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Single Core (Shielded) LV Cabling for Audio Equipment

Adrian B
Hi All


Quick question regarding suitable cabling for LV supplies to Audio equipment.....

I have a client (Sound Studio Designer) who is insisting on the LV power cabling to serve many different types of Audio equipment being shielded.  Firstly, I have never specified anything of the sort previously, normally specifying 6491 or 6242 etc., so is there something out there that is suitable?  Secondly, has anyone had any experience in this type of installation, and if so, what are the thoughts on single core or multi core cabling with shielding?  I personally would have thought that shielded singles would be the preference, but what do I know, I am quite obviously not a specialist in this field.


Any advice/ help would be hugely appreciated as always.


Thanks


Adrian B.
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    Where there are obvious flow and return currents, such as PSU + and - to a particular load, these are better under the same shield.

    So with that in mind, keep L and N to the same load together, not forming a big loop one turn mains hum generator.

    If magnetic pick up is the concern, twisted pair shielded is better than random bundle shielded, as the pick-up in alternate twists cancels.

    Generally fixed wiring in metal conduit is pretty much the best, but cannot be made leak proof.

    SY and CY may raise an eyebrow in wiring circles but are good enough for keeping trouble out of the wiring in EMP labs.


    Is there a shielding spec - how many dB better than a bare cable you need to be, or perhaps  an immunity spec such as no more than so many mV induced when illuminated by an external field of X volts per meter over a certain frequency range  ?


    ' be shielded ' is rather vague - how long is a piece of string. Some design authority input may be needed.

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    There are plenty of 'shielded' power cables about - something to BS 8436 would seem an obvious choice to me (usual caveats for reduced c.s.a. protective conductors) - or if you don't need the foil to act as a c.p.c. then some of the more traditional  "FP" types might do. Old school might be ordinary singles in steel conduit or trunking. Some might be tempted by SWA, but there's some suspicion that the gaps between the strands might let through some interference, depending on what sort of frequencies you need to block. SY and CY type flex are probably somewhere in between.


    Whether any of that is going to be particularly beneficial is another matter. Some might observe that if you're using a common DNO LV supply that's wending its way for several hundred metres through the neighbourhood picking up all sorts of noise from every installation on the way, shielding the last few metres might not make an awful lot of difference.


      - Andy.
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    Hi Guys


    Thanks for the very prompt responses, most appreciated.  Regarding the vagueness of 'Shielding', I quite agree and no, there are no stipulations as such so I cannot pass on any further information.


    As for comments regarding if it is worth while, seeing that yes, the incoming supply is a common DNO wending its way through both a residential and industrial area before serving the demise, this was something I raised 2 years ago when this (lengthy) project begun.


    What scares me somewhat is the fact that the so called 'Specialist' is also suggesting that an entirely separate Earth be installed, with no bonding to the incomer, and that when recording, the technician can switch over to this supposed clean earth and not utilise the incomer.  If I am correct, this is not common practice in the UK and not something that is acceptable!
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    TT earth islanding for the building or the rooms involved  is the nearest you will get to that.


    You can do it but you need to have an RCD up front, perhaps 100mA slow acting, and then from then onwards you then ignore the company earth, and use the local electrode. The two earth zones must not meet, so plastic radiator pipes or boxing in if parts of the building are to one earthing system and the rest to another.


    To add any value at all in terms of noise ingress you need to have some mains filtering on the circuits or perhaps just before the whole CU feeding the quiet zone.

    We tend to specify things like  these  on RF chambers, where we mount them on / stuck through  the chamber wall, with the dirty side out, clean side inwards  for receiver tests, and the other way round for transmitter or pulse power testing, but that may be overkill and you probably need to keep an eye on the earth leakage the filters cause.
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    mapj1:

    TT earth islanding for the building or the rooms involved  is the nearest you will get to that.


    You can do it but you need to have an RCD up front, perhaps 100mA slow acting, and then from then onwards you then ignore the company earth, and use the local electrode. The two earth zones must not meet, so plastic radiator pipes or boxing in if parts of the building are to one earthing system and the rest to another.




    The two earth zones may require separation by the DNO. It may not be achievable in a built up area, and might depend on the extent of the PME and arrangement of the distribution and other metallic services in the area.

    DNOs aren't always keen on two different earthing arrangements within a building either.


    An earthing approach to BS EN 50310 would, I believe, be far superior in most cases, and a TN system earthing arrangement is usually better.






    To add any value at all in terms of noise ingress you need to have some mains filtering on the circuits or perhaps just before the whole CU feeding the quiet zone.

    We tend to specify things like  these  on RF chambers, where we mount them on / stuck through  the chamber wall, with the dirty side out, clean side inwards  for receiver tests, and the other way round for transmitter or pulse power testing, but that may be overkill and you probably need to keep an eye on the earth leakage the filters cause.




    All single-phase supplies through a split-phase transformer would make a big improvement as well, but this would be costly. Just pulling a phase and neutral off the three-phase not always best for EMC

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    What scares me somewhat is the fact that the so called 'Specialist' is also suggesting that an entirely separate Earth be installed, with no bonding to the incomer, and that when recording, the technician can switch over to this supposed clean earth and not utilise the incomer.  If I am correct, this is not common practice in the UK and not something that is acceptable!



    Sort of separate "clean earths" aren't unknown but certainly very unfashionable these days. The usual arrangement was to have them connected to the main earth at one point (usually at the intake) but then kept well away from any nasty noisy equipment internally and connect to just the sensitive stuff. The trouble was it was very difficult to do properly - all too easy to cross connect to equipment with a 'dirty' earth, and besides often better results could be had by keeping the impedance of the earth connections between sensitive equipment low rather than trying to keep it free of "noise". The much more usual approach these says is to have a single earthing system but with as many interconnections as possible - so you end up more with a mesh or grid of earth conductors rather than just a tree or ring - lots of metalwork bonded (whether required for shock protection or not) and possibly screens of small cables paralleled with a much larger conductor to ensure they're no damaged by large (e.g. fault) currents as well as reducing the overall impedance.


    What you really don't want to do is have two intermixed parts of the same installation served by two completely separate earthing systems (e.g. one on the DNO earth and the other on a local rod) (well, unless you've got extraordinarily close control of the situation) - as the two earthing systems, for a number of reasons, can be at quite different potentials at times, and can give a serious risk of electric shock between them - and with no protection from RCDs or anything else.   As Mike says the only sensible way of avoiding the DNO's earth is to TT the complete installation. But even that might not always be helpful - if 'noise' has been introduced more or less equally onto all the DNO's conductors your equipment would only see the difference, which might be relatively small. Change to a separate earthing system and the equipment might now see the full extend of that type type of noise (between live conductors and PE).


      - Andy.
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    Transformer isolation will be good for low frequency junk, less useful for higher frequencies, RF tends to also be  a problem for audio as well as direct in -band break though, as it intermodulates to create audible responses.

    However any isolation will only be as good as the separation of pre- and post filtered wiring in any case, running them alongside for any length will largely undo the good. Hence shielded walls where it really matters, which is not here.


    I'm  not aware of any restriction requiring you to use the DNOs earth after the meter,  you can TT one house in a row, or indeed one room in a building, much as you would a garden shed.

    However, if there are shared services these do need some thought, and so does where the TT earth electrode goes needs to be far away from sources of current injection or removal from the terra firma, I suspect we are saying the same thing in different words.
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    Is this all to ensure that the sound does not get corrupted?


    One of the great ironies of life is that by the time that you can afford the best audio equipment, your ears and auditory pathways can no longer benefit from it. ?
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    mapj1:

    Transformer isolation will be good for low frequency junk, less useful for higher frequencies, RF tends to also be  a problem for audio as well as direct in -band break though, as it intermodulates to create audible responses.

    However any isolation will only be as good as the separation of pre- and post filtered wiring in any case, running them alongside for any length will largely undo the good. Hence shielded walls where it really matters, which is not here.




    If we are looking at how the electrical system deals with harmonics and reception of radiated emissions, a centre-tapped transformer (earth the centre-tap) is much better for single-phase supplies.






    I'm  not aware of any restriction requiring you to use the DNOs earth after the meter,  you can TT one house in a row, or indeed one room in a building, much as you would a garden shed.

    However, if there are shared services these do need some thought, and so does where the TT earth electrode goes needs to be far away from sources of current injection or removal from the terra firma, I suspect we are saying the same thing in different words.




    Well, whilst there are no restrictions per say, the DNO may have a separation requirement. Whilst to-date not often invoked, some of the installation practices employed thus far for EV are making DNOs think again about TT. If the TT is required for a caravan site or marina, the IET recommend 10 m underground separation between anything connected to the TT, and anything connected to PME.

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    Morning All


    All comments/ thoughts/ advice taken and thanks kindly for your input.  Help like this is invaluable.


    Regards


    Adrian

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