Single Core (Shielded) LV Cabling for Audio Equipment

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.

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.