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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Another question for Jon Risch - Shielding Romex posted by Mikenificent1 on September 26, 2000 at 20:14:17:
[ Why is the steel jacket of the romex not an effective shield? ]It is not that the steel is not a good shield, quite the contrary, it is the most effectve one for magnetic fields. However, there are two issues here that recommend against steel armoured romex.
One, the steel is in the form of a wrapped spiral, so as to make it flexible, like the gooseneck on a hi-intensity lamp. This often does not provide for the best RFI protection, as the spiral acts like an inductor, and greatly reduces the ability of the steel to drain the RF to ground. The higher resistance of the steel also makes for a poor RF shield too.
Two, the very close proximity of the steel will cause heavy current flow in the Romex to be slightly distorted, as the steel will be non-linear in it's reaction to this current. Add in the fact that the current draw is in short and very high current peaks (when the power supply tops off the filter caps through the bridge rectifier, at the peaks fo teh AC sine wave) rich in harmonics, and the steel is going to be pushed further into potential distorting action.
If the steel were spaced away from the wires even just a bit more, say another 1/4" all around, then this distorting action would be greatly reduced, but on such cables, they do not 'waste' any space, and the wires are in pretty intimate contact with the steel outer layer.
A mylar shield properly done (like line level cables), or a braided copper shield will at least provide good RFI and electrostatic shielding.
[ What about also wrapping the jacket in copper tape? ]
Same problem as the inductor formed by the steel spiral, inductance limiting the RFI effectiveness. Copper braid purchased separately, and slipped over the intended wiring would be the best for DIY from scratch.
[ There are four separate cables inside the jacket (red, white, black, blue). Assumming I could fit it into the plug some how, I was thinking of using 2 for the positive and 2 for the negative, and then run a separate shielded cable (maybe CAT5) outside of the steel jacket for the ground and then heat shrink everything together. ]
For minimal EM radiation, you could cross-connect the four wires if you do this, this will contain the EM fields as much as possible, reducing the amount of stray fields that would exist away from the wires.
One polarity would be on the wires at 12 o'clock and 6 o'clock positions, and the other on the 3 o'clock and 9 o'clock positions.Be aware that placing the ground wire, whatever is used, outside the shielding, will allow it to pick up RFI and interference too.
[ One thing I'm not sure about is if I attach the shields to ground or not? If not, then where and how? ]
If you do this, which may be against code, be sure to have the total resistance for the CAT5 cable equal to at least 12 gauge wire, which would be at least 4 bundles of 4 pair CAT5 wires.
To use any metallic covering as a shield, the most effetive shielding would be to connect it at the ground at the breaker panel, and not elsewhere. The CAT5 ground wire would of course, need to be connected at both ends. Again, this is probably against the electrical code, and you should realize that this may invalidate your home owners insurance.
Jon Risch
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Follow Ups
- Re: Another question for Jon Risch - Shielding Romex - Jon Risch 19:10:17 09/28/00 (2)
- actually, this cable would be used for power cords not my mains - Mikenificent1 21:37:26 09/30/00 (0)
- Jon, is aluminum MC cable OK to use for dedicated AC line? (nt) - Jerry P 06:53:58 09/29/00 (0)
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