In a message dated 97-07-30, Larry Barnette writes: << I'm looking for a standard that prescribes 3750VAC seperation from circuitry to be connected to telecom networks. >>
Larry: The history of the 3750 VRMS requirement is long and tortuous, but here is an abbreviated version: Long ago, before IEC 950 superseded IEC 380, the dielectric withstand voltage for "reinforced" insulation with a working voltage of 250 VRMS was 3750 VRMS. The BS 6301 safety requirements called out "supplementary" insulation for tip/ring to ground, and "reinforced" insulation from tip/ring to the AC power mains. In most products, the reinforced insulation was implemented in the AC power supply, and only supplementary insulation was required in the tip/ring interface. However, for modems where the power supply was not part of the product being approved (e.g., an aftermarket modem card for a PC), BABT insisted that the modem manufacturer provide the reinforced insulation in the tip/ring interface. Under IEC 380, this implied a 3750 VRMS dielectric strength (among other things). Thus, this special case under BS 6301 and IEC 380 led to the popular misconception that 3750 VRMS was universally required in the international market. This misconception lives on, even today. Today, the requirements are less stringent, but more complicated to explain. Under IEC 950, the dielectric strength requirement for "reinforced" insulation is 3000 VRMS, and for supplementary insulation it is 1500 VRMS. However, reinforced insulation is now almost never required in the tip/ring interface, and even the requirement for supplementary insulation has been deleted in Amendment 4 to IEC 950. Under Amendment 4, there are now three types of tip/ring circuits (TNV-1, TNV-2, and TNV-3), and the isolation requirements differ for each type. A conventional analog modem will be TNV-3, which now requires "basic" insulation. The dielectric strength requirement is 1000 VRMS in this case. My "brief history" has now become quite long, yet a lot of detail has been left out. Suffice it to say that 3750 VRMS is no longer a requirement anywhere, but the specific requirement that now applies can range from 3000 VRMS down to to zero, depending on the type of equipment involved. The gory details can be found in IEC 950 (as amended through Amendment 4). I hope that the above has been helpful. If you have questions about the dielectric strength requirement for your specific application, give me a call at 617-721-2848. Once you answer some basic questions about the type of equipment and how it is used, I can quickly tell you what requirement applies in your case. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
