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#21
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Hum from phone wires running next to mains?
"Ivor Jones" wrote:
wrote in message [snip] : : 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. That's STP (shielded twisted pair) and is not really worth it for most applications. There is a military spec. for it somewhere, I believe. It's also a different impedance to UTP so may not work correctly with all equipment. All outside plant telephone cable with the exception of the local drop cable is shielded. Inside a telephone office equipment room, T1 and higher speed data cables are all shielded if the cable extends between rows or for more distance in one row than 4 racks. STP is significantly expensive, and will not commonly be seen anywhere that it is not absolutely required. For example, it would make no sense to use it within a normal customer premise area, unless there is an equipment room with multiple rows of equipment racks. -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
#22
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Hum from phone wires running next to mains?
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--. I believe I can help here. Rightpondian "flex" Leftpondian "cord" -- Graham %Profound_observation% |
#23
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Hum from phone wires running next to mains?
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. To me, Phil's post illustrates how *similar* the our systems are, and I strongly suspect that the reason why we don't see REN 1 is that the approval rules specify the figure quoted should be an integer between 1 and 4. Powered devices like DECT base-stations and FAX machines don't need to supply any significant ringing current from the line, they just need to sense the AC waveform to trigger the ringing. REN is a bit of an anathema these days IMHO. A good get-out for the support drones. ( For the North Americans) What is unusual with the UK system apart from our "special" plug and receptacle instead of an RJ11, is the fact that we use a third wire which couples all the ringers in parallel to a capacitor in the master socket to which the incommer is connected. Many modern phones do not even use this "bell wire" and just use the A and B wires (tip & ring). -- Graham %Profound_observation% |
#24
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Hum from phone wires running next to mains?
Ivor Jones wrote:
wrote: [snip] 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. That's STP (shielded twisted pair) and is not really worth it for most applications. There is a military spec. for it somewhere, I believe. It's also a different impedance to UTP so may not work correctly with all equipment. It should deal with hum pickup on audio cables quite nicely. However shielded twisted pairs are considerably more expensive, and you have to be careful about generating ground-loops in the shield grounding. I replaced your non-standard (: quote markers with the normal ''. Please don't use thos non-standard characters. They foul up other software. -- [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 |
#25
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Hum from phone wires running next to mains?
"Graham." wrote:
.... snip about ringer specs on phones and lines ... For the North Americans) What is unusual with the UK system apart from our "special" plug and receptacle instead of an RJ11, is the fact that we use a third wire which couples all the ringers in parallel to a capacitor in the master socket to which the incommer is connected. Many modern phones do not even use this "bell wire" and just use the A and B wires (tip & ring). In North America again (I don't know about elsewhere) the normal phone uses 3 wires to connect to the two wires of the phone circuit. The yellow wire carries the ring signal. Just disconnect that and the phone won't ring, and the load is zero. -- [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 |
#26
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Hum from phone wires running next to mains?
"CBFalconer" wrote in message : : Ivor Jones wrote: [snip] : : I replaced your non-standard (: quote markers with : : the normal ''. Please don't use thos non-standard : : characters. They foul up other software. With respect, and without wishing to start a row, that's *your* problem. I use non-standard quote marks for a purpose. If your system can't cope with that, then it's up to *you* to do something about it. I have been using the quote marks I use for several years and you are the first to complain. Ivor |
#27
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Hum from phone wires running next to mains?
CBFalconer wrote:
In North America again (I don't know about elsewhere) the normal phone uses 3 wires to connect to the two wires of the phone circuit. The yellow wire carries the ring signal. Just disconnect that and the phone won't ring, and the load is zero. I don't recall any system in North America that put ring voltage on a separate wire. The yellow wire is generally not connected unless one 4-wire cable is used for two separate telephone lines. In North America the "normal" line uses only 2 wires. The audio signal is applied between the "tip" and the "ring" of a single pair. "Ring Current" and "Loop Current" are also applied between the Tip and the Ring of the same pair. Commonly used drop cable has four wires: Green is the Tip and Red is the Ring (positive and negative, repectively for the DC loop current), while the Yellow and the Black wires are not used. (Note that the DC voltages used by telephone companies are negative with respect to ground, hence for DC the Tip wire is at ground potential, and the Ring wire has a negative potential. But the Tip is not at ground potential for Ring Current or for the audio signal.) One configuration often seen includes a second line on the same cable, using Yellow and Black as Tip and Ring. Historically the Yellow wire was, for a few years, used for a small AC voltage (nominally 6.8 volts) to power a lamp circuit on some telephone set models. Another historical use had the Yellow wire as a ground for party line service from the old style mechanical switching systems (such as the Step or Stroeger switching systems once used by the Bell System and by Automatic Electric). On those systems the ring current was applied between either Tip or Ring and ground, which was supplied to the telephone set on the Yellow wire. -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
#28
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Hum from phone wires running next to mains?
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#29
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Hum from phone wires running next to mains?
"Ivor Jones" wrote:
"CBFalconer" wrote in message : : Ivor Jones wrote: [snip] : : I replaced your non-standard (: quote markers with : : the normal ''. Please don't use thos non-standard : : characters. They foul up other software. With respect, and without wishing to start a row, that's *your* problem. I use non-standard quote marks for a purpose. If your system can't cope with that, then it's up to *you* to do something about it. I have been using the quote marks I use for several years and you are the first to complain. Consider for a bit just how absurd that statement is... Are you posting your articles for your personal edification, or are they intended to be read by an audience? Who should you format them for, yourself or the audience? Your non-standard quote characters are *not* appreciated by the audience, and indeed the more sophisticated members that you might want to appeal to the most are the ones most likely to make use of software options based on the quote marks. What your formatting style does, is tell the reader what your priorities are, and that your ability to comprehend the effect is apparently impaired. -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
#30
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Hum from phone wires running next to mains?
krw wrote:
In article , says... 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. 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. 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) |
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