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TN-S RCD for TT system

Horace Horace

Hi im after some advice on a hot tub install. The house has a split load 16ed CU without non-rcd protected ways. I have run 30m of 6mm SWA from a spare way of the CU which (as i mentioned already) is DP RCD protected. I plan take the SWA to an insulated CU (enclosure) in the garden where I will isolate the TN-S earth (armour) by a nylon stuffing gland, and adopt a TT system for the hot tub with ocpd. I want a TT system in place to avoid anyone getting tingles due to earth potential differences im sure you understand this. My question really is if im gonna use a TT system for the hot tub, which obviously needs RCD protection, can I rely on the supply boards RCD for this. Ideally I would have wanted to take the SWA feed from a non RCD way (but there is none) or a switch fuse (but there is no room to fit a switch fuse) and have more local RCD protection. I seldom carry out TT installs so thought to ask as im unsure if an RCD on a TN-S system can be utilized to also protect a TT system ? Im assuming not but not but cant seem to fathom why it would not operate. Relying on the split load CU RCD would not cause too much inconvenience in the event of a trip as there is only a washing machine socket and cupboard light on that side of the board, similarly if I need to put another RCD in series with it more local to the hot tub. Thanks for any response

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  • 0

    Physically it'll be fine - modern RCDs work by detecting an imbalance between L and N currents and that'll be the same whether the escaping current finds its way back via the TN earthing system, a TT electrode or even directly to the general mass of the Earth.

    Regs compliance wise it's less clear. There is (to me a rather odd) requirement that earthing systems in TT systems should 'correspond' to protective devices (RCDs):

    411.5.1 Every exposed-conductive-part which is to be protected by a single protective device shall be
    connected, via the main earthing terminal, to a common earth electrode. However, if two or more protective devices
    are in series, the exposed-conductive-parts may be connected to separate earth electrodes corresponding to each
    protective device.

    which makes it sound like your setup might not comply (but still it's not entirely clear as half of what's protected by your RCD will be TN rather than TT). I suspect that 411.5.1 hails from the days of voltage-operated ELCBs where such an arragement makes a lot more sense.

       - Andy.

Reply
  • 0

    Physically it'll be fine - modern RCDs work by detecting an imbalance between L and N currents and that'll be the same whether the escaping current finds its way back via the TN earthing system, a TT electrode or even directly to the general mass of the Earth.

    Regs compliance wise it's less clear. There is (to me a rather odd) requirement that earthing systems in TT systems should 'correspond' to protective devices (RCDs):

    411.5.1 Every exposed-conductive-part which is to be protected by a single protective device shall be
    connected, via the main earthing terminal, to a common earth electrode. However, if two or more protective devices
    are in series, the exposed-conductive-parts may be connected to separate earth electrodes corresponding to each
    protective device.

    which makes it sound like your setup might not comply (but still it's not entirely clear as half of what's protected by your RCD will be TN rather than TT). I suspect that 411.5.1 hails from the days of voltage-operated ELCBs where such an arragement makes a lot more sense.

       - Andy.

Children
  • 0 in reply to AJJewsbury

    Thanks for the reply AJewsbury, I do speak and read English but often the regs (such as the 411.5.1) really confuse me how they are written. I read 411.5.1 and think well, there wont be any exposed conductive parts in this installation, the only conductive parts I can think of are the SWA glands which will be covered by their shroud, and possibly the water heating element which is not exposed ! There is the earth rod itself and its connection to the earth cable but this will be in a pit with a lid only removable by use of a tool; all the switchgear & equipment is class 2.

    I had a hunch that applying a single RCD to both TN and TT systems wouldn't sit well with the regs, maybe 411.5.1 confirms that but im not sure how to decipher it. I wonder if installing an additional RCD main switch at the outdoor enclosure would be better in this instance, even though it would be in series with the upstream RCD and offer no discrimination. As I said a trip would not necessarily cause any major inconvenience anyway. 

  • 0 in reply to Horace Horace

    Easily sorted. Just install another  new 30mA R.C.D. in a suitable enclosure near to the hot tub location. Job done. You then have two additional  lines of defence against electrocution. It has been suggested that R.C.D.s can fail in use, and not operate correctly when really needed.

    Z.

  • 0 in reply to Horace Horace

    In this case the exposed-conductive-part is the conductive hot tub water.

    Z.

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