Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!linus!philabs!cmcl2!seismo!rochester!rocksanne!sunybcs!kitty!larry From: larry@kitty.UUCP (Larry Lippman) Newsgroups: net.sci Subject: Re: Choice of voltage? Message-ID: <872@kitty.UUCP> Date: Fri, 14-Mar-86 12:26:59 EST Article-I.D.: kitty.872 Posted: Fri Mar 14 12:26:59 1986 Date-Received: Sun, 30-Mar-86 04:01:49 EST References: <232@ccnysci.UUCP> <1199@mit-eddie.MIT.EDU> <56@gilbbs.UUCP> Distribution: net Organization: Recognition Research Corp., Clarence, NY Lines: 75 Summary: -48 volts for telephone circuits: everything you always wanted to know In article <56@gilbbs.UUCP>, mc68020@gilbbs.UUCP (Tom Keller) writes: > In article <177@lanl.ARPA>, jlg@lanl.ARPA (Jim Giles) writes: > > >Along the same lines, why -48 volts for telephone lines. I understand > > >the negative voltage is to prevent loss of copper. But 48? Until the 1950's when polyethylene usage began to prevail, all outside telephone cables were constructed using paper-insulated wires within an outer lead sheath. Since the lead was obviously at ground potential, any moisture entering the cable would react with impurities in the paper pulp and result in a conductive solution which would cause the copper conductors to plate out on the inside of the lead sheath IF the copper were the ANODE. Since the copper conductors were in effect the CATHODE as a result of being at -48 volts, such plating would not occur. The relationship of copper to lead in the chemical electronegativity series is such that the lead - while acting as the anode - will NOT plate out on the copper. While moisture entering a lead and paper-insulated cable will still result in a leakage resistance fault with current flow between the conductors and the sheath, at least the damage to the cable will be minimized since the conductors will not start losing copper. Wet cable faults can in many cases be repaired by opening the defective section and "boiling out" the moisture with melted paraffin, along with the use of powdered dessicants and purging the affected cable span with dry nitrogen. The concerns about "losing" the copper conductors to the sheath are no longer significant with polyethylene-insulated conductors. However, at this time there is still a large amount of paper-insulated cable in service, and paper-insulated cable is still manufactured, since polyethylene has not yet been able to compete with paper insulation for maximum pair density in large pair-count (2,000+ pairs) cable. > > The original phone system was run on battery power (just like the > > telegraph). There were no AC power grids in those days (and not until > > the light bulb). The value 48 is a multiple of the potential of the > > lead acid batteries that they used (just as 6 and 12 are - you remember > > car batteries). I don't know why 48 was used instead of 12 or 24 or.... > > I suspect that if you were to make the calculations, you would discover > that 48 volts turned out ot be the most efficient compromise between boosting > the voltage to achieve longer un-amplified signal runs, and I**2/R losses in > the copper lines. Just a guess, mind you... The use of -48 volts for telephone service is a nice compromise of many factors, including safety, fault current limiting, loop resistance limits, etc. However, -48 volts was not always used. Many early manual and dial central offices used -24 and -36 volts. If my memory is correct, most of the early Western Electric panel offices used -36 volts - and this was well into the 1930's. While Western Electric tended to standardize on -48 volts rather "early" in the game, many other vendors of telephone apparatus - such as Stromberg-Carlson, Kellog, Automatic Electric, etc. - continued to use -24 and other voltages well into the 1950's. It is interesting to note that one of the early Western Electric "electronic" PABX's used -24 volts on the station loop. This was the 805 PABX, and was introduced as a low-cost alternative PABX in the early 1970's. One other point of trivia: In the real world, there is no such thing as a 48 volt central office. Virtually all central offices use lead-calcium storage batteries in a 24-cell string which are float-charged at 2.17 volts per cell, for a nominal battery voltage of 52.08 volts - NOT 48 volts. Some central offices use a "counter-EMF" cell to reduce the working voltage to approximately 50 volts; today, counter-EMF cells are solid-state rectifier diodes inserted for their forward voltage drop, but they used to be special chemical cells. Counter-EMF cells are configured to be shorted-out when the battery float-charger fails. ONLY in an AC power failure situation with loss of the office float- chargers, will a central office ever see -48 volts as the battery voltage. If you want to check this out, measure the on-hook voltage across tip and ring of a telephone line using a digital voltmeter. I bet it'll be at least 50.0 volts unless you have some awfully "leaky" cable pairs, or have non-standard service through loop extenders, line concentrators, subscriber line carrier, etc. ==> Larry Lippman @ Recognition Research Corp., Clarence, New York <== ==> UUCP {decvax|dual|rocksanne|rocksvax|watmath}!sunybcs!kitty!larry <== ==> VOICE 716/741-9185 {rice|shell}!baylor!/ <== ==> FAX 716/741-9635 {G1, G2, G3 modes} duke!ethos!/ <== ==> seismo!/ <== ==> "Have you hugged your cat today?" ihnp4!/ <==