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There are two sayings I keep hearing when talking about wiring e.g. home cinema or computer hardware;

  • Don't wrap data/network cables and power cables up in the same loop because the power cables will mess up the streams of your data cables
  • Don't loop power cables because it can produce an induction effect.

I'm interested in whether these two theses are still valid with the current coating and shielding of modern day data/network and power cables.

So, when I wire my home cinema and the power cables lie next to the data cables, will that cause any effect on the quality of video/audio I am likely to perceive?

And is wrapping three or four meters of power cables in a coil actually in any way dangerous, seeing how thick the coating and shielding is on them?

How else should you manage the power cables that are too long to just let them lie around? Maybe a different wrapping/looping technique?

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No, these draconian rules were a remnant of the Reagan era. The Obama administration has relaxed on it, so don't worry. –  Kaz Dec 5 '12 at 21:58

5 Answers 5

up vote 7 down vote accepted

Yes, its still valid.

Power and data cable coatings are insulation - designed to protect from electric shock and to avoid short circuits. They do not interfere with magnetic fields.

I do not know what kind of shielding you are talking about - I have never seen any modern power or data cables with shielding that blocks magnetic fields. Magnetic fields are quite pervasive.

In response to your follow up question:

The size and strength of a magnetic field is relative to the amount of current flowing through the wire within. For very low voltage wires, like network cables, bundling them together generally doesn't create a strong enough field to disturb the other lines in the bundle.

However, if you loop the cables, you end up running the same current through very nearly the same space - over, and over, and over - and this can amplify the magnetic field that's created, enough that it can interfere with other signals.

With power cables, they run much more current and so create much bigger fields. Big and strong enough to interfere with the tiny currents going thru the other cables. Looping them is even stronger and more problematic.

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But how can you actually do that? When you look behind a TV board or a computer desk, all cables tend to be mixed up with each other. How should you separate those? Even if you do, they will never be like to be more than a few centimeters apart, which doesn't seem like much of a "safety" against magentic fields? –  Florian Peschka Oct 23 '12 at 12:09
    
I will edit my answer to clarify. –  The Evil Greebo Oct 23 '12 at 12:11
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Data cables like Ethernet use a twisted-pair form of "shielding" in that they don't actually shield the data wires but twist them in pairs so that both wires receive the same induced signal. The transmitter sends opposite voltages down each wire of the pair and the receiver subtracts them which effectively negates the noise. However, if the induced voltage is large enough (say from being coiled many times with a power cable) then it can exceed the maximum supported by the tranceivers on either end and "blow it out". Tangled wires do not induce voltages like coils do. –  Brian White Oct 23 '12 at 12:25
    
Good point. Something worth reading up on for @FlorianPeschka - look into how transformers work. They demonstrate very neatly how strong looping powered lines can be. –  The Evil Greebo Oct 23 '12 at 12:27
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Oh I loop my power cables all the time. I've never found that the amount of interference is a problem with my equipment. I'm sure it would be if I were dealing with something fairly sensitive, but all my cables go to consumer electronics that aren't nearly so delicate. –  The Evil Greebo Oct 23 '12 at 14:36

Henry's laws of induction are still in effect as well as Hertz and Maxwell's observations on electromagnetic radiation. The less noise you induce into your data cables the better, especially now that stuff's going to Gigabit. The inverse square law says that distance is a good thing. Keeping things straight and not having loops reduces the magnetic field as you aren't creating a transformer coil. And if they do have to come close, having them cross at a 90 degree angle keeps inductive coupling down to a minimum.

Also, don't use metal drive rings for data cables, they can act like ferrite chokes and reduce signal strength. For POTS they're just fine, but for the frequencies used in Ethernet, not so hot.

So much for in the wall and ceiling.

Once it exits the wall, things get nasty. I just figure 8 loop the electrical cable with a tie wrap in the center and keep the data cables as far away as I reasonably can get them. There isn't an easy answer here, basically if you start having problems, move things apart until the problem goes away. USB cables have a Faraday shield, so they're less of a problem. This has worked pretty well on the 27 computer network I run.

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In addition to the other answers which correctly note that it is hard to block magnetism, it is a very bad idea to run power and data together because some unfortunate person in the future -- possibly a future version of yourself -- is someday going to try to hang a picture and drive a nail through the wire, just nicking both the data and power cables enough to connect the power to the data, and suddenly your XBOX has 120 VAC going to the video port instead of the power port. Just don't do it; keep high voltage and low voltage as far apart as you reasonably can.

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Even more so today than before with high speed data networks. Physics hasn't changed lately so follow the same rules. Whether you notice this in a home entertainment system could be debatable.

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Yes, physics laws haven't changed, however most video/audio signals are digital nowadays.

This means that even if the signal is slightly degraded in the cable, you will still get the maximum quality.

Unlike analogic transmission where each small signal degradation was causing a small loss of quality, with digital transmission you have a "threshold". When your signal is above the threshold, you got the maximum quality. When it drops below the threshold, you have no signal at all (or a very jammed one).

Also for audio you may use optical cables which are completely immune to magnetic fields from a power line.

For looping the power cables, it's not recommended, and can be dangerous if you roll many hundreds metres (there have been some cases of fire started by this).

However, if you need to do a couple of loops, I don't think it will have a significant effect.

Bottom line: magnetic fields decrease quickly with the distance, so if you can keep your video and audio cables a few centimetres away from your power lines, that should be good enough.

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