Why do PBX systems use -48 V?

Question

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?

Answer 1

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."

Answer 2

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.

Answer 3

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.

Answer 4

Also they use 48 V (regardless of polarity) because that is lower current so easier to distributed. Now there are tons of chips and brick for -48 V to 12 V/5 V, etc. conversion. And for UL and other safety limits it is easier to get approved if it is less than ~60 V.

Answer 5

Telephone lines originally were all LONG HAUL from the SLIC generating the V-borsht (battery, overvoltage, ringing, supervision, coding, hybrid and test access). And with line drop over long hauls the end phone line will experience much less. Normal phone lines being -48V On Hook at the Telco's SLIC(FXS). Where at the FXO (or phone) that same voltage may drop to as low as 20V. Where on OFF hook will pull the line to approximately 7-9V. Also note that using a simple doubler the On Hook is about 90V, which can produce enough current to move the physical clapper of the Ringing Bell.

Now a days. FXS to FXO are all mostly short in that they are kilometers. And the REN (Ringer Equivalency Number) load is so low, the FXO/phone devices sees nearly 100% of the voltages.

So it is best not to lick Tip/Ring while the phone rings.

Answer 6

In addition to the reasons given above, I also think the reason for using 48V in particular is that batteries come in 12V or multiples thereof (24V). It is easy connect four 12V or two 24V in series to obtain 48V.

Answer 7

My first chief engineer once explained positive ground: Having a point source for electron flow from battery to ground tended to reduce shot noise in equipment. There is one path for electrons to flow from source, rather than multiple paths as from a ground grid.