Max Current for our POTS lines

Max Current for our POTS lines

Post by John Lundgre » Tue, 23 Jun 1998 04:00:00



I went to our new building downtown last week and measured the line
current on four of the POTS lines.  This building is a block from the
central office, so the lines are no more than a few hundred feet long. I
measured around 55 mA, some slightly less and some slightly more.
Pretty much the same.

Were our main campus is, it's about 2 miles from downtown, so the line
current is around half that much, or 27 mA.  I thought that the T-1
circuits had to have an amplifier every 6 kFt, but apparently ours
don't.  Our new T-1 circuit comes right out of the LightSpan 2000 pair
gain muxer.  So it drops off a fiber from the CO that feeds the
LightSpan at T-3 speed.

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Max Current for our POTS lines

Post by John Lundgre » Tue, 23 Jun 1998 04:00:00




> >I went to our new building downtown last week and measured the line
> >current on four of the POTS lines.  This building is a block from the
> >central office, so the lines are no more than a few hundred feet long. I
> >measured around 55 mA, some slightly less and some slightly more.
> >Pretty much the same.
> Did you measure that current with, or without a phone in the loop ? That
> sounds more like what you might see with no phone.

OOps, I forgot to mention that I used a DMM on the current range.  Yeah,
just basically the DMM leads across the terminals, shorting the line.

Quote:> >Were our main campus is, it's about 2 miles from downtown, so the line
> >current is around half that much, or 27 mA.
> Sounds about right. 2 miles of cable would add about 400ohms to the
> loop.
>  I thought that the T-1
> >circuits had to have an amplifier every 6 kFt, but apparently ours
> >don't.  Our new T-1 circuit comes right out of the LightSpan 2000 pair
> >gain muxer.  So it drops off a fiber from the CO that feeds the
> >LightSpan at T-3 speed.
> ---

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Max Current for our POTS lines

Post by John Lundgre » Tue, 23 Jun 1998 04:00:00





> > I went to our new building downtown last week and measured the line
> > current on four of the POTS lines.  This building is a block from the
> > central office, so the lines are no more than a few hundred feet long. I
> > measured around 55 mA, some slightly less and some slightly more.
> > Pretty much the same.
> > Were our main campus is, it's about 2 miles from downtown, so the line
> > current is around half that much, or 27 mA.  I thought that the T-1
> > circuits had to have an amplifier every 6 kFt, but apparently ours
> > don't.  Our new T-1 circuit comes right out of the LightSpan 2000 pair
> > gain muxer.  So it drops off a fiber from the CO that feeds the
> > LightSpan at T-3 speed.

> > --
> You might want to seriously concider adding current limiting on those lines
> current over 35 mills is a potental source of trouble, with relay contacts,
> hook switches, etc. dr.d

No such problem.  Our PBX handles the incoming CO lines.

And it's under maintenance contract.  :-)

The diff between 27 and 55 mA isn't enough diff to cause any shortening
of the lifespan of a relay.

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Max Current for our POTS lines

Post by John Ri » Wed, 24 Jun 1998 04:00:00



>I went to our new building downtown last week and measured the line
>current on four of the POTS lines.  This building is a block from the
>central office, so the lines are no more than a few hundred feet long. I
>measured around 55 mA, some slightly less and some slightly more.
>Pretty much the same.

Did you measure that current with, or without a phone in the loop ? That
sounds more like what you might see with no phone.

Quote:>Were our main campus is, it's about 2 miles from downtown, so the line
>current is around half that much, or 27 mA.

Sounds about right. 2 miles of cable would add about 400ohms to the
loop.

 I thought that the T-1

Quote:>circuits had to have an amplifier every 6 kFt, but apparently ours
>don't.  Our new T-1 circuit comes right out of the LightSpan 2000 pair
>gain muxer.  So it drops off a fiber from the CO that feeds the
>LightSpan at T-3 speed.

---                      

John Rice   __|__    K9IJ  | "I speak for myself, not my employer".  
     ________(*)________   |
            o/ \o          | Living under a bridge on the Information  

 
 
 

Max Current for our POTS lines

Post by John Lundgre » Wed, 24 Jun 1998 04:00:00


[snip]

Quote:> > > You might want to seriously concider adding current limiting on those lines
> > > current over 35 mills is a potental source of trouble, with relay contacts,
> > > hook switches, etc. dr.d

> > No such problem.  Our PBX handles the incoming CO lines.
> > And it's under maintenance contract.  :-)
> > The diff between 27 and 55 mA isn't enough diff to cause any shortening
> > of the lifespan of a relay.
> > --
> I beg to differ I've seen  and had to service relays, hook switches and
> selector switches that excessive loop current fried, whall not as common in
> relays, it is still a cause of many mystery problems in C.O. circuits, having
> a maintance contract is well and good but for a small additonal cost you can
> prevent unesssary down time and frustration, of course there is the
> possablity that your switch has a currrent limiting circuit as an intrigral
> part of the C.O. circuits, if so you can count you self truly blessed. Most
> manufacturers dont bother. dr.d

The current in the CO side is nothing to the digital lines on our PBX.
I can draw an arc from them!  I measured the current for a few seconds,
and it was somewhere around an amp and a half!  Then the current
limiting took over and it fell to a few milliamps.  With a 300 ohm
resistor across the line it will continuously develop over 140 mA!  The
5 watt resistor I used got real hot!  No problem powering all those
buttons and LEDs on the sets.

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Max Current for our POTS lines

Post by John Lundgre » Fri, 26 Jun 1998 04:00:00



> > > > > You might want to seriously concider adding current limiting on those lines
> > > > > current over 35 mills is a potental source of trouble, with relay contacts,
> > > > > hook switches, etc. dr.d
> > > > No such problem.  Our PBX handles the incoming CO lines.
> > > > And it's under maintenance contract.  :-)
> > > > The diff between 27 and 55 mA isn't enough diff to cause any shortening
> > > > of the lifespan of a relay.
> > > > --
> > > I beg to differ I've seen  and had to service relays, hook switches and
> > > selector switches that excessive loop current fried, whall not as common in
> > > relays, it is still a cause of many mystery problems in C.O. circuits, having
> > > a maintance contract is well and good but for a small additonal cost you can
> > > prevent unesssary down time and frustration, of course there is the
> > > possablity that your switch has a currrent limiting circuit as an intrigral
> > > part of the C.O. circuits, if so you can count you self truly blessed. Most
> > > manufacturers dont bother. dr.d
> > The current in the CO side is nothing to the digital lines on our PBX.
> > I can draw an arc from them!  I measured the current for a few seconds,
> > and it was somewhere around an amp and a half!  Then the current
> > limiting took over and it fell to a few milliamps.  With a 300 ohm
> > resistor across the line it will continuously develop over 140 mA!  The
> > 5 watt resistor I used got real hot!  No problem powering all those
> > buttons and LEDs on the sets.
> But the above statment is refering to the station side of the PBX not the
> C.O. side, yes the digital sets draw a signifigant amount of current ...from
> the PBX not the Telco, I stand by my statment, at the top. dr.d

I had one telephone maker tell me that anything _under_ 35 mA was
considered low current.  The Art of Electronics, by Horowitz and Hill,
has a graph, I believe from the Bell Labs, which gives the permissible
voltages and currents for a POTS line.  It also shows the areas where
currents are NOT allowed.  Some of these areas are on the _low_ end of
the graph.  In other words the low currents are not allowed because they
resemble leakage currents and may cause the CO to go off hook.   On the
high end, there are areas where the current is limited by the CO
equipment, so the line should not go over this.  So, assuming that these
Bell System maximum currents are taken into account when the phone
instruments were designed and built, then there should not be any
problem with excessive current.

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Max Current for our POTS lines

Post by John Lundgre » Mon, 29 Jun 1998 04:00:00






> > > > > > > You might want to seriously concider adding current limiting on those lines
> > > > > > > current over 35 mills is a potental source of trouble, with relay contacts,
> > > > > > > hook switches, etc. dr.d
> > > > > > No such problem.  Our PBX handles the incoming CO lines.
> > > > > > And it's under maintenance contract.  :-)
> > > > > > The diff between 27 and 55 mA isn't enough diff to cause any shortening
> > > > > > of the lifespan of a relay.
> > > > > > --
> > > > > I beg to differ I've seen  and had to service relays, hook switches and
> > > > > selector switches that excessive loop current fried, whall not as common in
> > > > > relays, it is still a cause of many mystery problems in C.O. circuits, having
> > > > > a maintance contract is well and good but for a small additonal cost you can
> > > > > prevent unesssary down time and frustration, of course there is the
> > > > > possablity that your switch has a currrent limiting circuit as an intrigral
> > > > > part of the C.O. circuits, if so you can count you self truly blessed. Most
> > > > > manufacturers dont bother. dr.d

[snip]

Quote:> > considered low current.  The Art of Electronics, by Horowitz and Hill,
> > has a graph, I believe from the Bell Labs, which gives the permissible
> > voltages and currents for a POTS line.  It also shows the areas where
> > currents are NOT allowed.  Some of these areas are on the _low_ end of
> > the graph.  In other words the low currents are not allowed because they
> > resemble leakage currents and may cause the CO to go off hook.   On the
> > high end, there are areas where the current is limited by the CO
> > equipment, so the line should not go over this.  So, assuming that these
> > Bell System maximum currents are taken into account when the phone
> > instruments were designed and built, then there should not be any
> > problem with excessive current.
> > --
> > I grant you that low current can be a source of problems, it however is not common, the FCC, and Belcore standards require a minimum of 22 ma be supplier to the terminal equipment, this is based on several conventions that the standard, battery feed resistance is 400ohms, that the battery is 48 volts, that the line resistance is 1500 ohms or less, that the terminal resistance is 200 ohms. Obviously there is some vareation possable especaly with loop legnth/resistance

Fix your long line problem.  This above paragraph was on ONE single
line!  Limit it to 80 characters.

Quote:> and feed voltage, however keep in mind there is no mandated loop current
> *maximum*.

You, yourself just specified, above, that there was a maximum.  You said
48 volts divided by the sum of 400 ohms plus 200 ohms.  That gives 80
mA.  Don't try to weasel out of that one!

Quote:> As long as they deliver a minimum of 22ma their job is done, if
> your equipment is damaged it's not their problem. If you have a C.O. that
> will limit the current great, a lot of us don't. I've messured loop current
> of 54 ma with a phone in the loop. As far as low current resembling *leekage*
> this would be a senereo where a marginal loop current will cause unreliable
> opperation, hence the minimum of 22ma. Personaly I like to see a solid 27 ma
> minimum in the loop.

Then by your judgment all of our lines are marginal.  Our location has
lines that get a consistent 27 mA from the CO at the MPOE.  Maybe
slightly less when the currents travel thru the lines across our campus.

Quote:> Unfortunitly telephones and other terminal equipment are
> often designed by engineers who have no idea what the REAL world is like and
> got their paramiters from a book. I personaly think that any one who is going
> for an engineering degree should be required to work in the real world  for
> two years fixing other engineers junk befor they get their deploma. dr.d

Maybe one should get an engineering degree before criticizing engineers
for their inexperience with the real world.  Walk a mile in their
shoes.  That way, when you criticise them, you'll be a mile away, and
have their shoes.  ;-)

There's nothing wrong with parameters from a book, provided they mirror
the real world.  Unfortunately some of the parameters have to do with
"engineering economics" that, when emphasized, often cause shortcuts to
be taken.

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Max Current for our POTS lines

Post by John » Sat, 04 Jul 1998 04:00:00


You want a minimum of 20ma. Other than that a POTS line doesn't care. In
todays  day and age however I am surprised that you go some 50ma + on a POTS
line since they are usually regulated.

--
John

http://www.kiana.net


>I went to our new building downtown last week and measured the line
>current on four of the POTS lines.  This building is a block from the
>central office, so the lines are no more than a few hundred feet long. I
>measured around 55 mA, some slightly less and some slightly more.
>Pretty much the same.

>Were our main campus is, it's about 2 miles from downtown, so the line
>current is around half that much, or 27 mA.  I thought that the T-1
>circuits had to have an amplifier every 6 kFt, but apparently ours
>don't.  Our new T-1 circuit comes right out of the LightSpan 2000 pair
>gain muxer.  So it drops off a fiber from the CO that feeds the
>LightSpan at T-3 speed.

>--
>Please remove NO and SPAM from my email addr to reply.
>--







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Max Current for our POTS lines

Post by John » Mon, 06 Jul 1998 04:00:00


Nothing at all obvious about that other than maybe the regulation is
notworking properly. Even a dead short is usually regulated better than
that!



>> You want a minimum of 20ma. Other than that a POTS line doesn't care. In
>> todays  day and age however I am surprised that you go some 50ma + on a
POTS
>> line since they are usually regulated.
>Obviously the reason that the POTS line has 55 mA is that it's only a
>block from the CO.

--
John

http://www.kiana.net
 
 
 

Max Current for our POTS lines

Post by Alexjern » Tue, 07 Jul 1998 04:00:00


Are you sure that wasn't 50V he was measuring?

 
 
 

1. Max Current for our POTS lines

I went to our new building downtown last week and measured the line
current on four of the POTS lines.  This building is a block from the
central office, so the lines are no more than a few hundred feet long. I
measured around 55 mA, some slightly less and some slightly more.
Pretty much the same.

Were our main campus is, it's about 2 miles from downtown, so the line
current is around half that much, or 27 mA.  I thought that the T-1
circuits had to have an amplifier every 6 kFt, but apparently ours
don't.  Our new T-1 circuit comes right out of the LightSpan 2000 pair
gain muxer.  So it drops off a fiber from the CO that feeds the
LightSpan at T-3 speed.

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