Telephone Lines and Ground Loops

[Ron Hardin]

My telephone line developed an AC hum, which, thinking about it, left
me curious how ground loops are avoided.

If you drive two ground stakes into the ground about 20 feet apart,
you can measure about a quarter volt of AC between them. There's
AC floating around everywhere, and no two pieces of ground are the
same potential.

The telephone company would then seem to have an unsolveable AC hum
problem, and I wondered how they got around it.

They do ground the phone line boxes every couple hundred feet or so,
though I don't know if that affects the twisted pair directly; but
certainly they have the trouble at the miles-distant termination of
the line and your house, as to what potential the ground is at the
two places.

It can't be just isolation transformers, because they need the DC
circuit to detect off-hook, not to mention powering the phone on
your desk, if it's got any IC's.
--
Ron Hardin


On the internet, nobody knows you're a jerk.

[David Ballinger]

"Ron Hardin" > wrote in message
...
> My telephone line developed an AC hum, which, thinking about it, left
> me curious how ground loops are avoided.
>
> If you drive two ground stakes into the ground about 20 feet apart,
> you can measure about a quarter volt of AC between them. There's
> AC floating around everywhere, and no two pieces of ground are the
> same potential.
>
> The telephone company would then seem to have an unsolveable AC hum
> problem, and I wondered how they got around it.
>
> They do ground the phone line boxes every couple hundred feet or so,
> though I don't know if that affects the twisted pair directly; but
> certainly they have the trouble at the miles-distant termination of
> the line and your house, as to what potential the ground is at the
> two places.
>
> It can't be just isolation transformers, because they need the DC
> circuit to detect off-hook, not to mention powering the phone on
> your desk, if it's got any IC's.
> --
> Ron Hardin
>
>
> On the internet, nobody knows you're a jerk.

The answer to the unsolvable hum problem is "The Balanced Pair".
the hum exists on both sides of the line as long as the resistance each side
of the line is the same and the capacitance to ground equal; the hum cancels
itself out between the two sides of the line. If you listen from one side or
the other to ground they both hum. The signal you want to hear is between
the two sides of the pair. The only place that the circuit is actually
connected to ground is in the central office at the positive side of the
battery 48 volts or in these modern times,out in your neighborhood Demux
point. That little box on the back of the house or office that is grounded
only has a spark gap and a fuse in it for your protection, sometimes they
fail and cause the imbalance on the pair, usually the first thing the
telephone repairman checks after they hear the hum. You wouldn't believe the
stray voltages that can be induced on a pair of wires that run parallel to
HT power lines, both static and dynamic, real current flows!
Anyway, that is why telephone lines Tip & Ring are balanced instead of
unbalanced, if they could have done it on one wire $$$ they would have,
half the copper.
Now Fiber-Optics, that is another story.
Hope that helps,
Dave___________

[Ron Hardin]

David Ballinger wrote:
>
> "Ron Hardin" > wrote in message
> ..
> > My telephone line developed an AC hum, which, thinking about it, left
> > me curious how ground loops are avoided.
> >
> > If you drive two ground stakes into the ground about 20 feet apart,
> > you can measure about a quarter volt of AC between them. There's
> > AC floating around everywhere, and no two pieces of ground are the
> > same potential.
> >
> > The telephone company would then seem to have an unsolveable AC hum
> > problem, and I wondered how they got around it.
> >
> > They do ground the phone line boxes every couple hundred feet or so,
> > though I don't know if that affects the twisted pair directly; but
> > certainly they have the trouble at the miles-distant termination of
> > the line and your house, as to what potential the ground is at the
> > two places.
> >
> > It can't be just isolation transformers, because they need the DC
> > circuit to detect off-hook, not to mention powering the phone on
> > your desk, if it's got any IC's.
> > --
> > Ron Hardin
> >
> >
> > On the internet, nobody knows you're a jerk.
>
> The answer to the unsolvable hum problem is "The Balanced Pair".
> the hum exists on both sides of the line as long as the resistance each side
> of the line is the same and the capacitance to ground equal; the hum cancels
> itself out between the two sides of the line. If you listen from one side or
> the other to ground they both hum. The signal you want to hear is between
> the two sides of the pair. The only place that the circuit is actually
> connected to ground is in the central office at the positive side of the
> battery 48 volts or in these modern times,out in your neighborhood Demux
> point. That little box on the back of the house or office that is grounded
> only has a spark gap and a fuse in it for your protection, sometimes they
> fail and cause the imbalance on the pair, usually the first thing the
> telephone repairman checks after they hear the hum. You wouldn't believe the
> stray voltages that can be induced on a pair of wires that run parallel to
> HT power lines, both static and dynamic, real current flows!
> Anyway, that is why telephone lines Tip & Ring are balanced instead of
> unbalanced, if they could have done it on one wire $$$ they would have,
> half the copper.
> Now Fiber-Optics, that is another story.
> Hope that helps,
> Dave___________

Thanks. That's fairly scary, that my computer modem is at the end of a 5 mile
wire grounded only at the far end. Quite an antenna, and its own chassis ground
rignt nearby.

I'd assumed they'd have to be grounded at my end too, hence the ground loop.

--
Ron Hardin


On the internet, nobody knows you're a jerk.

[David Ballinger]

"Ron Hardin" > wrote in message
...
> David Ballinger wrote:
>>
>> "Ron Hardin" > wrote in message
>> ..
>> > My telephone line developed an AC hum, which, thinking about it, left
>> > me curious how ground loops are avoided.
>> >
>> > If you drive two ground stakes into the ground about 20 feet apart,
>> > you can measure about a quarter volt of AC between them. There's
>> > AC floating around everywhere, and no two pieces of ground are the
>> > same potential.
>> >
>> > The telephone company would then seem to have an unsolveable AC hum
>> > problem, and I wondered how they got around it.
>> >
>> > They do ground the phone line boxes every couple hundred feet or so,
>> > though I don't know if that affects the twisted pair directly; but
>> > certainly they have the trouble at the miles-distant termination of
>> > the line and your house, as to what potential the ground is at the
>> > two places.
>> >
>> > It can't be just isolation transformers, because they need the DC
>> > circuit to detect off-hook, not to mention powering the phone on
>> > your desk, if it's got any IC's.
>> > --
>> > Ron Hardin
>> >
>> >
>> > On the internet, nobody knows you're a jerk.
>>
>> The answer to the unsolvable hum problem is "The Balanced Pair".
>> the hum exists on both sides of the line as long as the resistance each
>> side
>> of the line is the same and the capacitance to ground equal; the hum
>> cancels
>> itself out between the two sides of the line. If you listen from one side
>> or
>> the other to ground they both hum. The signal you want to hear is between
>> the two sides of the pair. The only place that the circuit is actually
>> connected to ground is in the central office at the positive side of the
>> battery 48 volts or in these modern times,out in your neighborhood Demux
>> point. That little box on the back of the house or office that is
>> grounded
>> only has a spark gap and a fuse in it for your protection, sometimes they
>> fail and cause the imbalance on the pair, usually the first thing the
>> telephone repairman checks after they hear the hum. You wouldn't believe
>> the
>> stray voltages that can be induced on a pair of wires that run parallel
>> to
>> HT power lines, both static and dynamic, real current flows!
>> Anyway, that is why telephone lines Tip & Ring are balanced instead of
>> unbalanced, if they could have done it on one wire $$$ they would have,
>> half the copper.
>> Now Fiber-Optics, that is another story.
>> Hope that helps,
>> Dave___________
>
> Thanks. That's fairly scary, that my computer modem is at the end of a 5
> mile
> wire grounded only at the far end. Quite an antenna, and its own chassis
> ground
> rignt nearby.
>
> I'd assumed they'd have to be grounded at my end too, hence the ground
> loop.
>
> --
> Ron Hardin


Yes it is, now if you think about it, out on those poles going down the
road, the power wires are on top -- usually a ground wire with the hot wire
and or three phase underneath, then a little closer to earth the telephone
then the cable TV and if you live in town maybe traffic signal wires or
intersection video wiring. If lightening does hit the top wire, it is
usually a large spike you get at your house on the power side -- but your
modem line does make a nice return to ground. I believe in surge
protectors! Now, if you do get a close strike, figure where you are in the
voltage divider circuit.
Really close to home, nothing survives, very close to nuclear, distance is
the key to equipment survival. Best bet unplug until after the storm
passes, Bet #2 Insurance, Bet #3 save the guarantee that comes with a really
good surge protector, the one with the $250,000 policy -- read the fine
print & save the receipts for everything, back up your data, at least the
important stuff, live a good life and pray. On the other hand, you may be
ready for a new computer anyway, bigger, better, faster, cheaper,,, well
they don't get cheaper, but you do get a lot more features and speed.
You don't know what you'll miss, till it's gone - - - -
Dave____________

[Eeyore]

Ron Hardin wrote:

> My telephone line developed an AC hum, which, thinking about it, left
> me curious how ground loops are avoided.
>
> If you drive two ground stakes into the ground about 20 feet apart,
> you can measure about a quarter volt of AC between them. There's
> AC floating around everywhere, and no two pieces of ground are the
> same potential.

Correct.


> The telephone company would then seem to have an unsolveable AC hum
> problem,

Why do you think that ? They don't use ground at the consumer end of the line.


> and I wondered how they got around it.

By using balanced connections they avoid hum (and other) pickup..


> They do ground the phone line boxes every couple hundred feet or so,
> though I don't know if that affects the twisted pair directly;

It doesn't. If you grounded one side of the phone cable regularly it *would* hum
since it wouldn't be balanced any more.


> but certainly they have the trouble at the miles-distant termination of
> the line and your house, as to what potential the ground is at the
> two places.

That's why they don't use ground. It becomes irrelevant.


> It can't be just isolation transformers, because they need the DC
> circuit to detect off-hook, not to mention powering the phone on
> your desk, if it's got any IC's.

You need to learn how balanced circuits work. Note that balanced circuits don't
require a 'balanced signal' i.e. a signal in both 'phases', only the line
impedance needs to be balanced.
http://en.wikipedia.org/wiki/Balanced

Graham

[Eeyore]

Ron Hardin wrote:

> I'd assumed they'd have to be grounded at my end too

That would hum crazily.

Graham

[GregS]

In article >, Ron Hardin > wrote:
>My telephone line developed an AC hum, which, thinking about it, left
>me curious how ground loops are avoided.
>
>If you drive two ground stakes into the ground about 20 feet apart,
>you can measure about a quarter volt of AC between them. There's
>AC floating around everywhere, and no two pieces of ground are the
>same potential.
>
>The telephone company would then seem to have an unsolveable AC hum
>problem, and I wondered how they got around it.
>
>They do ground the phone line boxes every couple hundred feet or so,
>though I don't know if that affects the twisted pair directly; but
>certainly they have the trouble at the miles-distant termination of
>the line and your house, as to what potential the ground is at the
>two places.
>
>It can't be just isolation transformers, because they need the DC
>circuit to detect off-hook, not to mention powering the phone on
>your desk, if it's got any IC's.

One telephone I had that had a wall wart developed problems. The wall wart
developed a high leakage current causing hum on the line. Replacing it
solved the hum problem.

greg

[Ron Hardin]

Eeyore wrote:
> > They do ground the phone line boxes every couple hundred feet or so,
> > though I don't know if that affects the twisted pair directly;
>
> It doesn't. If you grounded one side of the phone cable regularly it *would* hum
> since it wouldn't be balanced any more.

The telephone guy said they had grounds in every box out back, that serves as
a terminal strip to drop off lines to a couple of houses, every couple of houses.

(The whole system is underground.)

What they grounded, he didn't say. Maybe it was a ground for pointy-electrode
lightning arrestors, I don't know.

What being underground doesn't avoid is huge induced voltages from lightning
strikes.

Apparently in one of the inter-box segments my balanced pair went unbalanced,
maybe water leakage completing a circuit to ground; switching me to another
pair fixed it. Now I apparently know more about it than the telephone guy,
who seemed a little weak on theory, though very good in practical matters.

I lost a modem once to a very distant lightning strike, when I had the drop
line wound around a ferrite torroid to prevent RFI from getting into the house
from the neighbors' dimmers, and the telephone line was a major source ; but
that made the lightning effect very much worse. So I took that off when I
replaced the modem, and went to an outdoor antenna system to fix the RFI instead.
--
Ron Hardin


On the internet, nobody knows you're a jerk.

[Eeyore]

GregS wrote:

> One telephone I had that had a wall wart developed problems. The wall wart
> developed a high leakage current causing hum on the line. Replacing it
> solved the hum problem.

That figures. It was becoming an (unwanted) parasitic ground connection.

Graham

[Eeyore]

Ron Hardin wrote:

> Eeyore wrote:
> > > They do ground the phone line boxes every couple hundred feet or so,
> > > though I don't know if that affects the twisted pair directly;
> >
> > It doesn't. If you grounded one side of the phone cable regularly it *would* hum
> > since it wouldn't be balanced any more.
>
> The telephone guy said they had grounds in every box out back, that serves as
> a terminal strip to drop off lines to a couple of houses, every couple of houses.
>
> (The whole system is underground.)
>
> What they grounded, he didn't say. Maybe it was a ground for pointy-electrode
> lightning arrestors, I don't know.

That sounds more like it. The principle of using balanced connections for telephony
haven't changed in what ? over 100 years ?

Graham