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Hot Tub Earthing

John Ford

I am hoping the wonderful experts on this forum can answer some questions about a hot tub feed. I have purchased a hot tub for my home and the supplier has said it must have a TT earthing system as there is a risk of if neutral is lost on the house PME system with a fault on the hot tub the water could be live (not sure I have explained that well, I am a simple mechanical engineer). The hot tub suppliers view is that all hard wired hot tubs should have a TT earth.

My electrician has looked at the exact location of the hot tub and has said that there is a metal outside light, an outside socket and an air source heat pump close to the hot tub all protected by the house PME  he has a concern that under an extreme set of cirumstances / faults it could be that the PME and TT would not protect you, hsi strong preference is to stick with PME for all devices/sockets/lights/air source heat pump with the same earth type. The socket and light are not less than 2 metres from the hot tub per regulations. He also commented that the TT earth is only there to protect in a extremely unlikely circumstance, that is, the neutral source to the house is lost, the live remains to the house and there is a fault in the hot tub. Is there a right or wrong answer to earthing to a hot tub or a single solution that presents the least risk?

My registered electrician has also commented that the regulations call for an impedance less than 20 ohms for a TT earth and that this is extremely difficult to achive with a single 1200m earth rod. He believes we would need multiple earth rods to get the impedance below 20 ohms. What is the experience of the forum on getting TT systems below 20 ohms?

This is a live 'difference of opinion' between the electrician and the hot tub supplier so I would appreciate your expertise quickly. Thanks!

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    sounds like neither side is especially well informed, this is a common issue.

    1) Regs do not require an earth resistance of 20 ohms for TT - the only time 20 ohms pops up is for the regulations on substations and network operators, where a 'nest' of buried grd and earth rods is proportionate to the size of installation. There is good reason for this, as in many soil types (what is yours ?) a resistance of less than perhaps 100-200 ohms needs multiple electrodes. Equally, in Essex clay you may see 30-50 ohms off one rod, in sand or dry gravel you may see 500 per rod.

    The limit for a 30mA RCD protected cct is 1600 ohms, but that is a kitchen skewer in a flower pot. More sensibly aim below 100 ohms and  double up if it comes out much higher than this.

    The stuff outside earthed to the PME and in reach is more problematic. Ideally it needs to be out of reach or moved to the TT island supply, but that risk is there even without the hot tub, or maybe you are not barefoot...

    There is not a lot of point in earthing your pool to be at the same voltage as the ground it stands on (and a moment's thought about stepping out of it wet and barefoot will say why that is good) if you then grab hold of a lamp post or something at a different voltage, still barefoot and wet.

    The alternative is an insulating floor, but it is hard to arrange that outside and  so not one of the standard solutions, except indoors, where PME is OK because all the earthed things you can touch are at the same voltage, even if that s not the same as the surface of the ground outside,

    Also note the Regs do not require blind obedience of makers instructions, but does require them to be 'taken into account' So follow them unless you are very very sure of the counter argument.

    But the PME earth does not wander that many volts from Terra-firma very often - there are not piles of dead bodies under street lamps each morning, however, when it does fail it can be impressive. An example of what can happen

    Mike

  • 0 in reply to mapj1

    Gosh! Mike has just quoted from the Daily Mail. Are you quite well Mike? What a red letter day.

    Z.

Reply
  • 0 in reply to mapj1

    Gosh! Mike has just quoted from the Daily Mail. Are you quite well Mike? What a red letter day.

    Z.

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