CBFalconer wrote:
In North America again (I don't know about elsewhere) the normal
phone uses 3 wires to connect to the two wires of the phone
circuit. The yellow wire carries the ring signal. Just disconnect
that and the phone won't ring, and the load is zero.

I don't recall any system in North America that put ring
voltage on a separate wire. The yellow wire is
generally not connected unless one 4-wire cable is used
for two separate telephone lines.

In North America the "normal" line uses only 2 wires.
The audio signal is applied between the "tip" and the
"ring" of a single pair. "Ring Current" and "Loop
Current" are also applied between the Tip and the Ring
of the same pair.

Commonly used drop cable has four wires: Green is the
Tip and Red is the Ring (positive and negative,
repectively for the DC loop current), while the Yellow
and the Black wires are not used. (Note that the DC
voltages used by telephone companies are negative with
respect to ground, hence for DC the Tip wire is at
ground potential, and the Ring wire has a negative
potential. But the Tip is not at ground potential for
Ring Current or for the audio signal.)

One configuration often seen includes a second line on
the same cable, using Yellow and Black as Tip and Ring.

Historically the Yellow wire was, for a few years, used
for a small AC voltage (nominally 6.8 volts) to power a
lamp circuit on some telephone set models.

Another historical use had the Yellow wire as a ground
for party line service from the old style mechanical
switching systems (such as the Step or Stroeger
switching systems once used by the Bell System and by
Automatic Electric). On those systems the ring current
was applied between either Tip or Ring and ground, which
was supplied to the telephone set on the Yellow wire.

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)