Path: utzoo!dciem!nrcaer!scs!spl1!laidbak!att!pacbell!ames!mailrus!tut.cis.ohio-state.edu!bloom-beacon!NADC.ARPA!prindle
From: prindle@NADC.ARPA (Frank Prindle)
Newsgroups: comp.dcom.telecom
Subject: Re: Three wire lines
Message-ID: <8806011249.AA04333@NADC.ARPA>
Date: 1 Jun 88 12:49:01 GMT
Article-I.D.: NADC.8806011249.AA04333
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Many homes are wired with 3-wire cable.  This supports a single phone line
with 2-party service.  The third wire is connected to ground (earth) and
serves two functions: it is the return path for ringing current for both
parties, and a fixed resistance from tip to ground (when the phone goes off-
hook) is used by the CO equipment to identify one party (i.e. the "tip" party)
for billing purposes.  Such a line requires a phone modified for tip or
ring party connection (rearrangement of the ringer connections and use of
an additional hookswitch contact pair).  Needless to say, this is virtually
impossible with any phone except the old AT&T 500s and 2500s and their
equivalents.  Modular cables for today's single line phones don't even carry
the third wire through to the phone!  Plugging a new phone into a ring party
jack might just work, but in a tip party jack, it will likely bill calls to
the wrong party if they go through at all.

This service is no longer offered by most LOCs, but customers who have kept
the service through the years must be permitted to retain it (it is charged at
a flat rate for local calling, usually several dollars a month cheaper than
individual line flat rate).  Due to the sparsity of the remaining 2-party
line service in major cities, most such lines are, it reality, private lines.
It is less of a bargain in light of the fact that only a very limited number
of phones can be made to work with it.  There is generally no service charge
if a customer willingly wants to do away with it; the LOC will gladly switch
you to private service, but you are on your own to rewire (or toss) the phones.

The three-wire house wiring will remain behind for centuries to confuse all
but the dedicated phone historians.  Four-wire wiring became the rule much
later (probably in the late 50s and early 60s) when lighted phones started
to appear ("It's little, it's lovely, and it lights!").

Sincerely,
Frank Prindle
Prindle@NADC.arpa

[thanks to Patrick Thompson of Cup.portal.com and Rich Wales of UCLA for
submitting similar responses to this question. --jsol]