Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!rutgers!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: power from phone line
Summary: Reasonble limits to on-hook current draw
Keywords: phone, power, remote
Message-ID: <3547@kitty.UUCP>
Date: 9 Dec 89 05:00:26 GMT
References: <14230@jumbo.dec.com> <3542@kitty.UUCP> <548@scorn.sco.COM>
Organization: Recognition Research Corp., Clarence, NY
Lines: 41

In article <548@scorn.sco.COM>, johnd@sco.COM (John Dubois) writes:
> >	If you are referring to a conventional subscriber telephone line
> >at the remote site which you dial using the public switched telephone
> >network, then you can obtain between 1 and 2 mA from the line when it
> >is on-hook.
> 
>      How much current can you be sure you can draw before the phone
> company decides the line is bad?  For example, if I was going to design a
> commercial product, how much could I use?  Is there a specific minimum for
> on-hook line impedance?

	The maximum on-hook current draw depends upon the type of central
office apparatus and the set point parameters for the Automatic Line
Insulation Test (ALIT) apparatus.  Most ALIT apparatus, however, checks
for leakage from tip & ring to ground, and NOT leakage from tip to ring.
Under most circumstances, 1 mA is a "reasonably safe" amount of leakage
(corresponding to a 50,000 ohm leak) which should not interfere with
dialing, ringing or supervisory signaling.  ALIT apparatus is generally
programmed for a leak threshold ranging between 50,000 and 150,000 ohms,
depending upon the telephone company practice and condition of the cable
plant.  The primary purpose of ALIT is to give early warning of cable
failures, and not subscriber line apparatus troubles.

	If you are going to design a commercial product, then you are
in trouble because Part 68 will not allow ANY usable amount of leakage
from tip to ring.

In article <1052@amc-gw.amc.com> richm@amc-gw.amc.com (Rich Moran) writes:
> This is fine if you aren't worried about FCC regs.  Part 68 specifies a min.
> 5M-ohm input impedance for DC on tip-ring at voltages below 100V.  Even with
> a 56V battery at the CO, that's less than a milliwatt.

	If the remote monitoring line is an off-premises extension (OPX)
of a customer-owned PABX, then it is exempt from any FCC regulations.  We
do not know exactly what the original poster had in mind, so we'll give
him the benefit of the doubt. :-)

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