface temperature. The range says what the component tolerates, not how hot the assembly gets.
• Adding parts to certified equipment. Extra heat sources change the temperature of the whole box; see the question on fitting components into certified enclosures.
• Copying a T-class from one component to the panel. The panel's class comes from its own assessment.

Related on excodex

• The X and U suffixes on Ex certificates: specific conditions of use and Ex components
• Temperature classes T1 to T6: matching surface temperature to autoignition temperature
• What does the U suffix mean, and can a U-certified component be installed on its own?
• Can I fit my own components inside a certified empty Ex enclosure?
• Ex marking check

Standards and further reading

• IEC 60079-0, general requirements: definition of Ex component, marking with "U", annex on requirements for Ex components (temperature classification not applied, operating temperature limits specified). Paraphrased from the public Indian adoption IS/IEC 60079-0:2007: archive.org (text)
• European Commission, ATEX 2014/34/EU Guidelines, 6th edition (January 2026), components and their marking: single-market-economy.ec.europa.eu (PDF)
• IECEx ExTAG/536A/CD (2019), draft decision sheet on suffixes "X" and "U" for Ex ec modules: iecex.com, with the compiled comments on the first draft, ExTAG/548/CC: iecex.com
• IEC 60079-14:2024, design, selection and installation, with a clause on the use of Ex Components (contents in the preview): standards.iteh.ai (PDF)
• Directive 2014/34/EU, Article 13, conformity assessment procedures, including components: legislation.gov.uk

This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.