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Extraneous conductive parts

Former Community Member
Former Community Member
Hello,


Thinking about a domestic dwelling with main bonding to incoming water and gas pipes (even though most are plastic nowadays); all the electrical circuits within the dwelling are protected by RCDs; the only extraneous conductive parts to the bathroom being copper water pipes and copper central heating pipes... Why would the copper pipes need to be main bonded one to another close to the bathroom (in an accessible place for testing, like in an airing cupboard)?


Designing an installation which uses the cold water pipe in place of a main bonding cable (having a cross-sectional area which is greater, after all), why would any of the other three pipes need to be main bonded to that cold water pipe, when all of the pipes connect at the boiler any how?! Yes, if you were to cut all of the pipes and to replace them with plastic pipes then you risk introducing an electrical potential into the bathroom should a fault occur, but wouldn’t anyone doing that plumbing be obliged to consider this risk at that time? 


Otherwise, there is no point considering the use of copper pipes to replace main bonding cables. In which case, it would be necessary to bond the pipes one to another just outside the bathroom and run the cable all of the way back to the consumer unit.


I must excuse myself for being indolent and not referring directly to the wiring regulations, which is from where these ideas stem.
  • 0

    Supplementary Bonding is no longer required, provided that all circuits are protected by RCDs

    Plus the other two conditions.


    and that all copper pipes (and any other extraneous-conductive-parts, for that matter), are ‘effectively connected by main bonding conductors to the earth terminal of the installation’ (page 93 of the On-Site Guide).

    Not ALL pipes. Only those that ARE extraneous-conductive-parts to the bathroom.


    Whether those pipes are ‘effectively connected’ can be tested by performing a ‘continuity test’

    From the pipe to the MET.


    (and rectifying matters, should anyone happen to have replaced a section of copper pipe with plastic, without taking necessary precautions to protect the ‘main bonding’...).

    Should a pipe have been isolated from every contact with earth by the insertion of lengths of plastic pipe that isolated pipe MUST NOT be earthed by being bonded unnecessarily.

    It would be better if ALL pipes were so isolated.


    The operation of an RCD when an earth  fault occurs depends upon all extraneous-conductive-parts being effectively connected to earth via the main bonding conductor (10mm2 for TN-C-S, see Table 4.4(ii), page 48).

    Not really, it only requires the relevant CPC. Bonding is to equalise potential between the parts -
    but not parts which are isolated and are NOT extraneous-conductive-parts.


    Apart from being required to be connected to extraneous-conductive-parts within 600mm of the entry point to the dwelling (eg. gas and water pipes),

    The Main Bonding Conductor shall be connected to the e-c-p AT the point of entry to the premises - where practicable.  Practicable means where it will achieve the desired result.

    The 600mm only applies  for internal meters (and even that is not actually a good idea) so does not apply at the point of entry .

     

  • 0
    The operation of an RCD when an earth fault occurs depends upon all extraneous-conductive-parts being effectively connected to earth via the main bonding conductor

    Not really - the RCD operates (trips) due to an imbalance of L and N currents - and that imbalance would usually return via the c.p.c. - so the bonding of extraneous-conductve-parts doesn't really make a difference to whether the RCD will operate or not (but the Earthing of exposed-conductive-parts does).


    What the RCD (and any other means of ADS) does do is make sure that the voltage on the c.p.c. (e.g. as a result of a L-E fault) doesn't differ from that on the MET by a hazardous amount for a hazardous duration - the main bonding (pretty much) does similarly for extraneous-conductive-parts (limiting the voltage difference from the MET) - so the combined effect is that it should be very unlikely you'd be able to receive a fatal shock between any exposed-conductive-part and any extraneous-conductive-part within the bathroom.


       - Andy.
  • Former Community Member
    0 Former Community Member
    Thank you to everyone. I like to tell my family that I am great on theory (not so great in practice). God help me!
  • 0
    If items located in the bath/shower room are above 2.25m, would they need to be bonded as part of the supplemental bonding system ? For example, the lighting that is switched via a pull cord with class 2 fittings and a 12v window fan ceiling mounted above 2.25m.

    As the 701.415.2 reg mentions "the location" and does not specifically reference "within zones". Does this mean the bathroom/shower room in it's entirety ?
  • 0
    The height or zones have nothing to do with bonding requirements. - other than they might not be simultaneously accessible with other parts.


    Class 2 items cannot be bonded but the regulations say the CPC of the circuit supplying them shall be connected to the bonding.

    This is unnecessary but just in case it is needed in the future for Class 1 items.


    The definitions of 'special location' are different in BS7671 (the room) and The Building Regulations.(within zones).
  • 0
    Apologies, I should have typed "connected to as part of the supplemental bonding system". Not "bonded".
  • 0
    I don't think there is any need to apologise and I wasn't correcting you, but merely pointing out the regulations' requirement for a CPC which is not needed and that it should be connected to the bonding even though this is not needed either.


    Even if it were Class 1 light fitting, it still might be out of reach and so not require bonding anyway.

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