slb@drutx.UUCP (Sue Brezden) (02/28/86)
> Does anyone know why 60hz was chosen as the standard frequency > for the AC power distribution system in North America?? Along the same lines, why -48 volts for telephone lines. I understand the negative voltage is to prevent loss of copper. But 48? -- Sue Brezden ihnp4!drutx!slb ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Nirvana? That's a place where the powers that be and their friends hang out. --Zonker Harris ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
crowl@rochester.UUCP (Lawrence Crowl) (03/02/86)
In article <107@drutx.UUCP> slb@drutx.UUCP (Sue Brezden) writes: >> Does anyone know why 60hz was chosen as the standard frequency >> for the AC power distribution system in North America?? >Along the same lines, why -48 volts for telephone lines. I understand >the negative voltage is to prevent loss of copper. But 48? I heard many years ago is that 48 volts was below the "high voltage" figure. This allowed the phone company to avoid paying "hazardous duty" pay. This is unsubstantiated rumor dredged from memory. More rumor is that the workers (union?) then tried to lower the "high voltage" below 48. The world is full of selfless people. :-( -- Lawrence Crowl 716-275-5766 University of Rochester Computer Science Department ...!{allegra,decvax,seismo}!rochester!crowl Rochester, New York, 14627
sukenick@ccnysci.UUCP (03/04/86)
>> Does anyone know why 60hz was chosen as the standard frequency >> for the AC power distribution system in North America?? > >Along the same lines, why -48 volts for telephone lines. I understand >the negative voltage is to prevent loss of copper. But 48? ^^^^^^^^^^^^^^ Negative with respect to what? If its negative to prevent some degradation (oxidation,perhaps?)of the copper, then what is the ground wire composed of? -george Alright, enough questions here! how about some answers! *)
jbs@mit-eddie.MIT.EDU (Jeff Siegal) (03/06/86)
In article <232@ccnysci.UUCP> sukenick@ccnysci.UUCP (George ) writes: >>Along the same lines, why -48 volts for telephone lines. I understand >>the negative voltage is to prevent loss of copper. But 48? > ^^^^^^^^^^^^^^ >Negative with respect to what? The ground, I would assume. >If its negative to prevent >some degradation (oxidation,perhaps?)of the copper, > then what is the ground wire composed of? This does not seem particualarly relevant, since its potential with respect to the ground would be very small. Jeff Siegal - MIT EECS
sgcpal@watdcsu.UUCP (P.A.ul Layman [EE-Device Physics]) (03/06/86)
In article <232@ccnysci.UUCP> sukenick@ccnysci.UUCP (George ) writes: >Negative with respect to what? If its negative to prevent >some degradation (oxidation,perhaps?)of the copper, > then what is the ground wire composed of? > >Alright, enough questions here! how about some answers! *) The answer was part of your question. Why do you think they call it *ground*. (i.e. dirt, water, maybe some PCB's etc.) PAul
dbb@aicchi.UUCP (Burch) (03/07/86)
Well, Four 12 Volt Lead-Acid Batteries in series will produce 48 volts. I gather that in the old days, all power for telephony came from big battery rooms, and 48 volts worked out both to be an even number of standard (1.5 V) cells, and to be strong enough to drive the speaking circuit for a good distance. When the Ring current is applied to Loop Start or Ground Start lines, it is still referred to by some as the "battery". -- -David B. (Ben) Burch Analyst's International Corp. Chicago Branch (ihnp4!aicchi!dbb) "Argue for your limitations, and they are yours"
jlg@lanl.ARPA (Jim Giles) (03/07/86)
>Along the same lines, why -48 volts for telephone lines. I understand >the negative voltage is to prevent loss of copper. But 48? 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.... As for -48 instead of +48 - well, they didn't really standardize on the proper polarity for signal lines back in those days. It was probably just an arbitrary decision. J. Giles Los Alamos
mc68020@gilbbs.UUCP (Tom Keller) (03/09/86)
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? > > 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... -- ==================================== Disclaimer: I hereby disclaim any and all responsibility for disclaimers. tom keller {ihnp4, dual}!ptsfa!gilbbs!mc68020 (* we may not be big, but we're small! *)
larry@kitty.UUCP (Larry Lippman) (03/14/86)
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!/ <==