Hi, Joe This is probably a silly question but do you need to meet SELV requirements for the circuitry behind the line interface? If it does not need to connect to anything else and is adequately isolated from user accessible parts and hazardous voltages you could declare it as TNV3, and the barrier is then purely functional rather than safety critical.
[email protected] wrote: > > 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 > > 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. > > > > 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
