From: floyd@tanana.polarnet.com (Floyd Davidson)
Newsgroups: comp.dcom.modems
Subject: Re: A question for any electrical wiring experts ?
Date: 10 Oct 1998 16:50:01 GMT

Denny Salatino <salatino@foxinternet.net> wrote:
>On 10 Oct 1998 13:23:42 GMT, thad@thadlabs.com (Thad Floryan) wrote:
>>Can I "fix" the problem by sending 120VAC (or higher) down PacBell's side
>>of the interface to burn-off any moisture/condensation and force-weld all
>>wire connections between me and the central office?  If nothing else, it
>>would be kinda neat lighting up all the wires going into the central office.
>>
>>:-)
>>
>>Thad
>**********
>You got some good answers for input here, but I like your last idea.
>I thought I was the only one that wanted to try that! but then you
>would end up with a one wire system, but a rather heavy line.
>Denny.
>


Pretty easy to do to!  Just ring the line, and that is exactly what
happens (100v AC on top of 50 VDC is applied to the line).



And, while I'm sure the above was all meant in jest...  it works to
some degree.  Leased line circuits for data are a relatively new
invention, and when they first began appearing a new situation for
telephone testboards came with them.  These leased line circuits were
the only circuits around that did not have some kind of high voltage
applied to the line at least once in awhile, and it became apparent
that it was possible for that condition to allow these circuits to
build up a charge on the line that would cause the circuit to fail.
Usually the first person to do _anything_ to the circuit would also
cause the problem to disappear with no trace of what or why!  Just
plug in a test set, and everything started working.  Very mysterious.
However, it wasn't always that easy.  And some testboard people learned
that before testing such problems it was a good idea to "pre-condition"
the line.

Your solution is _exactly_ what we did.  Old 2-wire testboards had
a cord to which ring voltage could be applied.  So that cord would be
jacked into a circuit, and the switched flipped to "RING" a couple
times.  That *always* worked.

Today all leased lines using cable facilities are equipped with
something called "sealing current" to prevent that problem.  It
applies a relatively small voltage (24 vdc) to the cables and
allows 10-15ma of current to flow at all times.

  Floyd

--
Floyd L. Davidson                                floyd@ptialaska.net
Ukpeagvik (Barrow, Alaska)