Falling leaves, but not standards
12 January 2015
Autumn is the usual time for the major meetings of the IEC TC31 standards committees. We had missed out on an invitation to meet with other committees during the IEC General Meeting in Tokyo, so were very pleased to have received an invitation from Underwriters’ Laboratories to meet at their Northbrook site, just outside Chicago.
We spent two full weeks there, watching the trees turn colour, with up to five concurrent meetings on some days, prior to the plenary meeting at the end of the second week.
With so much activity, it is only possible to give you a flavour of some of the meetings that I personally attended.
In Working Group 22, we turned our attention to preparing the starting document for the next edition of IEC 60079-0. The current edition was published in 2011, and the next edition should be ready by late 2017. There is a conflict between maintaining a stable standard and updating it to take account of technical developments. Generally a five to seven-year cycle is about right, but some standards do get reviewed more frequently and there are plenty of examples of standards (mostly of narrow scope) that have not changed in over twenty years.
Many of the changes for the next edition will be catching up with those made in the related 60079 series standards, but we did introduce the definition of “associated equipment” as distinct from “associated apparatus”. The former will apply to items such as Ex e motor control relays, while the latter remains directly related to Ex i systems.
Following the publication of IEC TS 60079-32-1 and IEC 60079-32-2, there is a change in the way that electrostatic issues will be handled, with a direct reference to the 32-2 document for test methods. Practically, this should not resultin much for manufacturers, as care is being taken to ensure compatibility with the requirements that have served us well for a good number of years.
In Working Group 27, dealing with all aspects of rotating electrical machinery across all protection concepts, we once again spent much of our time on the thorny subject of convertor drives. In order to avoid the impractical task of testing every motor with its convertor, it is necessary to identify the equivalence of different convertors and how the differences affect the performance of the motor. It seems that there is not even any universally-accepted definition of “switching frequency”, so we have our work cut out trying to bottom this particular issue. Convertor fed Ex d and Ex t motors can often be directly protected by stator temperature detection, but this cannot be the whole answer for Ex e and Ex n motors, where the temperature of the rotor is frequently the limiting factor.
MT 60079-7 reviewed the comments on the CDV version of the next edition of IEC 60079-7 and prepared the Final Draft International Standard (FDIS). Once the FDIS text is finalised, the scope for further changes is strictly limited, as a positive vote cannot be accompanied by comments. So, barring any serious objections, we have established the final text. This is being translated into French and both versions should be released in Q1 2015. A positive vote will result in publication in Q3.
Although the text may not be recognised as coming from the currently published edition, there are not many real changes affecting current Ex e equipment. The reason for the radical overhaul is the merging of the original IEC 60079-7 text with the requirements currently in IEC 60079-15 for Ex nA equipment. The combined standard will now deal with two levels of protection: Ex eb for use in Zone 1, and Ex ec for use in Zone 2. This completes the split up of IEC 60079-15, leaving just Restricted Breathing and Sealed Equipment to be retained in the next edition.
In the plenary session we had reports from all the working groups and maintenance teams. We learned that the final vote on the non-electrical standards will take place in Q2 in 2015, and this should allow IECEx to go full steam ahead to provide international certification for this type of equipment and also for assemblies comprising both electrical and non-electrical items.
As mentioned in my last article, there is a commercial need to provide certification for assemblies as completed units, and IECEx is developing an interim solution. TC31 agreed that a permanent solution was required from the standards writers, and will take the IECEx Decision Sheet as an input to the process of developing new standard requirements.
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