What does the U suffix mean, and can a U-certified component be installed on its own?
Abstract
A U after an Ex certificate number marks an Ex component: an incomplete part, such as a terminal or an empty enclosure, that is not intended to be used alone. It must not be installed on its own, only inside equipment that its manufacturer has assessed as a whole.
A U after an Ex certificate number, as in IECEx ABC 26.0456U or ABC 26 ATEX 0456 U, means the certificate covers an Ex component: a part of Ex equipment that is not intended to be used alone and needs further assessment when it is built into equipment. So no, a U-certified component must not be installed in a hazardous area on its own. It becomes usable only inside equipment that its manufacturer has assessed as a complete item, through the conformity route that applies to that equipment.
Short answer
- U = Ex component. IEC 60079-0 describes such an item as incomplete and not suitable for installation without further evaluation.
- Typical Ex components: increased-safety terminals, empty flameproof or increased-safety enclosures, many flameproof bushings, switch and push-button elements, encapsulated relays and contactors, ballasts for fluorescent lamps.
- A component certificate states that it is not an equipment certificate. It carries a schedule of limitations for the equipment manufacturer instead of specific conditions of use for the installer.
- Under ATEX, components carry no CE marking. They come with a written attestation of conformity and assembly instructions, not an EU declaration of conformity and instructions for use.
- Whoever completes the assembly becomes its manufacturer and must have the complete equipment assessed: an IECEx certificate under IECEx; under ATEX a notified-body certificate for category 1 and 2 electrical equipment, internal production control for category 3. This applies to panel builders and to end users alike.
- X and U never appear together on one certificate.
Why a component cannot stand alone
A component certificate proves only what the component itself does. An increased-safety terminal has been tested for its creepage distances, clamping and temperature rise at a stated current, but not for the enclosure around it. An empty flameproof enclosure has passed its pressure and flame-transmission tests, but nobody has yet assessed the heat produced by whatever goes inside, the holes for the cable entries, or the free volume left by the contents.
That is why component marking usually lacks a temperature class: the class depends on the final equipment. The European Commission's ATEX Guidelines (6th edition, January 2026) put the legal side in one line: the component is not ready for use, and its final conformity assessment is done together with the complete equipment into which it is fitted.
Equipment versus component at a glance
| Point | Ex equipment | Ex component (U) |
|---|---|---|
| Ready for use | Yes, following the instructions | No, must be incorporated |
| Information for the user | Specific conditions of use (if X) | Schedule of limitations, addressed to the equipment manufacturer |
| ATEX papers | EU declaration of conformity, instructions for use | Written attestation of conformity, assembly instructions |
| CE marking under ATEX | Yes | No (unless other EU law requires it) |
| Temperature class in the marking | Yes | Usually not; set in the final equipment |
| Installed by | Installer | Equipment manufacturer |
The schedule of limitations tells the equipment manufacturer and its certification body what must still be verified: for example the temperature class in the final enclosure, minimum dimensions of the hole that receives a bushing, maximum current or conductor sizes, and tests to repeat on the assembly.
Practical case: the panel builder's shortcut
A panel builder is asked for a local control station for Zone 1. It buys an empty increased-safety enclosure, Ex e terminals and two Ex d push-button elements, all with certificates ending in U, fits them together and wants to ship.
The station cannot be installed on the strength of those three certificates. The panel builder is the manufacturer of new Ex equipment and must:
- meet every item in each schedule of limitations, which usually means a temperature assessment of the populated enclosure and checks of the cut-outs and creepage distances;
- obtain a certificate for the complete station, for example an IECEx certificate of conformity, or under ATEX an EU-type examination certificate followed by a production module, because this is category 2 electrical equipment;
- apply its own full Ex marking and certificate number, and issue the declaration of conformity and instructions.
The component certificates make this step shorter: IEC 60079-1, for example, lets an empty flameproof enclosure with a component certificate be used in an equipment certificate without repeating all its type tests, provided its schedule of limitations is met. That route and its limits are covered in the related question on fitting your own components below.
Note: Check the suffix of items you plan to fit on site. Products designed to be fitted by installers, such as cable glands and blanking elements, are usually certified as Ex equipment (often with an X), not as components. If the number ends in U, the item is a component.
Common mistakes
- "All the parts are certified, so the assembly is certified." A set of U certificates is not an equipment certificate.
- Installing a component bought from a catalogue because it carries an Ex marking.
- Ignoring the schedule of limitations, for example using a terminal above its rated current or a bushing in a thinner wall than specified.
- Expecting a CE marking or declaration of conformity for an ATEX component; their absence is normal.
Related on excodex
- The X and U suffixes on Ex certificates: specific conditions of use and Ex components
- EU-Type Examination Certificates and Notified Bodies explained
- Can I fit my own components inside a certified empty Ex enclosure?
- Why does an Ex component certificate often have no temperature class?
- Standards spaces: IEC 60079-0, ATEX 2014/34/EU
Standards and further reading
- IEC 60079-0, general requirements: definitions of Ex component and of the U symbol, component certificates and marking (edition 8, 2026). Public copy of the 2004 edition: law.resource.org (PDF)
- Directive 2014/34/EU, Article 2(3) definition of components; Article 13 conformity assessment procedures by category and written attestation for components: legislation.gov.uk
- IEC 60079-1, flameproof enclosures, annex on empty flameproof enclosures as Ex components (public copy of the 2014 edition): law.resource.org (PDF)
- European Commission, ATEX 2014/34/EU Guidelines, 6th edition (January 2026), section 47 and Table 2, equipment and components: single-market-economy.ec.europa.eu (PDF)
- US Coast Guard OCS National Center of Expertise, Drill Down 29 on the U suffix and the schedule of limitations: dco.uscg.mil
- IECEx decision sheet DS 2023/002 on marking of Ex components used only by their own manufacturer: iecex.com
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.