Often the meaning of the supplementary letter is little known, misunderstood and misinterpreted. Not understanding the difference between component and equipment or thinking that a certificate with the final "X" is a sort of B-series certificate with strong limitations on the use of the equipment is completely anachronistic today.
by Andrea Battauz, R&D Project Engineer of Cortem Group
According to the ATEX directive, an electrical device of category 2 must be accompanied by what in the equipment market is generally referred to as "the ATEX certificate". The correct name of this document issued by the notified body is "EU-type examination certificate".
As shown in the figure, each EU type examination certificate is indicated by an identification code made up of:
Often the meaning of the supplementary letter is little known, misunderstood and misinterpreted. We try to deepen and clarify its meaning.
Let's begin by explaining the meaning of the supplementary letter “U”.The ATEX Directive applies to both “equipment” and “components”.
We find the following definitions:
The same Directive also requires that a category two electrical device must be equipped with an EU type examination certificate issued by a Notified Body. This document is also of two types and follows the same distinction: of equipment and of component. The component certificate shows the supplementary letter "U" and indicates that the certified device does not have an autonomous function and, therefore, must be part of an Ex construction whose equipment certificate will allow its use.
The same concepts reported here by the Atex directive are also present in the regulatory body, specifically in EN 60079-0 .
The supplementary letter “X” indicates that essential information for the installation and use of the device is included within the certificate. Normally, this type of prescriptions is listed in the use and maintenance manual but, in this case, recognizing their greater importance, they are also reported in the Atex certificate.
Some examples of "X" symbol requests in the certificate are: the case in which the ambient temperature of the equipment is not indicated on the marking and it is different from -20°..+40°  or when it is required that the installation area of the equipment has a low mechanical risk .
In addition to the cases listed above or to others which, objectively, require particular attention, the use of the "X" symbol has rather inflated in recent years. The Notified Bodies also require it to give rather basic indications on the maintenance and installation of the product such as, for example, the fact that explosion-proof joints must not be repaired in an explosion-proof construction or that it is mandatory to use IP66 cable glands in a IP66 marked equipment. It is clear that, with these premises, any explosion-proof device or with an IP66 requires the "X" in the certificate.In light of this trend, the common hearsay that requires the certificate with the final "X" to be accompanied by strong limitations on the use of the equipment is out of place.
Often and willingly, the seller or the buyer generally looks for a product with "ATEX certificate" without having in-depth knowledge of the current legislation.
Not understanding the difference between component and equipment or thinking that a certificate with the final "X" is a sort of B-series certificate is completely anachronistic today.
The legislation evolves, it is desirable that the technicians and professionals of the sector update themselves and, to do this, draw from the most reliable sources: the standards and directives.
Reference standards and bibliography
 the two letters are never used together | IEC 60079-0:2017 par. 29.10g)
 Directive 2014/34/EU Article 2 | 1)
 Directive 2014/34/EU Article 2 | 3)
 EN IEC 60079-0:2018 3.36 and 3.37 (Note that the presence of the same concepts in the IEC 60079-0 standard makes them valid also within the IECEx scheme)
 IEC 60079-0 Ed.7 5.1.1 IEC 60079-0 Ed.7 26.4.2