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Special Locations Pool Rebar Bonding

Steven Dwight

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?

Parents
  • +1

    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.

Reply
  • +1

    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.

Children
  • 0 in reply to mapj1

    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

  • 0 in reply to Steven Dwight

    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 !

  • 0 in reply to Steven Dwight

    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.

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