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Science Forum Index » Electronics - Equipment Forum » Hum from phone wires running next to mains?
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| Foxtrot |
 Posted: Tue Mar 04, 2008 12:43 pm |
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Guest
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I am in the UK and want to make several phone extensions.
QUESTION: I would like to know I this will increase the level of hum.
ISTR UK phones have a transformer and some other components to
neutralise hum but would that be good enough to prevent hum from a messy
setup like mine? Some details are below.
-------------------------
In my situation the phone extension wires and the mains wires will run
close to one other.
There will be about four or five additional extension phone sockets.
And in some phone sockets there will be a loose extension lead of approx
3 metres which will be almost ontop of curled mains flex
--
max four x-post groups:
uk.telecom a.c.hardware a.e.electrical sci.electronics.equipment |
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| Graham. |
| Posted: Tue Mar 04, 2008 3:13 pm |
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Guest
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"Foxtrot" <foxtrot@demo.invalid> wrote in message
news:Xns9A57AA15ACE8ED712E3@127.0.0.1...
Quote: I am in the UK and want to make several phone extensions.
QUESTION: I would like to know I this will increase the level of hum.
ISTR UK phones have a transformer and some other components to
neutralise hum but would that be good enough to prevent hum from a messy
setup like mine? Some details are below.
-------------------------
In my situation the phone extension wires and the mains wires will run
close to one other.
There will be about four or five additional extension phone sockets.
And in some phone sockets there will be a loose extension lead of approx
3 metres which will be almost ontop of curled mains flex
It is quite difficult to induce hum into telephone wiring.
Use twisted pair cabling rather than the flat ready-made
extension cables.
--
Graham
%Profound_observation% |
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| Guest |
| Posted: Tue Mar 04, 2008 4:42 pm |
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On Tue, 4 Mar 2008 19:13:07 -0000, "Graham." <me@privacy.com> wrote:
Quote:
"Foxtrot" <foxtrot@demo.invalid> wrote in message
news:Xns9A57AA15ACE8ED712E3@127.0.0.1...
I am in the UK and want to make several phone extensions.
QUESTION: I would like to know I this will increase the level of hum.
ISTR UK phones have a transformer and some other components to
neutralise hum but would that be good enough to prevent hum from a messy
setup like mine? Some details are below.
-------------------------
In my situation the phone extension wires and the mains wires will run
close to one other.
There will be about four or five additional extension phone sockets.
And in some phone sockets there will be a loose extension lead of approx
3 metres which will be almost ontop of curled mains flex
It is quite difficult to induce hum into telephone wiring.
Use twisted pair cabling rather than the flat ready-made
extension cables.
Exactly!
The phone company has millions of miles of cable running right below
power lines and hundreds literally touching each other in the jacket
of the cable. That little twist they put in the pairs is excellent in
isolating them from crosstalk.
Just don't use "straight through" door bell wire and you will be fine. |
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| Guest |
| Posted: Tue Mar 04, 2008 5:22 pm |
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In alt.engineering.electrical gfretwell@aol.com wrote:
| On Tue, 4 Mar 2008 19:13:07 -0000, "Graham." <me@privacy.com> wrote:
|>It is quite difficult to induce hum into telephone wiring.
|>Use twisted pair cabling rather than the flat ready-made
|>extension cables.
|
| Exactly!
| The phone company has millions of miles of cable running right below
| power lines and hundreds literally touching each other in the jacket
| of the cable. That little twist they put in the pairs is excellent in
| isolating them from crosstalk.
That twist is a great little means to ensure induced signals, whatever
they may be, are induced in equal amount on both wires, so they do not
contribute to the actual intended signal that is a differential between
those two wires.
However, a risk exists when two different pairs are present next to each
other and each pair is twisted at the same pitch. The signal carried by
one can end up being induced differentially on the other. So don't twist
those power lines, or if you do, twist them at a pitch with a ratio to
the phone line twist that is not a whole number.
CAT5 cable is an example. It has 4 different pairs twisting along. Each
of the pairs has a different twist pitch by design (unless you get some
cheap cable not manufactured correctly).
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-04-1516@ipal.net |
|------------------------------------/-------------------------------------| |
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| Foxtrot |
| Posted: Tue Mar 04, 2008 7:44 pm |
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Guest
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On Tue 04 Mar 2008 21:22:30, <phil-news-nospam@ipal.net> wrote:
Quote: In alt.engineering.electrical gfretwell@aol.com wrote:
| On Tue, 4 Mar 2008 19:13:07 -0000, "Graham." <me@privacy.com
| wrote:
|>It is quite difficult to induce hum into telephone wiring.
|>Use twisted pair cabling rather than the flat ready-made
|>extension cables.
|
| Exactly!
| The phone company has millions of miles of cable running right
| below power lines and hundreds literally touching each other in
| the jacket of the cable. That little twist they put in the pairs
| is excellent in isolating them from crosstalk.
That twist is a great little means to ensure induced signals,
whatever they may be, are induced in equal amount on both wires, so
they do not contribute to the actual intended signal that is a
differential between those two wires.
However, a risk exists when two different pairs are present next to
each other and each pair is twisted at the same pitch. The signal
carried by one can end up being induced differentially on the
other. So don't twist those power lines, or if you do, twist them
at a pitch with a ratio to the phone line twist that is not a whole
number.
CAT5 cable is an example. It has 4 different pairs twisting along.
Each of the pairs has a different twist pitch by design (unless
you get some cheap cable not manufactured correctly).
I do not have any technical knowledge of this area.
I would like to ask about a cable which has two or more twisted pairs
in it.
Is there is a greaterlikelihood of hum if I connect a "2 wire" phone
extension by using one wire from a twisted pair and taking the second
wire from a different twisted pair? |
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| Foxtrot |
| Posted: Tue Mar 04, 2008 7:51 pm |
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Guest
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On Tue 04 Mar 2008 19:13:07, Graham. <me@privacy.com> wrote:
Quote:
"Foxtrot" <foxtrot@demo.invalid> wrote in message
news:Xns9A57AA15ACE8ED712E3@127.0.0.1...
I am in the UK and want to make several phone extensions.
QUESTION: I would like to know I this will increase the level of
hum.
ISTR UK phones have a transformer and some other components to
neutralise hum but would that be good enough to prevent hum from a
messy setup like mine? Some details are below.
-------------------------
In my situation the phone extension wires and the mains wires will
run close to one other.
There will be about four or five additional extension phone
sockets.
And in some phone sockets there will be a loose extension lead of
approx 3 metres which will be almost ontop of curled mains flex
It is quite difficult to induce hum into telephone wiring.
Use twisted pair cabling rather than the flat ready-made
extension cables.
(As you suggest, I will not get the flat ready made extension cable
which I guess is made from flexible multi-stranded wires.)
Is the sort of cable sold in the UK specifically for domestic
telephone wall sockets (wuth single stranded wires) usually made up
as "twisted pair" in the way you are recommending? |
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| Ivor Jones |
| Posted: Tue Mar 04, 2008 7:55 pm |
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"Foxtrot" <foxtrot@demo.invalid> wrote in message
news:Xns9A57F1761E171D712E3@127.0.0.1
[snip]
: : I do not have any technical knowledge of this area.
: :
: : I would like to ask about a cable which has two or more
: : twisted pairs in it.
: :
: : Is there is a greaterlikelihood of hum if I connect a
: : "2 wire" phone extension by using one wire from a
: : twisted pair and taking the second wire from a
: : different twisted pair?
Why would you want to do that..? The answer is very probably, so ensure
that the pair of wires you use are twisted *together*..!
Ivor |
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| Floyd L. Davidson |
| Posted: Tue Mar 04, 2008 8:33 pm |
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Guest
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Foxtrot <foxtrot@demo.invalid> wrote:
Quote: On Tue 04 Mar 2008 21:22:30, <phil-news-nospam@ipal.net> wrote:
That twist is a great little means to ensure induced signals,
whatever they may be, are induced in equal amount on both wires, so
they do not contribute to the actual intended signal that is a
differential between those two wires.
However, a risk exists when two different pairs are present next to
each other and each pair is twisted at the same pitch. The signal
carried by one can end up being induced differentially on the
other. So don't twist those power lines, or if you do, twist them
at a pitch with a ratio to the phone line twist that is not a whole
number.
Twist the power lines all you like. You *can't*
physically twist them identically to that of a comm
cable *and* get the two pairs to snuggle up to each
other in a way that will create the problem described
above.
Regardless, it isn't "smart" to run a comm cable in
physical contact with power cabling. Even a couple of
inches separation is sufficient to significantly reduce
common mode coupling. And the fact that no hum is heard
when it is first installed is *not* sufficient reason to
accept such practice. The common mode voltage induced
on the comm cable may not be a problem at any given
time, but it means that in the future anything (such as
kinks in the cable, dampness, damaged insulation, etc)
that reduces the balance *will* cause excessive hum.
The higher the common mode induced voltage, the less
unbalance required to cause objectionable hum.
If you can avoid putting the two types of cable
together, you *should*.
Quote: CAT5 cable is an example. It has 4 different pairs twisting along.
Each of the pairs has a different twist pitch by design (unless
you get some cheap cable not manufactured correctly).
All multipair twisted-pair cable uses different twists
for each pair. That is identically true for bundled
pairs in telephone cable. Moreover, if there are
multiple bundles the bundles are swirled within the
jacket too.
Quote: I do not have any technical knowledge of this area.
I would like to ask about a cable which has two or more twisted pairs
in it.
Is there is a greaterlikelihood of hum if I connect a "2 wire" phone
extension by using one wire from a twisted pair and taking the second
wire from a different twisted pair?
That is referred to as a "split pair", and yes it will
cause problems. It commonly happens with CAT5 cabling
due to the different standards for pin assignments for a
DS1 interface and for 10BaseT Ethernet. Typically a DS1
cable will work for Ethernet if the length is short, but
if used for faster than 10baseT, it won't work at all,
even for a 6 foot jumper cable.
On large telephone cables split pairs invariably have
significant crosstalk (either hum or speech from other
cables).
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@apaflo.com |
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| CBFalconer |
| Posted: Tue Mar 04, 2008 8:35 pm |
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Foxtrot wrote:
Quote:
.... snip ...
Is there is a greaterlikelihood of hum if I connect a "2 wire"
phone extension by using one wire from a twisted pair and taking
the second wire from a different twisted pair?
Yes. The idea of twisted pairs is that an interference appears on
both lines, and thus tends to cancel itself. Separating the lines
makes it easy for unequal induction.
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.
--
Posted via a free Usenet account from http://www.teranews.com |
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| kony |
| Posted: Tue Mar 04, 2008 9:04 pm |
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Guest
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On Tue, 04 Mar 2008 23:44:11 GMT, Foxtrot
<foxtrot@demo.invalid> wrote:
Quote: Is there is a greaterlikelihood of hum if I connect a "2 wire" phone
extension by using one wire from a twisted pair and taking the second
wire from a different twisted pair?
yes |
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| Tom Horne |
| Posted: Tue Mar 04, 2008 10:53 pm |
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Guest
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Foxtrot wrote:
Quote: I am in the UK and want to make several phone extensions.
QUESTION: I would like to know I this will increase the level of hum.
ISTR UK phones have a transformer and some other components to
neutralise hum but would that be good enough to prevent hum from a messy
setup like mine? Some details are below.
-------------------------
In my situation the phone extension wires and the mains wires will run
close to one other.
There will be about four or five additional extension phone sockets.
And in some phone sockets there will be a loose extension lead of approx
3 metres which will be almost ontop of curled mains flex
Much will depend on the power and light wiring method used and the
quality of the cable used to carry your telephone circuits. What do you
mean when you say "flex". I suspect you'll be amused to learn that in
the USA that word is electricians short hand for flexible metallic
conduit. I doubt that United Kingdom "flex" is anything like Flexible
Metallic Conduit a photograph of which can be found at
<http://www.tradexpro.com/product_catalogs/machinery_electronics/cable_wire/in-commerce/product/picture_fullsize/flexible_metallic_conduit.html?env=img-106372-->.
The best way to reduce the amount of noise in telephone lines is to use
station cable that has the wire pairs continuously twisted around each
other. In this way any electro magnetic fields that might otherwise
induce an unwanted noise into the circuit is self canceling in the
twisted pair of wires. Even with good quality station cable best
practice is to maintain at least several inches of separation between
the telephone cables and the electrical power and light wiring.
--
Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison |
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| Tom Horne |
| Posted: Tue Mar 04, 2008 10:58 pm |
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Guest
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Foxtrot wrote:
Quote: On Tue 04 Mar 2008 21:22:30, <phil-news-nospam@ipal.net> wrote:
In alt.engineering.electrical gfretwell@aol.com wrote:
| On Tue, 4 Mar 2008 19:13:07 -0000, "Graham." <me@privacy.com
| wrote:
|>It is quite difficult to induce hum into telephone wiring.
|>Use twisted pair cabling rather than the flat ready-made
|>extension cables.
|
| Exactly!
| The phone company has millions of miles of cable running right
| below power lines and hundreds literally touching each other in
| the jacket of the cable. That little twist they put in the pairs
| is excellent in isolating them from crosstalk.
That twist is a great little means to ensure induced signals,
whatever they may be, are induced in equal amount on both wires, so
they do not contribute to the actual intended signal that is a
differential between those two wires.
However, a risk exists when two different pairs are present next to
each other and each pair is twisted at the same pitch. The signal
carried by one can end up being induced differentially on the
other. So don't twist those power lines, or if you do, twist them
at a pitch with a ratio to the phone line twist that is not a whole
number.
CAT5 cable is an example. It has 4 different pairs twisting along.
Each of the pairs has a different twist pitch by design (unless
you get some cheap cable not manufactured correctly).
I do not have any technical knowledge of this area.
I would like to ask about a cable which has two or more twisted pairs
in it.
Is there is a greaterlikelihood of hum if I connect a "2 wire" phone
extension by using one wire from a twisted pair and taking the second
wire from a different twisted pair?
That practice is known in the North American communications industry as
a split pair. It is usually the cause of a host of troubles of which
induced noise is only the most common.
--
Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison |
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| jasee |
| Posted: Wed Mar 05, 2008 4:40 am |
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Guest
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Foxtrot wrote:
Quote: On Tue 04 Mar 2008 19:13:07, Graham. <me@privacy.com> wrote:
"Foxtrot" <foxtrot@demo.invalid> wrote in message
news:Xns9A57AA15ACE8ED712E3@127.0.0.1...
In my situation the phone extension wires and the mains wires will
run close to one other.
There will be about four or five additional extension phone
sockets.
And in some phone sockets there will be a loose extension lead of
approx 3 metres which will be almost ontop of curled mains flex
It is quite difficult to induce hum into telephone wiring.
Use twisted pair cabling rather than the flat ready-made
extension cables.
(As you suggest, I will not get the flat ready made extension cable
which I guess is made from flexible multi-stranded wires.)
Is the sort of cable sold in the UK specifically for domestic
telephone wall sockets (wuth single stranded wires) usually made up
as "twisted pair" in the way you are recommending?
No
I don't know why it is still used as most people in the UK seem to have adsl
connections nowadays (not just phones) |
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| charles |
| Posted: Wed Mar 05, 2008 5:25 am |
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Guest
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In article <fqmbgd0kd7@news3.newsguy.com>,
<phil-news-nospam@ipal.net> wrote:
Quote: I used to see phones rated in terms of their "ringer equivalence" here in
the USA. These numbers were, for some phones, as low as 0.2. I do not
recall ever seeing one about 0.9. That would suggest to me that you could
readily have more than 4 phones on such a phone circuit. I never had any
reason to actually do a scientific test of this.
There is obviously a significant difference in the phone systems in the two
countries. Our phones have the bells in parallel and if thee are too many
the wrong impedance is presented to the exchange, and no ringing voltage
will get sent. I have never seen a UK approved phone with a REN less than
1, but there were plenty of 2s & 3s about at one time.
You can buy a REN booster ( a mains powered device) which allows many more
phones.
--
From KT24 - in "Leafy Surrey"
Using a RISC OS computer running v5.11 |
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| George |
| Posted: Wed Mar 05, 2008 7:40 am |
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Guest
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On Tue, 04 Mar 2008 21:22:30 +0000, phil-news-nospam wrote:
Quote: In alt.engineering.electrical gfretwell@aol.com wrote:
| On Tue, 4 Mar 2008 19:13:07 -0000, "Graham." <me@privacy.com> wrote:
|>It is quite difficult to induce hum into telephone wiring. Use twisted
|>pair cabling rather than the flat ready-made extension cables.
|
| Exactly!
| The phone company has millions of miles of cable running right below
| power lines and hundreds literally touching each other in the jacket of
| the cable. That little twist they put in the pairs is excellent in
| isolating them from crosstalk.
That twist is a great little means to ensure induced signals, whatever
they may be, are induced in equal amount on both wires, so they do not
contribute to the actual intended signal that is a differential between
those two wires.
However, a risk exists when two different pairs are present next to each
other and each pair is twisted at the same pitch. The signal carried by
one can end up being induced differentially on the other. So don't twist
those power lines, or if you do, twist them at a pitch with a ratio to the
phone line twist that is not a whole number.
CAT5 cable is an example. It has 4 different pairs twisting along. Each
of the pairs has a different twist pitch by design (unless you get some
cheap cable not manufactured correctly).
Very interesting.
Please can you advise on how these twisted pairs compare with
1. shielded audio cable
and
2. rf coax.
In case 1 both the wanted signal and the noise are in the audio frequency
range.
In case 2 the electricity supply noise contains harmonics of similar
frequency to the wanted rf signal. |
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