Replies

1. 
   Art_B | May 31, 2001 09:21pm | #1

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   A separate metal conduit (for the signal cable only, I assume you know NOT to put it in with the power cable) will cut the EMI by about 80 dB if grounded at each end to the user equipment chassis with compression fittings. About 55 dB with screw/socket fittings. If just to wall boxes and typical electrical grounding, it will cut interference only by about 20 dB due to the remaining cable to the equipment. Plastic won't do anything for you except provide a little separation.

1. 
   FredB | May 31, 2001 10:59pm | #2

   *Art is right on. But the whole thing depends on a good common ground for the cable run and the user equipment. If anything breaks the common ground the whole thing goes to pot. -20dB isn't that much. So, unless you really have to run them alongside each other don't. You are really better of to cross and reroute, even if need to add more cable.

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   Norm_Kerr | Jun 01, 2001 06:39pm | #3

   *Art, How does this apply when you are running them underground?If you put the signal wires in a metal conduit and the power ones in plastic, does the earth provide the ground or would you still want to keep them far apart?I ask because in the past, posters have suggested that using one metal pipe for the underground was enough and I thought they could then be laid in the same trench. But since I only want to do this once, and since future technology may make it increasingly important, I want to make sure!How far apart is usually enough? Maybe in the same trench they could be separated by a foot or two pretty easily...

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   Art_B | Jun 01, 2001 08:14pm | #4

   *Norm:Things can get really hairy to calculate for the buried case, since the ground conductivity comes into play and changes with temperature and moisture content. Like Fred said, its all in the "grounding" - read high frequency connection- to the equipment chassis, NOT earth. In extreme cases, a piece of metal pipe in dry soil wihout any connection to equipment chassis can be worse than nothing. If you are convesant with electronic calculations I'll e-mail you the transfer impedance formula, etc.

1. 
   Robert_Macaione | Jun 01, 2001 09:51pm | #5

   *I'd be concerned if this were a high speed data traffic situation, but IMHO 20 feet exposed should not cause any noticeable conflicts in a home environment. What kinda applications is this going to support?

2. 
   Art_B | Jun 01, 2001 10:32pm | #6

   *Robert has a good practical point. Be careful of what and how you ask, you might get more than you care to hear and cause concern where there likely isn't any. An example: a recent satellite up/down link system has 275 feet of 10 MHz RS422 signal line, 300 VDC line, and 3 phase power line all in the same big conduit - although each cable has dual foil shields except only a braided shield on the power. There was some concern from eye-chart analysis that the data system would not work and need to be changed to fiber, but it works hard wired.

3. 
   FredB | Jun 02, 2001 06:07am | #7

   *Robert the problem is that not only must the install protect against interference within the structure, it must protect against RFI from without the building. Without good grounding and adequate separation this 20 feet could just become a great antenna for every squawk in miles. RFI can come from loose lines in the electric utility, legal and illegal radio transmitters, touch-lamps, and the list goes on. Since most consumer electronic gear is very poorly shielded itself you could end up with real problems.What kind of problems? Unuseable portable telephones, noise on the stereo, computer network unstable, computer printer that randomly prints and on and on. How likely is it you will have problems? Who knows? But the opening is there and problems have resulted from this kind of installation. So, good grounding, good routing and good separation is well worth the effort.

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   dan_smith | Jun 02, 2001 06:07am | #8

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   I have to run cat5 caable alongside 120v ac for about 20 feet...If I put it in plastic or metal conduit will this stop electrical interference..Leviton has no data on this..it is for structured wiring in a new house..
   thanks