Special Locations Pool Rebar Bonding

Hi all, in a domestic pool installation, the contractor has formed the concrete base of a swimming pool, linked to the main house, and not bonded the slab rebar. My understanding is that the contractor will now need to expose the rebar and bond back to the main earth terminal. My concern is this will affect the integrity of the pool slab (future weak spots and leaking) and also whilst trying to achieve complete rebar continuity could result in many holes. I believe there is insulation and screed going on top of the slab (I maybe wrong), could a secondary earth nest be sat on top of the slab or would this create more issues by in essence adding secondary extraneous conductive parts?

  • This needs to be considered along with your installation earthing question.

    engx.theiet.org/.../domestic-2-phase-system-and-additional-single-phase-supply

  • you wont need complete re-bar continuity, just two widely spaced points of connection that you can do a ring-round meter test to. The steel to steel contact does not need to  be great, after all the resistance of the concrete is in series between it and terra-firma, and the same concrete envelopes all the rebar thus linking any poor twizzle wire contact. Actually, unless the rebar is folded up the sides in a way that means accessing it is easy, beating holes in a swimming bath is a really bad idea....
    Can you get away from PME earthing and the need to earth the rebar altogether ?
    Mike.

  • I'm probably being dim ... but where has this requirement for bonding rebar come from? Unlike 705 I can't see any particular reference to rebar in 702. There is a reference to an earth mat or electrode providing a resistance of 20 Ohms or less when using PME, but there doesn't seem to be any guarantee that the rebar would achieve that (if there's a plastic sheet under the slab, it's probably quite unlikely). Is the assumption that the rebar is an extraneous-conductive-part? If it's not accessible from within the installation (either directly or via something conductive, e.g. damp concrete) is that the case here?

       - Andy. 

  • There is a reference to an earth mat or electrode providing a resistance of 20 Ohms or less when using PME, but there doesn't seem to be any guarantee that the rebar would achieve that (if there's a plastic sheet under the slab, it's probably quite unlikely).

    See also G12/5. It's also commonly done for ground floor shower blocks in sports clubs etc ... and cattle sheds.

    No "firm requirement", but if there is a grid it should be bonded.

    There is no guarantee that such a grid will prevent "tingles" from diverted neutral currents - it depends on the magnitude in the area. A TT approach might be recommended where this is practicable (e.g. suitably separated outbuilding).

  • Hi, thanks for noticing the two posts are linked. We advised our client that we may have to go down the 'TT' route, as I personally wasn't comfortable with two supplies into the one same building. (especially as the primary supply is two phase which I am not completely familiar with). I should mention, for clarity the pool hall is internal forming part of the main building, it is not remote from the building. 

    I've not had chance to liaise direct with the DNO designer and query with them how they believe the the correct way to earth site based on them providing 2 supplies.

  • Hi Mike, thanks for your reply.

    I am considering a 'TT' system as previous experience leads me to this method when 2 supplies are provided to one site. The client is a civils contractor would you believe, whose concrete base and retaining pool walls have been formed to the highest standard. The suggestion that we may need to expose the rebar to ensure bonding, therefore forming holes where potential leaks may form went down like a, well I think you can guess how it went down...

    The electrical contractor is visiting site this week with a well know earthing & lightning protection specialist, I'm interested as to their understanding of what's required.

    I've been referring 'to bond or not to bond: domestic swimming pools' wiring matters by Paul Harris.  

    Would the introduction of a 'TT' earthing system therefore not require bonding of the pool rebar? I understood that the rebar is classed as extraneous therefore requiring a link back to the main earth terminal.

    Steve

  • Hi Andy, thanks for your reply.

    You are correct, I am assuming that the rebar is classed as extraneous therefore requiring a link back to the main earth terminal.

    Its the damp concrete that worry's me, and how porous it may be.

    I'll need to ask the contractor if a plastic sheet has been installed as part of the slab build up, I would expect so.

    Steve

  • Hi Graham, thanks for your reply

    The pool forms part of the main house, one building. 

    My thoughts were to provide a 'TT' earthing system due to the proposed 2No DNO supplies into the property. (although until I speak with the DNO I'm not entirely sure if their second supply is terminating into a GRP enclosure on the site boundary).

    To provide a TT system feels like the correct method as we are then in control of a single earthing system for the entire property, not a hybrid of two earthing systems that are linked (which I'm not sure is compliant)

    Steve

  • Just to say there's quite a bit of 'experience' in the US with pools and the electric tingles.

    Mike Holt has a number of videos on his NEC code channel (https://www.youtube.com/@MikeHoltNEC, among others) and carefully staged experiments on their types of earthing, bonding & leakage, along with notes about hazards to animals and the tingle being especially 'detected' by pregnant women.

    Then again, many of the Florida pools are outside !

  • Arguably, at least in a TT situation, there is no credible way the concrete and rebar can introduce a different potential to system earth - though it may be a few volts off the system neutral derived earth, and as it is 'out of reach' - go on you push your hand through the concrete, it is about as important to bond it as the door frame, i.e. not at all..

    The need for bonding it it arises when the touchable poolside hardware, steps pumps lights etc is at some elevated system earth potential - i.e. the PME neutral voltage, when without the rebar grid  the water could be  some  ten volts off, being loosely held at terra-firma earth potential. The rebar connection 'lifts' the pool so it all bounces up and down together with system neutral voltage, 

    Farms with dairy herds have the same problem - cows are long wheelbase and the contact surfaces tend to be wet with 'conductive solutions' at both ends.  Cows won't be comfortable being milked if there are significant voltages between the milky bits and the snuffly bits that touch the guard rails.
    Given most farms in the UK are older buildings repurposed and extended over many years, the solution in the UK is almost never to have a rebar grid under the brand new milking parlour, but instead to TT the lot, and have hierarchical RCDs of increasing time delay towards the origin.
    You can sometimes see a similar approach with TT installations within houses where the local sparks do mostly farm work- folk are creatures of habit. (Parts of Hants where I live are quite rural and there are villages with rows of semi-detached houses near me that are all TT on overhead singles.  All that varies is if the incomer is a 1970s VOELCB, a 1980s 30mA RCD, or a modern 100mA time delay S-type RCD. Southampton has majority of PME and TNS however.)

    Mike.