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Some thoughts on IEC 60079-28 (prevention of ignition from optical radiation)


14 January 2019

After the September IECEx meetings in France, October saw us arrive in Busan, South Korea, for the General Meeting of the International Electrotechnical Commission (IEC). Technical Committee TC31 is one of the bigger IEC committees and we held meetings on 11 days over the two week period.  

Ron Sinclair MBE
Ron Sinclair MBE

For the first time, I learnt that there are approximately 600 individuals registered as experts or National Committee representatives across the various committees involved in TC31.  My best guess is that over 150 of those people were in Busan at some time during the fortnight, with about 80 present at the final two day plenary meeting.

The resources expended in creating the IEC 60079 and ISO 80079 standards are phenomenal.  Those who use the standards owe a real debt of gratitude to those who put in the effort to ensure that they are all up to date and fit to keep people safe in a world of changing technology.

The TC31 plenary is essentially a means for the various Working Groups and Maintenance Teams to report on their current work or to seek permission for a particular way forward.

I had been at the Maintenance Team meeting for IEC 60079-28 earlier in the fortnight.  This standard, relating to the prevention of ignition from Optical Radiation has proved an awkward child, since an attempt was made to increase its scope beyond the original brief of lasers and optical fibres.  As the second (current) edition was being prepared, the scope was specifically increased to include some types of LED light sources.  The revised scope of the second edition was then issued as an official Interpretation Sheet (I-SH) for the first edition.

It is now realised that the way the scope was changed brought in some LEDs that need not be considered, but also failed to bring in some other light sources that could be as problematical as high power LEDs.  The real problem is when the light source is either collimated (as with a laser) or brought to a focus in a hazardous atmosphere, irrespective of the actual form of the light source.

Work is starting on the next edition of the standard, which should better fit the understanding of the problems.  But to solve the immediate problem, MT 60079-28 sought the permission of the TC31 plenary meeting to yet again change the scope of the standard by way of an I-SH.  TC31 accepted the request and agreed that the document should be circulated to National Committees for acceptance.  We don’t yet know the outcome of that circulation, and whether or not the scope is “clarified” in the intended way will depend on the votes of the National Committees, when received.

Historically, ignition by the intensity of the light beam was never considered.  The traditional sources of illumination generally produced sufficient heat for that to be the overriding problem.  LEDs, though, are different, in that the way they and their luminaires are constructed separates the light from most of the heat, which is dissipated at a comparatively low temperature in a heat-sink.  This is necessary in order to ensure that the LED junction temperature is kept low enough to avoid early destruction.

The ignition mechanism considered is that of the intense light beam falling on an “absorber”, such as dust particles in the atmosphere, causing the absorber to significantly exceed the auto-ignition temperature of the atmosphere.  If the optical power is above the safe level, protection can be either by keeping the atmosphere or the absorbers away from the beam (for example using the techniques of Dust Enclosure Protection “Ex t” or Restricted Breathing “Ex NR”) or by containing a possible explosion (using a flameproof enclosure “Ex d”).  In all cases, care must be taken to ensure that there is not a focusing point of sufficient intensity outside the enclosure.

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