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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. ------------------------- 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 |
#2
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Hum from phone wires running next to mains?
"Foxtrot" wrote in message ... 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% |
#4
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Hum from phone wires running next to mains?
On Tue 04 Mar 2008 21:22:30, wrote:
In alt.engineering.electrical wrote: | On Tue, 4 Mar 2008 19:13:07 -0000, "Graham." | 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? |
#5
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Hum from phone wires running next to mains?
On Tue 04 Mar 2008 19:13:07, Graham. wrote:
"Foxtrot" wrote in message ... 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? |
#6
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Hum from phone wires running next to mains?
"Foxtrot" wrote in message
[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 |
#7
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Hum from phone wires running next to mains?
Foxtrot wrote:
On Tue 04 Mar 2008 21:22:30, 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*. 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. 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) |
#8
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Hum from phone wires running next to mains?
Foxtrot wrote:
.... 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 |
#9
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Hum from phone wires running next to mains?
On Tue, 04 Mar 2008 23:44:11 GMT, Foxtrot
wrote: 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 |
#10
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Hum from phone wires running next to mains?
Foxtrot wrote:
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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