Why telecom system works on -48 vdc not on +48 vdc?

Question

Why telecom system works on -48 vdc not on +48 vdc?

Answer 1

Great question. If you search around the internet, you'll find a lot of good information mixed with a lot of nonsense. The real reason has to do with galvanic corrosion.

In the old days, -48 vdc was used to 'wet' (provide power as well as data) voice telephone wiring. The wires needed to be wet to supply power to telephone handsets, so they could offer touch tone dialing and to provide power to the microphones. Once the standard was established for voice lines, phone companies needed power supplies to provide the -48 vdc. Once they had them, it was natural to use it to supply other equipment as well.

So the question comes back to, why is -48 vdc better for wetting buried voice lines than +48 vdc? The answer is simple. Buried lines occasionally have cracks in their insulation. A positive voltage will cause the line to 'suck in' electrons while a negative voltage will cause the line to 'spit out' electrons. (Electrons have a negative charge.)

If the line sucks in electrons, which a positive voltage would do, the nearby ions (formed from the salts that are dissolved in ground water) that were electron deficient would corrode the line. This is called 'galvanic corrosion'.

The scheme telecom wiring uses is called 'cathodic protection'. It is discussed in two paragraphs in the Wikipedia article I cited. The short version is that if +48V were used, the +48V line would corrode.