Electronic – Why do PBX systems use -48 V


It seems common that PBX and other telephone hardware use a positive-ground power supply, where the "hot" line is at -48v. What's the reason for that?

Best Answer

I remember this coming up many years ago in the alt.telecom newsgroup and I managed to find it for you (aren't I kind?):

Why most telecommunication equipment use -48V supply voltage

In summary (from the thread):

"From a book I've been reading lately (Instruction in Army telegraphy and telephony, vol 1, 1917), the reason is for fault tracing. An earth fault will tend to decrease in resistance, i.e. tend towards a dead earth, if the earth is positive with respect to the conductor, thus enabling it to be located."

"48V (or in the UK, 50V) seems to be arbitrary, many of the earlier CB systems of the Post Office used 22 volts or 40 volts. The automatic systems in some early exchanges of the Siemens 17 type used 60 volts IIRC.

48 to 50V may have been a happy medium (remembering that years ago, telecommunication companies were VERY conservative, and standardized across their entire network), allowing the use of long thin lines, but not risking electrocution of linemen or overheating on short circuits."

"A negative voltage is really a positive earth potential. If your positive conductor i(+) is earth, you can't short it to earth. It can be shorted to the exchange earth connection if it comes into contact with a suitable conductor in the cable, but as this 'earth' is the negative battery terminal (technically) you don't get the massive current flow to earth for a conductor to earth. The only way you can get massive current flow is if you short the pair together or put the positive earth to a foreign wire connected to the negative battery terminal."

"corrosion reduction—the leakage to earth that would occur if insulation were damaged opposes the corrosion."

"Why negative? AFAIK to reduce electrolytic corrosion of buried cables, which were lead-sheathed."

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