Brian, You appear to be correct about the hole sizes, but the maximum hole size permitted by Table 4D is fairly small (2mm) and won't make you best friends with your metalworker as a punch that size is going to wear out very quickly, and to provide significant ventilation, you are going to require a lot of 2mm holes.
Another approach that is permitted is to have larger holes, but fit a mesh of specified dimensions across the apertures. This may be a more economic solution. You could also fit some form of baffle plates, which again may be more economic and effective in terms of ventilation. One of the critical factors in this problem is that you require double or reinforced insulation between parts at hazardous voltage and accessible conductive parts. You base plate is an accessible conductive part, so your insulation must meet the requirements for double or reinforced insulation. However, if you are relying on an air gap to provide reinforced insulation, you must ensure that even under fault conditions, the gap that you have provided cannot be compromised. For example, could a detached wire fall across the gap to a point where the separation is less than the minimum, or could a printed wiring track that overheats and fuses under fault conditions peel off and bridge the gap? You may want to consider fitting a thin insulating layer between the underside of your printed wiring board and the base plate. Good luck with the solution. This is why I have a fundamental dislike of accessible conductive parts on Class II constructions!! Best regards Neil R. Barker CEng MIET HonFSEE MIEEE Manager Quality Engineering e2v technologies (uk) ltd 106 Waterhouse Lane Chelmsford Essex CM1 2QU UK Tel: (+44) 1245 453616 Fax: (+44) 1245 453571 Mob: (+44) 7801 723735 P Please consider the environment before printing this email. From: Brian O'Connell [mailto:[email protected]] Sent: 13 September 2007 16:58 To: [email protected] Subject: Class II construction question Good People, Investigating (for me) new territory. Customer requires Class II unit, but with a metal base plate. Target standards for the end-use device are UL/EN60950-1 and UL 1310. I am not certain if I can allow vent holes under the PCB traces of a circuit at hazardous and mains voltage levels. According to 4.6, I can allow openings in the enclosure if the hole is less than a specified diameter (table 4D), and if the spacing from the opening to the 'exposed' haz V is > 13mm (4.6.4.2), and if for any fault condition that no ejecta creates a fire hazard. Other that the generic requirements of 2.9.4, nothing else can be found. But, I seem to remember a requirement that there can be no opening (period) that is directly beneath a haz V for a Class II device (because a short to the chassis will not conduct to protective earth). Can someone please advise on the requirements for vent holes in Class II construction ? Thanks much. luck, Brian Sent by E2V TECHNOLOGIES PLC or a member of the E2V group of companies. A company registered in England and Wales. Company number: 04439718. Registered address: 106 Waterhouse Lane, Chelmsford, Essex, CM1 2QU, UK. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
