Coating may be a solution but be careful. Application of coatings will be assessed for 1. Ingress of air / bubbles (not solid insulation) 2. Adhession strength (won't scrape off) 3. Guarantee of full contact adhession (no surface air gaps) 4. Ensure coatings are resistant to Petroleum spirit and water (i.e. not soluble) Different labs have differing interpretations on this subject! Adding a coating may not be as easy as first appears!
Bill Ellingford. Approvals. Motion Media Technology Ltd ---------- From: PURWIN[SMTP:[email protected]] Sent: 16 February 1999 16:07 To: [email protected] Subject: Re: EN60950 TNV CIRCUITS Yes, you can apply epoxy or conformal coating over the areas that have reduced creepage on the boards than effectively you can claim reduced creepage see appendix R2 in the UL1950 At 03:51 PM 2/15/99 -0500, you wrote: >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] > Janusz Purwin Xedia Corp 119 Russell St. Littleton Ma 01460 Tel 978-952-6000 ext 166 Fax 978-952-6090 email; [email protected]
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