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Dedicated mains again ...

Discussion in 'audio' started by jevy47, Sep 12, 2018.

  1. Mike Reed

    Mike Reed pfm Member

    Interesting. Not sure why you wouldn't want an RCD (or better, an RCBO). Surely doesn't affect s.q., does it? I think RCDs are one of the better progressions in electrical safety.
     
  2. Mike Reed

    Mike Reed pfm Member

    More for convenience, as far as I can glean. It saves having to route several hefty cables, sometime quite a long way, from user to supply. The other, and I'd imagine, more likely scenario, is when external routing is required and multiple hefty T & Es are not poss., so a 25mm2 (ish) armoured cable would be employed. With the C.U. close to the kit (or not, depending on circumstances), you can then choose how you want to attach your mains leads.
     
  3. coupe-sport

    coupe-sport pfm Member

    Yep. My electrician told me that this would be changing in the 18th edition. Going non RCD was what was recommended and could be safely done so that's what i did.
     
  4. Blackmetalboon

    Blackmetalboon pfm Member

    Apologies in advance for the long post!

    It’s not as simple as that as Type B, C & D MCB’s have very different time/current tripping characteristics. It needs to be ascertained that the earth fault loop impedance of the circuit is low enough, failure to do so could result with a MCB not tripping within the required time or worse still, not at all.

    It’s always worth checking what size conductor the socket is specified to take. I can’t find a direct link to the information, but it is in there!
    Here’s a pdf for MK;
    http://www.mkelectric.com/Documents...ifications/T03 LOGIC PLUS Tech403,405,382.pdf
    Crabtree; https://www.electrium.co.uk/media/20150521124826_0_CrabtreeWA.pdf
    Contactum; http://contactum.co.uk/content/x2356

    If you managed to dig out the relevant technical specifications you’ll see nothing about the terminals accepting 10mm cable.

    Mike is on the right lines, installing an earth rod (TT Earth) can, under certain fault conditions, potentially raise your earth to mains voltage.

    See my above reply, your electrician will be able to advise you on wether or not it’s possible.

    From July 2015 all socket outlets installed in a dwelling must be RCD protected.
    https://electrical.theiet.org/wirin...rements-for-rcd-protection-of-socket-outlets/

    See the above answer. There has been a slight change to this in the 18th Edition, but is unlikely to affect a domestic installation.
     
    Mike Reed likes this.
  5. Mike Reed

    Mike Reed pfm Member

    Well, that's one way to shut the power off !:D. Good post, though. Not sure if PME earthing is a combination of using the neutral back to the sub-station and another, more local, earth as the name implies, or simply the neutral.
     
  6. BilliumB

    BilliumB pfm Member

    What's the best radial cable configuration to use?

    I'm just about to fit a dedicated radial (I'm on TT earthing). One thing I'm concerned about is to use the most suitable cable - I'm not convinced Twin and Earth is the best option due to the significantly smaller cross section of the earth (CPC) wire, eg 4mm vs 10mm for the live and neutral in typical 10mm T&E. With star earthing etc it seems crazy not to use a cable with a much larger earth. I guess a key issue will be the impedance of the earth wire to higher frequency noise.

    A cable with a twisted live/neutral pair would also seem to be more appropriate for noise rejection.

    I know some people use large copper screened CY data control cables for radials, but understand that these don't formally meet the regs - probably because they don't have the correct approvals, rather than them being intrinsically unsafe.

    Other possible options are split concentric (often used for power feeds in blocks of flats etc, and meets the regs), or the Irish version of T&E that uses a full size insulated earth wire (however with the insulated cpc, this may not meet UK regs).

    Any thoughts / experience?

    Cheers. Bill
     
  7. Blackmetalboon

    Blackmetalboon pfm Member

    With regards to PME, this link should explain it clearer than I could!
    https://www.voltimum.co.uk/articles/principles-protective-multiple-earthing
     
    Mike Reed likes this.
  8. Mike Reed

    Mike Reed pfm Member

    As I've got a combination of 10 and 6 mm2 T & E, as do friends of mine, I really think that's all you need. The earth core is more than adequate, and if you're on TT, you'll be able to add a spike to the hifi end if you can be bothered. Far better to have multiple radials (even 6mm2 if routing is a prob.) in a multiple ganged c.u., pref. with RCBOs. One big cable split into hydras, double sockets etc. is undoing what you're trying to achieve, in effect. I have one radial per piece of kit (and/or a spare). Also more convenient for isolating kit separately for that odd occasion.
     
    Andrew C! likes this.
  9. Mike Reed

    Mike Reed pfm Member

    Bit technical for me, but basically, adding a TT earth to a PME system is optional and dependent upon the installer's analysis. I would guess that adding a TT might be the norm, though. This is my understanding from scanning the article. The 'multiple' bit presumable refers to the multiplicity of earth bondings en route to the grid (not just the one) and not to whether a TT has been added.
     
  10. Blackmetalboon

    Blackmetalboon pfm Member

    Yes, on the supply cable from the substation to your electrical supply head. The Neutral and Earth are the same conductor which is split in the properties electrical supply head to give you separate Live, Neutral and Earth conductors from that point onwards.

    Adding an earth rod/stake to your installation isn’t the norm, in the link I provided earlier, this is done on the supply side by the Distribution Network Operator (DNO) who is responsible for the electrical supply to your home.

    You can add an Earth stake/rod to your PME installation but it is rather pointless and an additional expense. It is classed as an extraneous conductive part, basically it is bringing in its own earth which isn’t part of the installation and could be of a different potential. It is for this reason that metallic water & gas pipes are bonded to earth at your main earth terminal (MET). So there is no difference between a metallic water/gas pipe and an earth rod in this instance, all need connecting back to the MET. Not doing this is where the potential problems can occur.
     
    Mike Reed likes this.
  11. mikebirtill

    mikebirtill pfm Member

    you can make a part of your installation tt with its own earth. usually done for external buildings and car chargers in a domestic setting. so i can't see any reason why you couldnt do this for ones hifi sockets other than theres really no point
     
  12. Curtis

    Curtis pfm Member

  13. martin clark

    martin clark pinko bodger

    If the incoming supply has PME available you really shouldn’t. It’ll provide a much worse functional Earth, and it is a really bloody daft idea.
     
    Mike Reed likes this.
  14. Blackmetalboon

    Blackmetalboon pfm Member

    True, it can be done when exporting the Earth may not be possible/advised. An external building would then be using the TT earth and any extraneous conductive parts would have to be connected back to the MET of this installation.

    Having a separate TT earthing system for your hifi sockets could give you a potential difference between earths. As it’s unlikely that the hifi sockets will be the only form of electrical points in the room and you also have to consider other metallic parts that could be bonded to the MET of a PME installation.
     
  15. AndrewM

    AndrewM pfm Member

    There have been incidents where the external PME earth on a supply has been compromised: eg careless use of a JCB, cables brought down in a high wind or even copper thieves making off with earthing equipment. The net result being that a subsequent short from live to neutral even outside your house looking for a way to ground can go into the house earth wiring and look for a way to ground! In the past, the connection at the consumer unit from the earth terminal to the water pipe provided a back up level of protection, but with the advent of modern plastic water supply pipelines that is now most illusory.
     
  16. BilliumB

    BilliumB pfm Member

    Hi Mike, thanks very much for the response.

    I honestly don’t know if you’re right or wrong about the 10mm T&E earth conductor size (4mm^2) being more than adequate - it’s a very long time since I did my electronics degree and I wasn’t brilliant at it then! I guess trying out the alternative is the only way to actually find out.

    However, simply from thinking through the issues, I do wonder if the large majority of the benefit from a dedicated mains radial setup is down to the effective creation of a ‘Technical Ground’ separated from all other interfering electrical equipment all the way back to the property entrance where it is bonded to the incoming earth (be it PME or TT), the lowest impedance ‘point’ in the earth system at the property. This would suggest that much of the benefit in going from say a 4mm T&E radial (CPC size 1.5mm^2) to a 6mm (CPC 2.5mm^2) or a 10mm (CPC 4mm^2) radial might be down to the CPC conductor size. I really must get hold of an oscilloscope to investigate further.

    Logically moving from simply connecting to a normal 2.5mm T&E ring circuit to a 10mm T&E radial should benefit the grounding setup by:

    1 - removing a large ground loop created by the ring circuit.
    2 - removing the voltage impact on the ground reference of earth leakage currents from all the other electrical kit connected to the ring circuit.
    3 - reduction in the impedance of the ground connection to the incoming property earth (PME or TT), through increasing the earth or CPC conductor cross sectional area from 2 x 1.5mm (ie total of 3mm^2) in a 2.5mm T&E ring to 4mm^2 in a 10mm radial (and from removing the added impedance resulting from a multitude of CPC to CPC connections in the ring at each socket).

    While the the reduction in impedance going from 3 to 4mm^2 is obviously worthwhile, I can’t help thinking that going from 3mm^2 to 10mm^2 would be even better. I then wonder if cable configurations other than ‘flat’ T&E might also be beneficial. My problem now is finding someone who has actually tested these thoughts!

    Cheers. Bill
     
    Last edited: Sep 16, 2018
  17. sls4321

    sls4321 pfm Member

    How does all this compare to using regeneration? I bought a PS Audio P3 years ago, put it in and forgot about it. Gives clean power with low impedance.
     
  18. AndrewM

    AndrewM pfm Member

    I can see the advantages of your points 1 and 2. Less so 3. Under normal circumstances in theory the earth connection is at 0 volts so any stray voltage/current leakage caused by impedance variations in the various connected ground planes is going to be extremely small, and I doubt the increased cross sectional area would make much difference unless the cable runs are very long.

    The single heavy radial used in conjunction with a hydra/multiple socket arrangement does have the advantage that any earth loop caused by two or more connected items each with mains earths plus a link via a signal cable screen grounded at both ends is kept as short as possible.

    For my own fairly simple system, with a run of about 5 metres from the consumer unit, I installed two radials using 6mm twin and earth each terminating in a plain unswitched MK double socket. Unless you are planning to run some large class A monster amp , imho anything more than that is overkill.
     
  19. BilliumB

    BilliumB pfm Member

    My concern is that any electrical noise on the ground reference will cause impact to sensitive parts of the system, possibly at the millivolt level.

    I’m interested to hear of actual experience from reducing the ground conductor impedance, which of course, is never going to be zero especially to higher frequency noise voltages.

    My thought is that improvements to this part of the system could make an impact vastly out of proportion to the cost involved - especially as I’m putting in a new dedicated radial anyway.

    Cheers. Bill
     
    Dossjohn likes this.
  20. mikebirtill

    mikebirtill pfm Member

    yes it is.
     

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