Purwin, Bill, Allen Thanks for your comments.
With respect to coatings, I had thought of this but as Bill stated, there are too many things to also meet as mentioned by Bill. Coating might work for an unpopulated PCB but when components are on already, I cannot see how I can guarantee that everything is completely covered. With respect to adding a hard wired Earth ground, I had mentioned that my system is classified as Class II equipment, since we only connect to hot and neutral of the AC mains and do not connect to earth at all. I am worried about adding an earth ground as it could create more problems than it solves. With respect to working voltage, I had stated that my TNV circuits connect directly to the analog Telecom Network. My working voltages are derrived, as stated from a 28 Vrms input which is then converted to 40 VDC at worst case. What is actually used to power my TNV circuits is only 20 VDC maximun, as my TNV circuits only use power to drive relays and only uses 5 VDC to run ICs and opto-isolators. The 5 VDC is derrived by putting the 20 VDC through a 5 Volt regulator which in turn then feeds my TNV components. The reason I was not sure if I could say TNV-1 was because although my working voltage at worst case is 40 VDC or 28 Vrms, my TNV are connecting to the Telcom Network, so I assumed that this depended not on my equipment but on the public network. The only thing I could come up with is the classic working voltages for a TNV-3 of 70.7 Vpeak and 120 VDC to determine my creepage and clearance requirements. I have taken the public network as unknown, which I believed force me to say TNV-3. For TNV-1, many have told me this was done primarily to cover the new digital technology such as ISDN, E1, T1, etc. Although the CO is -48 VdC, possibly up to 57 VDC, one could almost say it is in the range of SELV. But I am not digital and I do not know how far I could push it. If I could get a way with TNV-1, working as SELV, then all I would realy need is only operational insulation and also be able to use one of the three clauses of section 5.5.4 of EN60950 for operational insulation. It had also been mentioned to me, unfortunately I cannot remember who told me and so please excuse me, that if I drill a hole or slot between the track and pad, such that creepage, which is measured along a surface, must now go around the slot and hence would increase my creepage. I might also have to add some insulating material sticking up at the slot to ensure I got clearance since it is measured through air. I thought this was a great idea but since I use a double sided PCB, and thanks to Murphy's law, there were tracks on the underside of the PCB, exactly where I would have placed the slots. This may not work for me now but I believe this is a great solution that should be kept in mind in case anybody else could have a similar problem. To summarize then, I think I have TNV-3 which needs basic insulation between TNV and SELV. If I could somehow be able to classify as TNV-1 within limits of SELV then I would only need operational insualtion between TNV and SELV. This would solve my problem. Any further comments welcomed. Regards Joe Schembri [email protected] RCIC - http://www.rcic.com Regulatory Compliance Information Center
