Electric shower installation - RCD confusion! - PLEASE HELP!

Discussion in 'Electricians' Talk' started by likes_electricity, Mar 9, 2005.

  1. likes_electricity

    likes_electricity New Member

    Dear all,

    I soon intend to install a 9.5kW electric shower to the home of a close friend whose bathroom I will be renovating. I have had previous experience with home electrics and have also worked with a qualified electrician rewiring a house once before, so I am not completely new to the subject. I have read the IEE installation method described on some DIY web sites and understand fairly well the proceedure for making such an installation. However, I am confused over one particular point of the installation.

    Whilst I understand the need for the use of an RCD unit in the connection of this device to the mains, I am not quite certain on how to use the unit.

    My friends CU is a fairly modern Wylex unit. It has a maximum rating of 100A on the main switch and is split into 2 sections, 1 with the mains ring and immersion heater on an RCD and the other with the remaining circuits such as the lights, cooker, etc.

    There are 3 empty fuse blocks (spares) on the side of the RCD. Whilst I understand that the shower unit must have its own 45A RCD, I am not sure whether to fit this on 2 of the 3 spare slots (shifting lower ampages across accordingly), or wether I should upgrade the existing RCD on the CU and add the shower to this through a 45A MCB or whether I should install a RCD unit seperate to the CU and have the electricity company link it to the meter.

    The other thing on my mind is that I will be installing a bath with a spa system and am not sure if this too will require a separate RCD or if it can be placed on the rail of the existing RCD in the CU.

    Please help if you have the answer. Thank You.


    P.S. I intend to get the final installation checked by a certified electrician before turning on the power, but I still want to get it done right so as to avoid timewasting.
  2. squeaky

    squeaky New Member

    LE,
    you have got a little confused. The shower requires its own 45A MCB protected by an RCD. Fitting a 45a mcb into one of the spare slots on the rcd protectd side of the CU is way to go. I take you are using 10mm T+E, that the earth bonding in the bathroom is present and correct and that you are fitting a double pole 45/50A switch to turn the shower on/off?
  3. likes_electricity

    likes_electricity New Member

    Thanks squeaky

    I did get a little confused, and what you say does make sense as long as I make sure the total load does not overload the existing RCD.

    Also, yes, I will be installing the shower using 10mm^2 wire, protected by metal shielding where it is in the wall. As I understand this must be run through a 45A Dipole switch or pull cord outside the shower room and then to the MCB as you have stated.

    Whilst the earth bonding in the house is probably lacking as the bathroom seems very old, I had intended to run a new 4mm^2 earth cable across all pipework and the bath itself, back to the earthing block on the meter.

    Thanks again. Please post any more advice or suggestions you have.
  4. likes_electricity

    likes_electricity New Member

    back to the earthing block on the meter.

    I mean to the main earthing block NEAR the meter, sorry.
  5. Addie

    Addie New Member

    the pull cord doesn't have to be out side the bathroom
    it only has to be far enough away so no one can touch the body of the switch whilst using the bath or shower and IP rated for that zone
  6. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Anyone any advice on running the earth wire from the bath and pipes back to the earthing block at the CU ?

    Handyandy - really
  7. sinewave

    sinewave Screwfix Select

    > Anyone any advice on running the earth wire from the
    bath and pipes back to the earthing block at the CU
    ?

    Handyandy - really


    Yep, take the easiest route! ;)
  8. kraftwerk

    kraftwerk New Member

    does a shower definately have to be protected by an rcd,are there any exceptions
  9. sparkp

    sparkp New Member

    not as long as your test results gaurantee 0.4s disconnection time
  10. sinewave

    sinewave Screwfix Select

    A shower doesn't need to be on a RCD at all, according to the Regs.
    But I wouldn't dream of installing one with out RCD protection, so you can please yerself!
  11. ChubbyPhaseWire

    ChubbyPhaseWire New Member

    not as long as your test results gaurantee 0.4s
    disconnection time

    ?
  12. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Anyone any advice on running the earth wire from
    the
    [/b]bath and pipes back to the earthing block at the
    CU
    ?

    Handyandy - really


    Yep, take the easiest route! ;-)

    Sorry, mate. Perhaps I wasn't too clear there.
    What I should have said is, Isn't it true that you should not take the earth bonding from the bathroom to the CU,
    but bond all to each other and take the incoming water pipe earth to the CU ?(Only)

    Handyandy - really
  13. sparkp

    sparkp New Member

    not as long as your test results gaurantee 0.4s
    disconnection time

    ?

    just quoting what my nic inspector told me when querying why i put the shower circuit on the rcd side of the split load board as i normaly do
  14. sinewave

    sinewave Screwfix Select

    Depends what you mean by the incoming water pipe.

    If you mean the incoming water pipe to the bathroom then that will be effectively the same as connecting the bonding in there back to the C/U wont it?

    Complicated area this, as if the main equipotential bonding of the water is done at source (as it should be) say in the kitchen, under the sink, and the water supply to the other services in the house is all in copper then effectivly it's all bonded together and at the same p.d.?

    So why not run bathroom bonding direct to the MET?
  15. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    As I see it (and remember I'm just trying to further my understanding of it for my own satisfaction)

    as if the main equipotential bonding of the water is done at source (as it should be) say in the kitchen, under the sink, <

    Then you will have in the bathroom(if you send that back to CU directly) THREE separate routes to an earth source.
    Pipes to CU, pipes through house pipes to CU and natural earth. Doesn't this introduce more possibility of potential difference ?

    Handyandy - really
  16. Kai

    Kai New Member

    It is SAFER in fact.

    My Immersion heaters have THREE paths to earth, all metallic, the CPC in the Night and Day Circuits, (Cross Bonded between the FCU's with 2.5.sq.mm. earth lead inside the mounting patress), AND a 10.0.sq.mm bonding cable between all the metalwork and run back to the main earthing terminal in the CU. The Zs for that setup is 0.36ohms, on a PME system, giving a 666amp earth loop current, large enough to pop the B16 Breaker/operate the 13amp FCU, within 0.4 seconds, in the event of a fault.

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