In a message dated 96-06-07, Barney McNamara writes: > The question is the safety classification of the POTS lines coming out of our box. > We generate a voltage over SELV limits to ring the attached telephone equipment.
Barney: Since the POTS line coming out of your adapter provides a power ringing signal, it will be classified as a "TNV" circuit according to IEC 950. I assume that the SELV circuit in your router is an "earthed SELV" circuit. If the SELV circuit is not earthed, the answer to your question is different from the one described below. The USA standard UL 1950 (third edition) and the European standard EN 60950 are both based on IEC 950, and both use the definition of a TNV circuit. In either standard, you can connect your TNV circuit directly to an earthed SELV circuit, provided that you comply with the provisions of clause 6.2.1.3. In general, this is fairly easy to do. Depending on the interpretation of the test lab you use, you may be required to demonstrate that your earthed SELV circuit meets the requirements of clause 2.5.11 for ground integrity, at the point where your TNV circuit connects to the earthed SELV circuit. The applicable test current for this test is debatable, but a conservative interpretation would be to use 25 amps. Also, you should be aware that your ring generator circuit must meet the requirements of Annex M in UL 1950 and in EN 60950. I hope that this gets you headed in the right direction. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
