I have no choice but to run telephone cable within six inches of electrical cable. Is there a way to shield the telephone cable. Thank you.
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You can put it in conduit or just use 2 conductor shielded cable. May not need it though, standard phones have a notch filter between 300Hz - 3KHz for this very reason.
Greg,
My training, (9 years in communications electronics,) says that the reason for the notch pass filter is to reduce the bandwidth to that small part of the audible spectrum that carries 90% of the information in the human voice.
The fact that it also blocks 60hz hum is a bonus. 300hz being 2+ octaves above 60hz makes it a fair hum filter.
SamT
I missed that first comment.
I hope that they DON'T have a notch filter in the 300-3k band. It would nto be much of a phone if it did.
And while the codec at the CO would have a filter I don't think that the hybrid network in the phone is really much of a bandpass filter. But yess it is optomized for 300-3k.
I don't savvy phones that much, at least ones without a carbon microphone, but the only purpose of modifying 300hz-3khz is because that is the bulk of human voice spectrum.
SamT
Exactly, that is why you would not want a notch filter.
Yeah, what you want is a "band-pass" filter, not a "notch" filter. They are the opposite things.
See If I an clear the "meassage waiting".
Apparently someone addressed a message to me and then deleted it, but since it is deleted I can read it.
So this thread comes up each time as having an unread message for me.
EDIT - no that does not work.
Edited 8/30/2004 10:09 am ET by Bill Hartmann
I wasn't aware of a standard "notch filter" in telephonic devices but...I've thought about this potential problem too, but a notch filter at 300-whatever doesn't filter 60cycle AC hum. Did you mean the filter was a bandpass which implies the device would only "hear" between 300Hz and whatever, well beyond 60hz?
Scrappy,
Ya, ya, I meant bandpass, I was typing too fast and not thinking....
Moose,
I also have the same problem as you do. In my basement, I have some telephone cable running right next to electric. I have no problems with my telephone lines.
My telephone cable is Cat5e
My electric is in piped conduit.
Not sure what electric you are running and if it is in conduit? bx?
Cheers!
Dark Magneto
For how long a distance?
Use twisted pair cable (and of the "cat" series) and unless it is 100's of feet you won't have any problems.
Right, one should never use the flat phone cable for permanent wiring -- always use round cable -- and it's best to use a twisted-pair cable (Cat 3 or Cat 5).
What's the diff between the round style of phone cable and flat - other than the latter is what one is far more likely to come home with HD?
Wasn't aware of diff and now's the time to know.
Power wiring generates a magnetic field, and the higher the voltage, the stronger the field. Devices with coils, such as motors, transformers, and ballasts generate strong magnetic fields. The magnetic fields can cause interference with communication signals.
Round cable is less prone to picking up interference than flat cable. Twisted pair cable (in which each wire of a pair within the cable is twisted around the other) is less prone than plain round cable. Shielded twisted pair (in which there is a foil shield around the wires) is less prone than unsheilded twisted pair.
I am working on house that has the main power line runing from a sub panel to the main breaker panel across the garage ceiling. The problem is (imho) that both the cat5 and the cable tv wire are runing parrallel to this main, maybe 6" from it for 15' or so. Do you think such a high powered cable could cause problems with the data or am I overly concerned? Nothing has ben hooked up so we cant test any thing yet.
Shouldn't be a problem. Any closer, though, and one would begin to be concerned, since the coupling between wires tends to follow an inverse square relationship.
I don't have the education or experience to answer whether you are too close or fine. Sorry.
What you want is Twisted Pair wire. Each pair of wires are twisted a couple of times/inch. That way any induced magnetic fields will cancel out.
If they aren't twisted the wiring can act like part of a transformer and pick up induces noise from the power wiring.
Flat telephone wiring is NOT twisted.
Generic round "telephone" cable may or may not be twistd.
"Cat" cable is all twisted pair.
Edited 8/14/2004 4:40 pm ET by Bill Hartmann
We should clarify "flat". The sort of flattish, oval regular telephone cable may contain twisted pairs. There is another kind of "flat" phone wire, more used on office renovations in the 70's, that is about a half inch wide but only a sixteenth or so thick. The wires are parallel, and the cable is only one wire thick. It was glued right to the wall and painted over. It looked like a piece of rubber ribbon with longitudinal striations.
Not to add any confusion, but there is a twisted pair flat cable also. But it is only used within electronic equipement.
Flat telephone wire in NOT twisted.
Thanks for catching that. I tend to think faster than I type.
First off, the best cable is twisted pair. This means that the two wires of the pair are constantly changing orientation relative to each other, so that each picks up almost exactly the same amount of hum and other inductive noise. The receiver (be it telephone or computer) measures the DIFFERENCE between the two wires to detect a signal, so any "common mode" noise is canceled.
(Needless to say, this cancelling effect is only maintained if the two wires in a pair are kept together at splices and when connecting a given signal. "Crossed pairs" can cause all sorts of problems.)
Even if the wires aren't twisted, with a round cable the wires are all closer together and will tend to "randomize" their positions over several feet. So they will tend to pick up equal (and cancelling) noise/hum.
A flat cable has wires that are in a fixed relationship to each other and, especially if one tries to be "neat" and keep the cable untwisted, will tend to have one wire consistently closer to a given source of interference than the other wire in the active pair.
I don't think it is a big deal unless the elect/phone are within a few inches of each other. Also, if the wires cross in roughly a perpendicular fashion, the "cross talk" is greatly reduced if not negligible. When you may get into trouble is when the 2 wires run parallel to each other and are within an inch or so of each other.
Six inches isn't likely to be a problem. If you want to be sure, run one or the other in conduit, or place a piece of steel flashing between them.
Long parallel runs (where wires are within about 3" of each other) should be avoided, and it's most important to keep phone and Cat5 cable away from fluorescent ballasts, doorbell transformers, motors, and other things with a strong magnetic field.
Another important thing to avoid with Cat5 is kinks in the cable.
There should be no need to shield twisted-pair telephone cable. Which is the only type of telephone cable you should be putting in a wall. The non-twisted cable designed to be used between wall plates and the phone itself for short distances (<25') should not be used in a wall.
Think about it: Outside the house, that telephone cable has run thousands of feet in parallel with high-voltage distribution lines, without any shielding.
Properly balanced twisted pair is highly resistant to hum pickup. If you do hear interference, it is usually due to some fault, such as one of the wires in the pair being grounded, which will ruin the hum-rejection capability of the cable.
that telephone cable has run thousands of feet in parallel with high-voltage distribution lines, without any shielding
Well, not entirely. The above-ground phone cable is run on the lowest position on the power poles, or farthest from the highest voltage lines on the pole. The telco cable is in a foil-wrapped gas-tight sheath, too (the telco uses nitrogen gas to prevent oxidation of the copper conductors, thus the need for gas-tight).Occupational hazard of my occupation not being around (sorry Bubba)
The nitogen gas is not to prevent oxidation, but to dry out the cable.
The cables used to be lead coated and used a paper rape for insulation. When water got into a crack they would use the nitrogen to dry out the cable. In most case I think that it was only used as needed.
But I remeber seeing tanks for a long time in some locations so I think that they might have kept some pressured that they had continous problems with.
But with a the plastic insualted cables a nitrogen tank is very rare.
Well, the distance from the telephone cable is not that much more than the distance between the individual high-voltage couplers. So the field strength at the telephone cable is significant, and as I mentioned, extends for thousands of feet.
Telephone calbe installed now is not sheilded. Not that it would matter if it was. At voice frequencies, the coupling is mostly magnetic rather than electrostatic, and simple shielding won't do much to stop magnetic coupling.
Bill's right about the gas injection being used to prevent water ingress. Here is wet western oregon, the cables are kept under positive pressure with dry air to keep the water at bay. The cable vault in the basement of a telephone Central Office is a sight to see.
The thing that prevents inductive pickup in telephone wires is the fact that they are twisted, combined with the fact that the telephone is a "differential" device -- ie, it operates on the voltage difference between the wires, not the voltage difference between a wire and ground.
Since the wires are twisted, about once every 3-4 inches, any inductive pickup in one wire will be almost exactly matched by the inductive pickup in its "twin". So the net pickup is virtually nil.
Electromagnetism (electric plus magnetic) is a single force. Whatever blocks it, blocks all of it.
Twisting conductors does not prevent a transfer of energy from a third conductor. It does prevent inductively transfering energy between the two twisted wires.
The electromagnetic field from earths hi tension lines extends a light year from the conductor for each year that conductor has been in use.
SamT
You're right -- twisting doesn't prevent inductive pickup in the absolute sense, but it does keep the inductive pickup from causing problems.
If it's me, I use cat5e cable for the run. It's a bit more expensive, but well worth it. It can be used for phone with no problem... and FWIW, can be converted for data if the need ever arose in the future (4 twisted pair, that is).
If you are going to need to run the cable in any venting-type spaces (duct work, return air, etc), be sure the cable is plenum rated.