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Thread: How do we earth shielded cable?

  1. #1
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    Default How do we earth shielded cable?

    These days we are being subtly encouraged to buy shielded cable for everything, including home telephone extensions. Shielding must be better, right? But the received wisdom , if you search for it, is that using shielded cable which is not properly earthed is worse than using no shielding at all, supposedly because the foil and drain wire can act as an antenna if not earthed.

    BT has a strict prohibition against touching their wires, and they have invented a type of Network Termination Equipment called the NTE5 termination box to allow consumers to wire their own extensions in a relatively foolproof manner. Has anyone ever seen an earth connection on the NTE5 faceplate? Has anyone even seen an earth connection on the older master Line Jack Units? Has anyone seen any instructions or advice on how to use shielded cable in conjunction with any of BT's termination boxes?

    If they have, I would really like to know.

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    Moderator toscal's Avatar
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    I have used cat5 for my phone connection from the master socket( I live in Spain so no NTE5 socket boxes) to my patch panel in the garage. The cable is shielded but I haven't connected the shield to anything at either end.
    Trying to earth equipment that doesn't have an earth connection can lead to all sorts of problems, especially if the earth at one end isn't from the same source as the earth at the other. More of a problem when working with equipment using 220 volt generators, especially if more than one generator is involved . Normally in a house your earth is all from the same source.

    These links may be of interest
    http://web.ukonline.co.uk/freshwater/pst1.htm
    http://web.ukonline.co.uk/freshwater/rj45.htm
    Last edited by toscal; 23rd June 2008 at 12:39 AM.
    IF YOU CAN'T FIX IT WITH A HAMMER, YOU'VE GOT AN ELECTRICAL PROBLEM.
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    Automated Home Legend TimH's Avatar
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    A large proportion of my experience is in industrial automation where shielding is considered standard, and I concur with the observation that poorly-installed shielding can be worse than none at all. As Toscal states, the convention is to only ground the shielding at one end of the cable. This is usually at the final control cabinet at the DCS/PLC (versus the field instrument end) but the screen must be continuous through all intermediate terminals boxes etc.

    I've been fortunate(?) to have seen side-by-side installations where some cables were shielded and properly and others were not. It clearly can make a difference and the sheilding can reduce interference in some environments (4-20mA analogue signal cables running within 300mm of 6kV motors etc.).

    For home use though I'm much more sceptical... With only 240V and relatively low loads being swtiched, the prospect of interference is pretty low, IMHO. So unless it's specifically recommended by the manufacturer of a particular product, I wouldn't bother with screened cable at all

    HTH,

    Tim.

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    Thank you, Toscal and TimH for your replies.
    My motivation for posing the question stems from having bought some screened cw1308 cable for wiring telephone extensions in my home, and then finding that I did not really know how it could be matched to BT's connection boxes. The answer seems to be, that I should simply dump the screened cable and buy unscreened twisted pair (UTP) cable instead. BT's old equipment was all designed before the days of home computing, long before we all started worrying about stray radio-frequency interference. I keep being told that twisted pair cable gives adequate protection from outside interference, without the problems that may come by attempting to screen it. I presume, the same must apply to the use of CAT5e cable for the connection between an ADSL router and the RJ45 socket on BT's NTE5 faceplate. Again, we have a choice between screened or unscreened cable and, again, it might actually be better to opt for the unscreened variety.

    I wonder, does anyone have direct experience of when using screened cable up to a domestic BT outlet really did make a significant difference? ... and, if the screen was earthed, how was it done?

  5. #5
    Moderator toscal's Avatar
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    I think good cabling practices are probably more or just as important as shielding.
    Never run data cables parallel to power cable. Always cross power cables as close to perpendicular as possible.
    Avoid kinks in the cable as you're pulling, and don't pull too hard during the install. I always pull through about 1m more than is required. This is then cut off during final termination. It may seem wasteful but its this 1m that gets the abuse during the pulling or laying. Some drops of washing up liquid on the cable works quite well in lubricating the cable making it easier to pull.
    As a rule of thumb keep the bend radius similar to the circumference of a CD. This is easier to visualize than saying a bend radius of 50mm
    Never have more than 2 bends in a cable route, or mount a junction box after the second bend.
    IF YOU CAN'T FIX IT WITH A HAMMER, YOU'VE GOT AN ELECTRICAL PROBLEM.
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  6. #6
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    I have just come across this excellent little article on Radio Frequency Interference (RFI):

    http://svconline.com/mag/avinstall_u...ontrolling_rf/

    Now, of course, I am worrying about all the new low-wattage, mercury-vapour fluorescent light bulbs I have installed in place of the incandescent ones the government doesn't want us to use any more, for the sake of energy efficiency. Still, if they were a problem, I suppose someone would have mentioned it by now. We will just have to keep our cables away from them.

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