In a message dated 2/18/99, Bill Ellingford writes: > Ted is of course correct if the unit is a simple device without ports for > interconnection to other apparatus and no user contactable parts. > Such a case may apply to a simple (POTS) telephone. There are no ports, no > user accessible parts (other than insulation) and, if mains powered (such as > loudspeaking) has a power supply barrier between mains and TNV. In such a > case, the power unit would be hardwired or use carefully selected plugs etc. > to prevent user contact at the power (low v) connector. > The telephone body insulation is the barrier between user and TNV. In this > case, all secondary circuitry is declared at the appropriate TNV level.
Joe: I just want to reinforce the comments made by Ted Warren and Bill Ellingford. I have used this approach successfully on three different product designs. The key is to make sure that none of the circuitry inside the plastic box is accessible. If the only connections are RJ-11 jacks and the power jack, most of the attention will focus on the power jack. The simplest solution is to hard-wire the power cord, but you can also use a connector if it is adequately recessed to avoid being accessible to the appropriate test probe. I have found that many test labs are skeptical of this approach until I explain the rationale and we review the relevant clauses in EN 60950. The knee-jerk reaction is to assume that an isolation barrier is required, because this is true of the typical case, such as a modem with an RS-232 port. However, the non-isolated approach is perfectly legitimate under the circumstances described above. Perhaps this approach will provide a way for you to salvage your existing design. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (voice) 781-721-0582 (fax) [email protected]
