Tip and Ring - Line Voltage

Line Voltage

The telephone company maintains large battery systems that supply DC line voltage for the operation of analog telephone service (POTS) at customer locations. The voltage supplied is a compromise between operational needs for reliable service and safety precautions for customers and service personnel. The length of the line to a customer telephone interface presents a resistance across which the central office voltage experiences a drop and therefore the voltage at the customer site may vary. The nominal value is 48V, but the central office common battery is adjusted to between 50 and 52 volts.

In the middle 20th century, long loops in many rural areas of North America used range extenders, which operated at 100 or 130 volts to ensure reliable signaling. Some rural switching systems were designed to apply range extenders internally and thus share a few extenders among many lines, while for other lines, one extender was applied externally per line.

Originally, the potentials on the wires were positive with respect to earth (ground). This is called negative ground, since the negative side of the battery is grounded to earth. Telephone companies discovered that with positive voltage on the copper wires, copper wires experienced corrosion due to electrolysis. Operating in reverse, positive ground (negative voltage on the wires), the copper is protected from corrosion, a process called cathodic protection.

To ring the telephone to alert a subscriber to an incoming call, about 90 volts of 20 Hz AC current is superimposed over the DC voltage already present on the idle line. Historically a variety of frequencies have been used, however.