John, It is good to hear someone bringing up these points. Not all digital loops are dry or have just -48 VDC for sealing current.
Examples are HDSL and fractional T1 service that have remote units located inside the customer premises. They typically receive -137 to -185 VDC from the C.O. between the pairs. The customer should not see those voltages due to isolation, but those circuits are brought into the customer premise and terminated in a single device with output to the customer. They are almost always owned by the Telco, but that is changing as people set up private networks. For Europe, we had to lower the voltage to 113.5VDC (<120 peak), and this affects the distance we can go, but EN60950 limits us to the 120V peak without considering the telco interface a power supply and having to meet alot of additional (impractical) requirements. Jim Wiese ADTRAN, INC. [email protected] ---------- From: treg-approval To: treg Subject: Re: BS EN 41003: 1997 now available List-Post: [email protected] Date: Thursday, February 13, 1997 5:37PM >Date: Thu, 13 Feb 1997 14:24:37 -0500 >From: Frank McCaughey <[email protected]> Frank, >The 130 Vdc used on the older DS-1 lines was fed, simplex, down the >Tx and back up the Rx pair, to power the repeaters. Only the longest >loops (several km) were fed +130/-130, to make 260 Vdc pair-to-pair. >For equipment used on customer premises, there would likely be no >repeaters between the customer location and the CO, and even if >there were, they would likely be fed from the CO, not the customer >premises. >As the number of repeaters increased, the powering was fed >-48V/Ground >-48/+48 >-130/Ground, and so on. >So the case where 130 Vdc would appear on customer premises would be >extremely rare, only in a specially-engineered situation, and where >other special protective measures should be taken. Actually, the span-repeater power should _never_ appear on the customer premises, this is one of the distinctions between DSX-1 and DS-1. DSX-1 (the "cross-connect" point) is supposed to be "behind" an "office-terminating-repeater." I suppose in the current deregulated U.S. market this isn't quite the certainity it used to be, since the OTR might be built-into a piece of CPE. >In other words, the case is unusual enough that a footnote would be >more than adequate in any list of requirements, rather than tar the >entire system with the same brush. I worked for GTE for many years, and in their network, 130 VDC is commonplace -- I don't know about the rest of the world. ________________________________________________________________ John Combs, Senior Project Engineer, ITS/TestMark Laboratories Email: [email protected] URL: http://www.testmark.com
