Aaron: Thanks for bringing up this topic. I have wrestled with these issues as well, and would like to share my interpretations with the group. I would like to have some discussion on this topic, because the question you raise has serious implications for the design of various terminal adapters. For reference, I have re-printed your three scenarios at the end of this note. My interpretations are as follows:
For Scenario 1, basic insulation is required. This is because a typical PC does not have a "permanent connection to earth" as required to escape the requirement for basic insulation. The exemption works quite well for a traditional PBX, but not well for the typical PC installation. It is my impression that many vendors are not providing basic insulation in their POTS adapters (such as ISDN terminal adapters, cable telephony adapters, etc.). I would be interested to know whether they have found a legitimate way around the requirement, or whether they are simply unaware of it. For Scenario 2, the same requirement as Scenario 1 applies (basic insulation). In addition, some other requirements apply that derive from the exposure to lightning and power cross. These additional requirements can be identified by looking for the differences in requirements between TNV-2 and TNV-3. Scenario 3 is complicated. If there are additional ports on the equipment (such as an RS-232 port) that make the internal SELV circuits accessible, basic insulation is still required. One way around this is to ensure that the internal SELV circuits are not accessible. This would be possible if the equipment had no additional ports (an unlikely scenario), or if all of the additional ports were isolated with basic insulation. A plastic or floating metal enclosure would also be required. Attention must also be given to the connector for low voltage power from the wall unit (preferably hard-wired). If you can manage to implement all of this, the internal low voltage circuits do not need to be isolated from the TNV-2 or TNV-3 circuits, even though the equipment does not have a "permanent connection to earth." I have used this approach successfully on a couple of products. Well, that's a brief summary of my interpretations for the three cases you cite. I look forward to hearing from other treggers regarding their interpretations and opinions on this topic. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (voice) 781-721-0582 (fax) [email protected] (email) ------------------------------------------------------------------------------ - Excerpts from original posting: Scenario 1: If the equipment is the PC card, the SLIC is drawing power from PC bus (earthed SELV) with step up DC-DC converter to provide the voltage to drive the SLIC to provide ringing signal (which is usually higher than SELV definitions). This port will be classified as TNV2 for internal building wiring so as the SLIC IC itself as there is no isolation inside the IC. (Discussion clauses is refer to clauses in EN60950) a) Does this POTS port (TNV2 circuit) require to have BASIC INSULATION (creepage and clearance) from earthed SELV (Clause 6.2.1.2 & table 19)? Clause 6.2.1.2 also said that "BASIC INSULATION is not required.....". Is it fair to say that BASIC INSULATION is not required since TNV2 and SELV are both connected to protective earth (PE) as the SELV of PC mother board and any ports on PC is connected to PE. The SLIC is also connected to PE in TNV circuit. b) Test procedure of clause 6.4.2, TNV2 port and (c) of Figure 18 do not apply. Is my intepretation in this scenario is correct. Scenario 2: Same as above, but this TNV ports may be wired outside of the building (ie. inter building connection). What is the requirement that applies here since it is subjected to lightning surge and power cross. Is it same as scenario 1 above but the equipment must pass ITU-T K.20, FCC part 68 lightning surge and power cross testing? Scenario 3: If the equipment is a stand alone equipment using DC Double insulated Power Plug Pack (Output 9-112VDC). In this case, even that the SLIC is drawing power from SELV circuit, SELV is unearthed and there is no protective earth is provided for both SELV and TNV circuit. Is it fair to say that BASIC INSULATION must be provided betweeb TNV and SELV.
