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Hum from phone wires running next to mains?

Discussion in 'Electronic Equipment' started by Foxtrot, Mar 4, 2008.

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  1. Foxtrot

    Foxtrot Guest

    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
     
  2. Graham.

    Graham. Guest

    It is quite difficult to induce hum into telephone wiring.
    Use twisted pair cabling rather than the flat ready-made
    extension cables.
     
  3. Guest

    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.
     
  4. Guest

    In alt.engineering.electrical 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).
     
  5. Foxtrot

    Foxtrot Guest

    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?
     
  6. Foxtrot

    Foxtrot Guest


    (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?
     
  7. Ivor Jones

    Ivor Jones Guest

    [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
     
  8. 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).
     
  9. CBFalconer

    CBFalconer Guest

    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.
     
  10. kony

    kony Guest

    yes
     
  11. Tom Horne

    Tom Horne Guest

    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_ca...exible_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.
     
  12. Tom Horne

    Tom Horne Guest

    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.
     
  13. jasee

    jasee Guest

    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)
     
  14. George

    George Guest

    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.
     
  15. Brian Cryer

    Brian Cryer Guest

    Can't comment on the hum ... but it looks like others have.

    One contribution I would make is that you are aware that your phone service
    will support 4 REN and that each phone is normally 1 REN, meaning that you
    can have a maximum of 4 phones. My parents had more of this and whilst from
    their perspective it seemed to work (they could call out), it stopped people
    from calling in because their phones stopped ringing.

    So be aware that if you are adding four or five additional phone sockets
    that you won't be able to use all of them (at the same time).
     
  16. Guest

    | On Tue, 04 Mar 2008 21:22:30 +0000, phil-news-nospam wrote:
    |
    |> In alt.engineering.electrical 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.

    I don't have specific data on the quality of noise immunity. I'd bet that
    kind of research has been done. It most certainly would vary by quality of
    construction of the cables in question.

    RF coax comes in various levels of quality based on a stated shielding
    percentage. I've seen lows of 60% all the way up to 100%. The latter
    could be a foil, or a solid metal encapsulation (quite a variety of
    different coax types with this).

    I've seen cables, including CAT5, with both twisting _and_ shielding around
    the whole cable assembly. I don't know how much the effectiveness works
    together. I have not had a case where I would consider using it.
     
  17. Guest

    | |>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.
    |
    | Can't comment on the hum ... but it looks like others have.
    |
    | One contribution I would make is that you are aware that your phone service
    | will support 4 REN and that each phone is normally 1 REN, meaning that you
    | can have a maximum of 4 phones. My parents had more of this and whilst from
    | their perspective it seemed to work (they could call out), it stopped people
    | from calling in because their phones stopped ringing.

    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.


    | So be aware that if you are adding four or five additional phone sockets
    | that you won't be able to use all of them (at the same time).

    Or check your phone specs for an REN or ringer equivalence number.

    If you want to put DSL on your phone line, I also suggest a splitter at
    the entrance of the phone line and a separate NON-branching higher grade
    (e.g. twisted pair) wire for the run from the DSL side of the splitter
    to the intended connection.
     
  18. charles

    charles Guest

    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.
     
  19. kony

    kony Guest

    It depends on how old the phones, or these days with modern
    electrically powered phones, cordless/etc, the REN, number
    may be very low per phone. IMO, no good reason not to get a
    cordless phone these days as some are dirt cheap, except
    it's nice to have at least one non-electric in case the
    power goes out.
     
  20. Ryan Weihl

    Ryan Weihl Guest

    I just looked at my phoneset, a wireless extension by Uniden.
    the base is rated at 0.08 REN. We have a regular phone and 2 of these
    wireless base stations with 2 sets each, so everyone has a set handy
    and the neighbors can listen in too.


    --
     
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