Joe, In my opinion TNV 3 is required. In the strictest sense the safety barrier at telco interface should be 3 mm and/or two layers of qualified insulation to pass EN60950.
In actual practice 2.5 mm will pass creepage, but two layers of insulation are still required if tracks are on opposite side of board. This means tracks on opposite sides of the barrier must have two layers of PCB between them. In my experience the use of conformal coating for insulation is not reliable and a manufacuring nightrmare. Regards, Duane J. Marcroft Telecom Consultant Peer Communications [email protected] >For one of my analog telephone systems, I have a problem with respect to >EN60950 and TNV circuits which I hope someone can help me with. > >The complete system is categorized as Class II equipment which consists of >a separate power supply connected to our analog telephone network >interfaces. The interfaces are enclosed in a plastic housing. > >The power supply consists basically of a transformer that has reinforced >insulation with minimum 3KV dielectric withstand voltage. It would be >connected to an AC supply of 220 Vrms, 50 Hz. The AC mains voltage is >then stepped down to a lower AC voltage of maximum 28 Vrms. This lowered >AC voltage is then fed to and terminates on our system which then converts >it via a full wave rectifier circuit to a maximum of 40 VDC output. > >Since my DC output is fed from a transformer with reinforced insulation >and the maximum output is 40 VDC, I consider this a SELV circuit, actually >an unearthed SELV circuit since we do not connect to AC ground on our >power cord, two-prong only. > >This SELV circuit is used to power my analog TNV circuits which connect >directly to the analog Telecommunication Network. From my understanding >of EN 60950, my TNV circuits would be classified as TNV3, since it is >exposed to any surges from the telecommunication network whose >characteristics may or may not be known. The working voltages would thus >be 70.7 VAC peak and 120 VDC which exceeds the limits for a SELV circuit. >This is why I do not call them TNV-2? I wonder if I am correct. I also >wonder if I could consider my TNV interfaces as TNV-1? > >EN60950 states that between TNV circuits that exceed the limits of SELV >and unearthed SELV circuits, I must have Basic Insulation. Using the >secondary circuit clearance table requirements for mains up to 300V, >Pollution degree 2, and the creepage table requirements for Material >Group III at a working voltage of 120 VDC I get: > > Clearance : 1 mm > Creepage : 1.5 mm >I have no problems with clearance, assuming I correctly chose it. The >problem is with creepage on my PCB. > >The terminals and circuitry, Tip/Ring, that connect directly to the >telecommunication network are isolated from my SELV circuit by a >transformer that provides Basic Insulation. > >The ground of my SELV circuit, in some locations, comes close to the >Tip/Ring terminals that connect directly to the telecommunications >network, The track separation, creepage, between my SELV ground PCB >track and the Tip/Ring pad is in some cases as low as 0.9 mm, which would >thus fail the 1.5 mm creepage requirement. > >I have done dielectric tests as per section 6.4.2.2,that is, a) 1.5 KVAC >between the shorted telecom network connectors and telephones and 1 KVAC >for cases b) and c) with no problems. > >As can be seen then I do not meet the creepage requirements between TNV >and unearthed SELV, but I do meet the electric strength requirements. > >I am not sure where I stand now with my system. Can someone help me? Is >there anything I can do, apart from redesigning the PCB, to overcome the >creepage problem? I also have systems that are already built and in stock. >What can I do with these? > >Any help would greatly be appreciated as soon as possible. > >Regards > >Joe Schembri >[email protected] >Tel: (905)264-1505 >FAX: (905)264-1510 > > > >Joe Schembri >TCR >1 Whitmore Rd. #21-23 >Woodbridge, Ontario, Canada >L4L 8G4 > >Phone: +1 (905) 264-1505 >Fax: +1 (905) 264-1510 >e-mail [email protected]
