Why do PBX systems use -48 V?
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."
Electrically it makes no difference if the ground is positive or negative the components will be connected accordingly. However in the early discovery days when electric trams and telephone cables were laid. They were first connected as negative ground and it was found over a short time that corrosion and eating away at cables was a serious problem. Especially for the delicate telephone cables which where insulated by paper wrapped around each wire. In the UK, tests were done over period of time using both systems and significant improvement was found with the positive ground system. Due to the long life that was expected from these cables it was obvious that positive ground at the telephone exchange had to be the new standard. It still is today, world wide all telcos still use the minus 48 volts to the subscribers house and positive ground at the exchange battery. Now as for car electrical systems, I can only assume that the lack of longevity of the car made positive ground an inconvenience and in the mid fifties after the war world trade and standardisation was required hence the 12volt negative ground system.
As an ex-Bell employee, this was my first question as my tendency (from school) was negative ground. The post referencing the trams nailed it. It was found positive voltage cable corroded much faster as also mentioned in a post above - it's basically electrolysis. -48 volts was used to minimize resistance loss on cables from the central office to the customer and PBX / station equipment on the customer site used -24 V. I've been out a while - I'm not sure if subscriber equipment still uses -24V anymore.