On 07/07/2016 10:36 AM, Pat wrote:
On Wed, 06 Jul 2016 13:09:35 -0700, mike wrote:


There are two wires. One is grounded, the other has -48 volts.

Not true. POTS lines are balanced twisted pairs. Neither side is
supposed to be grounded (except for old party line ringing schemes).


Hello, and it's the voice frequency (~3 Hz to 3 kHz) part of a 2-wire
common battery telephone line that's balanced to (earth) ground to
minimize the introduction of common-mode noise. Repeating coils and/or
chokes allow the common battery to supply loop current to subscriber
sets while not short-circuiting the voice frequencies on the wire pair.
Party-line ringing systems aside, it's been common practice in the U.S.
to connect the positive terminal of the central office battery to earth
(I'm told the reason for this was originally as an anti-corrosion
measure). From a DC perspective one of the twisted pairs is earthed
(but only at the CO). Measurement with a DC voltmeter at a subscriber
set location from the Ring (red) side of the line to ground (something
leading back to earth such as a metal water pipe or ground prong input
of an AC utility receptacle) will provide confirmation. Just don't
leave the meter hooked up this way very long as it will most likely
upset the voice-frequency line balance and cause considerable hum to be
introduced. Sincerely,

--
J. B. Wood e-mail: