Rich
With respect to actual testing of the materials in the enclosure, that was also impractical because there was (still is) a wide range of lengths and diameters, which were always very well populated because they were (are) made as compact as possible for the intended applications. Therefore we were pretty happy that the same principles as outlined in my previous email did apply across the range. Also, something not mentioned in that email was that the calculated typical worst-case case outer shell temperature rise due to internal ignitions was less than 1C – which meant that there was no risk of that rise causing ignition of any surrounding material. The odd thing that I noted very clearly at the time was that the standard did not include any concept of a real partially- or fully-sealed enclosure where any internal fires could be contained within the enclosure without external flame spread or other related hazards. That, I thought (still do) was a major oversight by the standard developers who had adopted the 60950 enclosure requirements almost verbatim and without full consideration of the very wide range of equipment to which the standard might then be applied. In response to your last para, those were self-certification jobs to EN 61010-1:2010 – which, fortunately, allows the risk assessment approach to issues where you can’t comply with chapter and verse of the standard! So it was down to us to decide whether the construction was adequate – and we were! John E Allen W. London, UK From: Richard Nute [mailto:ri...@ieee.org] Sent: 21 May 2016 18:32 To: 'John Allen'; EMC-PSTC@LISTSERV.IEEE.ORG Subject: RE: [PSES] fire safety test methods for different country standards Hi John: Thanks for your comments. In the end, the “solution” was a different sort of pragmatic approach because the boards were always enclosed in hermetically sealed high pressure (10,000 psi+) / temperature (180C+) -resistant stainless steel tubes which have very little free air volume inside them. That means that there is very little free oxygen for component fires to use, and calculations proved that ignitions involving all the flammable material within the enclosures would exhaust that oxygen well before fires could develop, and also the way the enclosures are built and sealed means that flames or flammable material could not escape unless there had first also been very substantial external physical damage. This is another option. Build a fire inside the equipment and see what happens. I use a fire-starting tablet or pellet (I’ve forgotten the name). An enclosure with minimum openings that would allow replenishment of oxygen will suffocate the fire once the internal oxygen is used up by the fire. The enclosure does not need to be sealed. Usually, such construction will not have very much empty space and therefore relatively little initial oxygen to feed the fire. (I used such testing to prove that a circuit fire would not ignite an HB enclosure.) This is another situation where one can show that a fire will not spread very far beyond the initial fuel. Whether or not this is accepted as compliance with the standard will depend on the certification engineer and the policies of the certification house (and your ability to convince them that the construction is adequate). Rich - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
