Consumer Unit selection/layout and Bonding queries

Discussion in 'Electrics UK' started by markocosic, 3 Feb 2013.

  1. markocosic

    markocosic

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    Following on from this thread:

    http://www.diynot.com/forums/electr...btree-starbreaker-backward-compatible.353859/


    New meter being fitted 20/02/2013 by DNO.

    I'm going to supply some 25mm^2 tails and a switchfuse (cheaper than an isolator switch) and ask the engineer nicely if he wouldn't mind wiring to the switchfuse rather than the CU, and replacing the DNO's 16mm^2 cutout<>meter cables with 25mm^2.

    On TheKeymeister's suggestion I'll send the DNO a photo of the cutout, and see if they fancy swapping it for a spunky new one with an 80A or 100A fuse rather than the existing 60A fuse.

    Switchfuse:

    http://www.tlc-direct.co.uk/Products/CGMSF100.html


    The existing CU is obsolete, with blanking plates missing and no RCD protection on some circuits. A new one, fully populated, is relatively inexpensive:

    http://www.tlc-direct.co.uk/Main_Index/Consumer_Units_Index/MK_17_Consumer_Units/index.html

    It comes with 1x 40A, 4x 32A, 2x 16A and 3x 6A MCBs.


    I can swap one of the MCBs for a 50A affair to take a 10.5 kW shower at a later date:

    http://www.tlc-direct.co.uk/Products/MK5950S.html

    As the cable size at 10mm^2 and either in it's own surface trunking, in uninsulated floor void, or buried in solid plaster, hence adequate, I could fit this from day one, correct?


    Option 1:

    Run the shower (50A, 10 mm radial), immersion (16A, 2.5 mm radial), and lights, (2* 6A, 1.0 mm radials) from one RCD.

    Run the hob (40A, 6 mm radial), sockets (2* 32A rings), and electronic miscellaneous (alarms and CH, 1* 6A, 1.0 mm radial) off the other RCD.

    This avoids lights going out when you're drilling through an electrical cable at the top of a ladder/jabbing a knife in a toaster at 4am, and splits the load across the (63A only) RCDs.


    Option 2:

    bernardgreen suggested keeping the shower separate from the main CU in the linked thread.

    Run the hob (40A, 6 mm radial) and lights, (2* 6A, 1.0 mm radials) from one RCD.

    Run the sockets (2* 32A rings), immersion (16A, 2.5 mm radial), and electronic miscellaneous (alarms and CH, 1* 6A, 1.0 mm radial) off the other RCD.

    Run the shower off it's own 50A RCBO mounted in the same CU:

    http://www.tlc-direct.co.uk/Products/MK7939S.html


    Option 3:

    As above, but take the output of the switchfuse to a double pole connector block/henley:

    http://www.tlc-direct.co.uk/Products/MK1100.html

    Then run an entirely separate CU fed by 25mm^2 tails just for the shower.

    I'm less keen on this at you've now got three "isolator" switches, only one of which is guaranteed to kill your circuit whereas with 1&2 either main switch kills everything. More work/expense too.


    Over to you, experts! Which would you recommend? Something different again?



    Bonding:

    At present, there's a pipe earthing strap clamp around the lead-sheathed incomer that runs to the E7 time clock (underneath the DNO's seals) and direct to the earth bar in the CU. There is no separate MET. No idea whodunnit.

    There is an earth lead dangling in the cupboard, which ran from the CU earth bar to nowhere. I'm guessing that this used to bond a gas meter, and this used to live under the stairs but now lives outside. The current gas meter is not bonded at all, and is fed by a plastic sheathed pipe.

    The water supply is a 15 mm OD copper pipe from underground. This is bonded to the hot water supply. The straps appear dirty, and they've used the strap as the connector across two clamps rather than a piece of wire. The earth cable runs back to the CU earth bar.

    The CH is not bonded directly. It does have a bond via the hot water pipes <> heating coil in the HW cylinder <> boiler and CH pipes/radiators. I haven't got the gear to check impedances and haven't had this checked yet.

    All the earth cable is 10mm^2 (same size at the shower cable). The meter tails are 25mm^2 but the cutout<>meter cables are run in 16mm^2.


    The pipe strap around a lead sheathed incomer looks sketchy to me. Is it the DNO's responsibility to provide a good earth?

    As I understand the regs, 10mm^2 is ok with 16m^2 meter tails, but if I have these swapped for 25mm^2 by the DNO chappie then all the earth bonding needs to be 16mm^2, correct?

    I should bond cold, hot, and CH pipework (big metal towel rail in a bathroom with an electric shower). It might not need it/might bond adequately through the pipes but the delta cost is pennies and it'll save egg on face when the LABC checks it all out.

    Gas meter bonding...

    It's outside and the boiler, bonded by both it's (tiny) electrical supply and indirectly via it's flow/return pipes, is inside the kitchen about a metre away. I'm not too fussed about the safety implications of bonding this, as there's nothing within 2 metres that's powered except for the boiler's own (earthed, and fused at 3A) electrical supply. What are the rules though? Running 16mm^2 to this would be a right royal PITA, as its in the opposite corner of the house to the electrical gubbins.



    Other:

    -The house had an attic conversion with 4 double sockets daisy-chained (radial, or spur off a spur off a spur, how would you describe?) and hooked into the upstairs lighting circuit. Can I add a 13A fused faceplate to the ring main and hang the lot off the output of that? It'll be quick/dirty/good enough for a hairdryer.

    -Is a shed fed in 2.5mm^2 by a 13A fused faceplate on the ring acceptable?

    -Is a double outside socket fed by a 4mm^2 super off the ring acceptable? (it's the other side of a wall to an existing socket)

    -Kitchen fitters have been present. Amongst many other kludges they've moved the cooker switch and to do so joined the supply with chocolate boxes, plastered over it, and stuck tiles over the top. I understand that connections need to be accessible now. The supply runs through the ceiling/floor void, then down the kitchen wall. Am I ok to cut this cable upstairs, and fit a junction box under the floorboards in the bedroom. (this is accessible by lifting the carpet/lifting a marked/non-screwed floorboard - adequate?)



    Many thanks for your patience and sage advice if you've got this far!


    For your horror/amusement I've provide a "double socket conversion" by the previous (Polish) owners:

    [​IMG]
    IMAG0696 by markocosic-yahoo, on Flickr

    [​IMG]
    IMAG0695 by markocosic-yahoo, on Flickr

    Hey! They used tape at least... :rolleyes:
     
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  3. flyingsparks

    flyingsparks

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    Thread too long.. Gave up reading after the 4th line sorry.
     
  4. streetlighter

    streetlighter

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    Well the ultimate option for circuit protection would be each circuit that requires RCD protection fed from an RCBO with them all fitted in a main switch consumer unit, cost is the issue here an RCBO can cost between £20-60 each but it is the best option avalable.

    If you are thinking of MK consumer unit wickes have them on offer at the moment a dual RCD split load with 10 MCBs is £46 inc VAT

    That socket conversion is a right mess, makes you wonder what else the previous owners have messed up with !
    I would get an Electrician to carry out an Electrical Installation Condition Report before even starting work on this installation.
     
  5. markocosic

    markocosic

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    Good tip on wickes, thanks!

    Most of electrical install is reasonable, I've had floorboards up and all the faceplates off. Cooker bodge, attic socket bodge, double socket bodge - all easily reversed. Bonding upgrade and rcd all circuits and it'll be to current specs as far as I can see.

    Check gets done when labc visit, no need for eic now as far as I can see - fixes needed are clear enough?
     
  6. EFLImpudence

    EFLImpudence

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    As said, too long so I shall stick to Earthing and Bonding.
    Learn the difference.
    Only parts which are already earthed may require bonding.
    This should be bonded from your side of the stopcock to the Met.

    No need.

    Not likely to require it but should be tested to determine if it is extraneous.

    No need.

    You will need to get one.
    Parts requiring bonding cannot be determined by looking or describing.

    That's because the DNOs don't waste money on oversized conductors.

    Get them to apply the correct connector clamp.
    BS 951s should not be on the sheath.

    No. Just because you are fitting larger tails than necessary does not mean you have to increase the size of the bonding.
    Obviously if the supply is increased to 100A things will change.
    On TN-S systems you can use the adiabatic equation to determine the Earthing and, hence, Bonding conductor sizes.

    If all circuits are protected by RCDs and Main Bonding is correct and disconnection times are met then no supplementary bonding is required.
    Do not apply bonding to isolated metal parts - more testing.

    As the water it should be bonded where the pipework enters the premise to the MET.

    Boilers do not require bonding.
     
  7. OwainDIYer

    OwainDIYer

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    I would split the lights and sockets across two RCDs, lights for A and sockets B on one RCD and lights for B and sockets A on the other.

    Then if the lights go out you still have power for table lamps etc.

    Put the loft light on the downstairs lights, as if you're in the loft you may be working on the upstairs light circuit.
     
  8. markocosic

    markocosic

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    Earthed - connected to the ground or connected to something connected to the ground

    Bonding - ensuring that all of these are tied together and to the MET to minimise any difference in potential

    I'll have used the terms interchangeably as it's not the day job - sorry - but bonding is what I'm asking about!


    I understand what you're saying with regards minimising the number of connections/use of materials, especially where you've RCDed everything, but where labour is "free," materials are "cheap," but the cost of failing an inspection is large I've got different priorities to a pro.

    Got it. Must bond CW and Gas by the rules. However HW is connected to CW by the mixer taps (all copper pipework). Boiler primary loop is connected to HW via hot water cylinder. CH loop is connected to boiler primary. Towel rail and rads via CH loop.

    It *is* connected electrically and *is* in a bathroom. It *may* meet impedance requirements through it's own connections, especially as all is RCDed and provided that they work the disconnect times will be short enough to protect, but as I've not go the kit and the delta cost to bond is about a tenner versus 180 notes for LABC to visit twice I'd like to hedge that bet.

    This is ok by the rules, albeit wasteful in materials, right?



    That was my thinking: wire for 100A supply pre-emptively. If the DNO do change it FOC or for a reasonable fee then it's all ready to roll without re-work/re-inspection. Tenner extra to do so; faff of 16mm^2 cable is the same as for 10mm^2.


    Two phonecalls so far...

    Phonecall #1 told me to naff off and have the electrician fit an earth rod/make it a TT system as "it wasn't their job."

    Scratched head and asked Google some.

    For phonecall #2 I asked "What they would like me to do with their extraneous conductive part that's simultaneously accessible with the house equipotential zone?" Oh, and by the way the cable has been visibly crushed by a pipe clamp with an earth that runs into your Economy 7 meter under the seals and was presumably fitted by the board. "That sounds like it should be a TN-S or TN-C-S earth, but we'd normally provide a terminal with that." They're now inspecting. ;)
     
  9. ban-all-sheds

    ban-all-sheds

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    Do those priorities include deliberately making things more dangerous?
     
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  11. markocosic

    markocosic

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    No - hence asking if there's a reason not to bond these. Not "is it required? but "would it be bad to?" A pro would want to minimise material costs; labour too if paid by the job rather than hour. I'm less fussed by the material/time cost but want to avoid the re-inspection.


    Is having a high Z bond to the HW/CH (via hot water cylinder etc) safer than the low Z bond? No bond is ideal, but that's not what I have. (it is all electrically connected at present)


    Line of reasoning A says that you'd get a lower fault current through a person in the event that the shower/light fitting/extractor fan become live and the bonding of everything in the bathroom is rubbish. (or better yet completely isolated)

    Line of reasoning B says that you'll get a lower fault current through a person in the event that the radiators/taps/sink become live through a wiring<>pipework short/fault. (as it'll be tied to earth/voltage can't rise as much)
     
  12. ban-all-sheds

    ban-all-sheds

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    It would be if they were not extraneous-conductive-parts.


    No - a pro would want to do it properly.


    Possibly.


    It's earthing which guards against that, not bonding.


    That's not what bonding is for.
     
  13. EFLImpudence

    EFLImpudence

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    Extraneous parts will be earthed by THE ground (planet)
    Exposed parts will be earthed by the circuit CPC - to ensure operation of the CPD.
    Main bonding - Yes.
    Supplementary bonding is to equalise potential difference in the event of a fault - between simultaneously accessible parts.
    It is only applied in a location, i.e. a room, namely bathrooms etc. where extra hazard exists because of the body being wet.

    If you wish to apply supplementary bonding as if RCDs were not installed then that is up to you.
    It must still be done correctly - not connecting things with g/y wire willy-nilly because it is there and is metal.

    Yes, as said.
    Irrelevant unless in a bath/shower room etc.

    Presumably they are in the bathroom so can (if not isolated) be bonded to other extraneous parts and the CPCs of all the circuits of the location.

    I suppose that should have started with 'If', then yes.

    Yes, but only in the 'special locations'.
    It is not needed elsewhere, e.g. kitchen.

    With 100A TN-S supply - 10mm² Earthing conductor and 6mm² Main Bonding is adequate but I appreciate that you may get an inspector who doesn't know how to work it out and only reads the dreaded 'guides'

    It is their responsibility to maintain the earth connection.
    You should not tamper with the incoming cable.
     
  14. JohnW2

    JohnW2

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    It is presumably very likely that the CH system plumbing will have a low impedance path to earth via the (hopefully already bonded) water supply piping. What sort of testing would you propose should be undertaken in order to 'determine whether the CH pipework etc. is extraneous', and how would you interpret those tests?

    Kind Regard, John
     
  15. EFLImpudence

    EFLImpudence

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    Bearing in mind that the question and reply were relating to main bonding.

    As you know it cannot be determined if the central heating pipework is extraneous without disconnecting it from the boiler and the rest of the pipework.
    However, unless any of it is buried in the ground, or similar, it will not be.
     
  16. JohnW2

    JohnW2

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    Not by testing, no - which is why I was surprised when you suggested testing. In practice, as you go on to say ...
    Exactly. In practice, one has to determine whether it is 'extraneous' (which it very rarely will be) by inspection, since there is no practical way of doing it by testing (without dismantling the plumbing!).

    Kind Regards, John
     
  17. EFLImpudence

    EFLImpudence

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    It is possible a new boiler may be fitted.

    It is a long enough question already without asking anything else.
     
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