Hum from phone wires running next to mains?

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.
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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
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uk.telecom a.c.hardware a.e.electrical sci.electronics.equipment
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It is quite difficult to induce hum into telephone wiring. Use twisted pair cabling rather than the flat ready-made extension cables.
--
Graham

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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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In alt.engineering.electrical snipped-for-privacy@aol.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).
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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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[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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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*.

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.

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).
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Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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On Tue, 04 Mar 2008 23:44:11 GMT, Foxtrot

yes
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Foxtrot wrote:

... snip ...

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.
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[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net
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says...

Twisting also makes the loop area low (average over a long stretch is nil). Separating them makes a large loop, increasing the size of the antenna.
--
Keith

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That is not a valid analysis. It is a transmission line, not an antenna.
Consider that the effect, both for relatively small gauge cables, such as the ubiquitous 26 gauge used today, is *exactly* the same as the effect on the open wire lines used in the 30's and 40's with several inches of separate between a pair of much larger copperclad steel wires. And while the twist on some cable is measured per inch, on typical telephone cable it is measured in many inches per twist, and on those old open wire lines it was in hundreds of yards per twist.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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It sure as hell is. Open up the loop and it makes a *wonderful* antenna.

...and open-wire transmission lines won't pick up stray noise?
--
Keith

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It's a "wonderful" antenna regardless. But it's a single conductor long wire antenna. Changing the spacing is merely changing the effective diameter of the single conductor. To get any other effect requires spacing that is significant in terms of wavelength (greater than perhaps 1/8th of a wavelength, for example).

It picks up as much, or as little, as unshielded twisted pair of smaller gauge and closer spacing. That's the point... there isn't any difference. In either case what you have is a single conductor longwire antenna, not a loop antenna, until the spacing is a significant fraction of a wavelength.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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Absolute nonsense.

Bullsnit. Try reading your EE100 text again.
--
Keith

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Actually, that's why it works so well as a balanced transmission line.

I'd suggest studying transmission lines and antennas. Start with Kraus.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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Sure, it's a transmission line for the t-wave on the line. It's also an antenna, with the gain proportional to the area of the loop. Try running that open line next to a power line.

Get real Floyd!
--
Keith

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On Thu, 06 Mar 2008 15:40:31 -0900, snipped-for-privacy@apaflo.com (Floyd L. Davidson) wrote:

I have built many twin lead antennas for VHF use. The distinction is not so clear as you are advertising.
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In fact, it is. What is an folded dipole? As opposed to a loop?
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Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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...and they work rather well as antennas, just as any open line. Folded dipoles don't work so well if you twist them, though.
--
Keith

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There is a fundamental difference between a folded dipole and a loop antenna. It is exactly as I suggested above.

You apparently have no idea what a folded dipole *is*, in theory. Consider another similar construction, which does not change anything in the same way that a loop does: multiwire rhombics. The effects are the same as experienced with a folded dipole (the two conductors equate to one larger conductor).
The claim that separation between the two wires of a twisted pair (or even an untwisted parallel pair) transmission line has the effect of a loop antenna is false.
The idea that this is covered in "EE100" is equally ridiculous, and the dismissal (in a different message) of my suggestion to read the work of Kraus, where it is in fact discussed in detail, suggests that some people really should read Kraus.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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