Why are (most) telecom systems supplied with -48Vdc? [duplicate]

Question

Many telco systems we install require a -48Vdc power supply. A ballanced PS of -48Vdc / + 48V dc I can understand. But only -48Vdc, why is that?

Answer 1

48VDC is the voltage that the POTS (Plain Ordinary Telephone System) works on. It's high enough to allow long thin wires to be used from the CO to the subscriber, and not so high it wastes a lot of energy or presents too much of a shock hazard. That's the voltage when the phone is on the 'hook'. It's the voltage of 24 lead-acid cells in series. In the old days telephones were required to be extremely reliable and to work even in case of a power failure (dedicated land lines are still that way, though the equipment connected to them may not be).

-48VDC is used rather than +48VDC relative to earth in order to reduce the electrolytic corrosion (via cathodic protection) that will occur if the wires get wet.