Path: utzoo!utgpu!water!watmath!clyde!att!rutgers!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: crossing phone lines
Summary: Central office battery voltages
Message-ID: <2716@kitty.UUCP>
Date: 27 Sep 88 04:41:08 GMT
References: <7921@cit-vax.Caltech.Edu> <2689@kitty.UUCP> <657@eplunix.UUCP>
Sender: nobody@cs.Buffalo.EDU
Organization: Recognition Research Corp., Clarence, NY
Lines: 60

In article <657@eplunix.UUCP>, raoul@eplunix.UUCP (Otero) writes:
> > 	In the on-hook (i.e., idle) state, connecting the two lines exactly in
> > parallel (tip1-to-tip2 and ring1-to-ring2) will, in general, be undetected
> > by the central office apparatus since there will be no current flow.
> 
> No such luck. Unless the voltage matching is perfect, which it never is,
> a small DC current will flow from one line to the other. This will 
> mess up both your lines, and you will not be able to use either. Ma
> Bell will also usually disconnect the lines within about a week of this
> sort of nonsense, thinking there is a short somewhere (there is!).

	With one rather rare exception (two lines, each on a dedicated
loop-extender), for all intents and purposes there will be no detectable
current flow if the two lines are connected in parallel when on-hook.
CO switching apparatus must be able to operate in the presence of leakage
resistance caused by cable insulation conductance (often aided by water!).
In general, CO apparatus should be able to dial at maximum loop resistance
(usually 1,500 ohms) with a "leak" of 50,000 ohms (1 mA of spurious current
flow), and should not be able to pull dial tone with a 10,000 ohm leak
(5 mA of current flow).  At a typical maximum loop resistance of 1,500 ohms
plus a maximum of 500 ohms station set resistance, one has at least 25 mA
of current flow in order to pull dial tone.  5 mA of current flow is small,
when compared to the minimum amount to dial in a normal situation.
	Typically, at least 5 mA of current is required before the CO
apparatus will detect an off-hook condition.  Bear in mind that this
threshhold is intentional, so as to prevent spurious dial tone requests
when outside cable plant is only "slightly wet".
	Assuming a situation "close" to the CO with two 250 ohm loops, for
a 250 ohm ring-to-ring resistance, a CO battery voltage differential of
1.25 volts between each line relay (or current sensor) would be required.
While this may seem like a small voltage differential, it really is quite
large when comparted to CO power distribution critera.  Good CO power
distribution practice goes to great lengths using really large gauge wire
to keep the impedance of the 48-volt battery distribution as low as possible;
the primary reason reason is to minimize noise and crosstalk.

	Incidently, it is common to see a CO voltage of greater than 48
volts.  Virtually all CO's use a 24-cell battery string, with a typical
"float" voltage of 2.17 volts per cell, resulting in a nominal 52.08 volts.
While a counter-EMF cell may be used to drop this float voltage (the
counter-EMF cell is shorted when AC power fails), a battery voltage
between 50 and 51 volts is most common.  The battery voltage may even
be a volt or two higher if an "equalizing" charge is being placed across
the battery string.

> However, don't take any of our word for it. Just short them together
> at the phone jack and see.... Try some real scientific method.

	If you want to be real scientific about it, use a milliammeter to
measure the current flow.

	One other exception to my statement above would be if the two lines
were served by different exchanges using physically different switching
apparatus within the same CO building (like XBAR and ESS); this is not
uncommon in large cities with multiple exchanges in the same CO building.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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