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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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    the electrician says that the loss of neutral at the network transformer without also losing live would be very unlikely.

    To put some numbers to that - DNOs report broken N events across the country at the rate in the region of one per day (with each event affecting potentially many customers) - or to put it another way, an average customer runs the risk of it happening about once in 1000 years.

    As well as broken N events, PME 'earth' can naturally be quite a few volts different from true earth under normal conditions too - so there can be a risk of 'perceived electric shock' in any event - not enough to electrocute, but enough to make people feel very unsafe and perhaps jerk back with an increased risk of other accidents - collisions, slips and falls etc.

    Overall the risks are such that it's actually against the law to use PME Earth for either caravans or boats, and the wirings regs ban it for electric vehicles unless mitigating (e.g. o-pen) devices are used. Likewise for swimming pools it's recommended against unless there's a buried grid of some sort to keep the ground surface close to PME earth potential.

       - Andy.

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  • 0

    the electrician says that the loss of neutral at the network transformer without also losing live would be very unlikely.

    To put some numbers to that - DNOs report broken N events across the country at the rate in the region of one per day (with each event affecting potentially many customers) - or to put it another way, an average customer runs the risk of it happening about once in 1000 years.

    As well as broken N events, PME 'earth' can naturally be quite a few volts different from true earth under normal conditions too - so there can be a risk of 'perceived electric shock' in any event - not enough to electrocute, but enough to make people feel very unsafe and perhaps jerk back with an increased risk of other accidents - collisions, slips and falls etc.

    Overall the risks are such that it's actually against the law to use PME Earth for either caravans or boats, and the wirings regs ban it for electric vehicles unless mitigating (e.g. o-pen) devices are used. Likewise for swimming pools it's recommended against unless there's a buried grid of some sort to keep the ground surface close to PME earth potential.

       - Andy.

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