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That’s the ticket! Marking issues

19 February 2015

SGS Baseefa General Manager Ron Sinclair MBE is chairman of BSI Committee EXL/31 and the IEC's Cenelec TC31 committee. Here he looks at some of the issues relating to the marking of Ex Equipment.

I have mentioned before my alarm at some of the strange information that gets put out in the various LinkedIn forums.  Recently, there has been a discussion on the subject of marking of Ex Equipment, and I thought it would be relevant to bring some of my thoughts to the readers of HazardEx , while also mentioning some additional problems that I see from the point of view of a certification body.

A number of years ago, IEC 60079-0 and its predecessors referred to the need for marking to be permanent and legible.  However, none of the standards were able to come up with objective criteria for either legibility or durability, so the “requirement” was dropped from the text of the standard.  Tests that would have been appropriate for a 10 MW motor on the deck of an oil rig would not be appropriate, say, for a hand-held, intrinsically-safe, instrument. 

However, the “need” for appropriate legibility and durability was not removed.  Conformity to the marking requirements is an integral part of the standard.  Therefore, if the marking cannot be read, the equipment is effectively non-conforming, and all bets are off.  It is incumbent on the manufacturer to choose a marking method that is appropriate for the expected conditions of use, so that the equipment can remain in conformity.

The standard specifies the content of the marking in great detail (no less than 10 pages in Clause 29 of the current edition of IEC 60079-0) but, in this clause, omits to make reference to the other clauses of the standard that might be relevant when choosing a particular form of marking.  So what else needs to be considered?

All or part of the marking may be cast or moulded into the enclosure.  This is an excellent method, in many respects, but does require extra steps in the manufacturing QA process to ensure that only compliant product can be dispatched with the marking.  For ATEX, the application of the CE Marking is supposed to be the last act when the product is placed “on the market”, so the QA system has to robustly ensure the equivalent security of this requirement.
Metal labels can be secured with screws or drive pins.  However, the attachment method must have been evaluated for conformity with Ingress Protection requirements (particularly for increased safety) and/or safe hole depth (particularly for flameproof equipment).

Plastic or thin metal labels can also sometimes be secured with screws, but more frequently with adhesive.  More than 30 years ago, this could be problematical, but as modern adhesives have developed, this method can be more secure than using screws.

But what else is critical about the label material?  If it is a metal label, possibly aluminium, the material will have to comply with the same restrictions on light metals as the rest of the enclosure.  Therefore unprotected aluminium cannot be used on equipment with Explosion Protection Level (EPL) Ga (ATEX Category 1) which might be installed in Zone 0.

If a metal label is to be mounted on a plastic enclosure, the maximum size of the label is constrained by the maximum capacitance limits intended to avoid electrostatic charging and ignition.  Not too much of a problem for EPL Gb and Gc Equipment of Groups IIA and IIB (or Group I or Group III), but quite a limitation for equipment of EPL Ga (for all Groups) or Group IIC (for all EPLs), where the maximum permitted capacitance is only 3 pF.  It is not possible to give an absolute size limitation, as location in respect of the earth plane during the test is a factor, but anything over about 10 cm2 needs to be tested.

This leaves us with plastic labels which, of course, also have to meet the requirements for non-metallic enclosures in respect of mitigating the risk of electrostatic ignition.

Questions have been asked about the minimum size of the label text.  The standards are silent on this subject, but the ATEX Directive confirms a minimum height of 5mm for the CE Marking.  There are also minimum sizes given in IECEx Guide 05A in respect of the IECEx Licenced Mark.
So who fits the label?  This must be done before the equipment is dispatched from the factory as it is the label that provides traceability.  Where this is not possible, the attachment needs to be done under the QA procedures of the manufacturer. 

I have heard of manufacturers supplying loose labels to the purchaser.  This will not do, as there is no traceability and no guarantee that the label has, in fact, been attached to the right equipment.

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