All I can add to this is that the wiring regulations state that the manufacturer's instructions must be followed. So, if the manufacturer states that a TT install is required for their kit, then a TT supply you shall have.

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

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

Z.

Hi Mike,

Thanks for the detailed response. 

The supplier seems to be focusing on the risk of losing neutral with live remaining rather than the wall light and socket that are a short distance away. Naturally supplier has stated minimum distances for light and power socket. On the other side the electrician says that the loss of neutral at the network transformer without also losing live would be very unlikely. The outside light and socket are not within reach and the air source heat pump will be 1.25 metres away from the hot tub.

The base ground is sand/gravel and currently a 10mm / 3/8 earth rod is around 1.1 metres in the ground. 

So in summary you are suggesting that would be sensible to have a TT earth on the hot tub?

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.

So in summary you are suggesting that would be sensible to have a TT earth on the hot tub?

From what you have said, if you can step out of it onto bare earth, yes. But as a separate thing, when you stand next to and touch the heat pump, what are your feet on ? If it is bare earth, then easier mitigations than rewiring it may be some gravel or concrete paving slab - one discourages bare feet and drains water away quickly, the other acts as a current limiting resistor in series with the body. Neither are in the big book of best practice, but are ways of turning a small risk of a bad shock into a small risk of just an annoying one.

There must be no metal connection between the house earth and the TT earth, and where they come close - probably in a wall mounted box near the  hot tub then one or other should be insulated as potentially live part.
Common to use an armoured cable, where the armour connects to the supply earth, and operates a fuse or breaker if the cable is found by pick axe or drill, but at the load end the armour is 'gapped' and a plastic box and gland are used. From that point on the earth wiring is relying on the grounded electrode(s).
There must be an RCD before or at the transition to the TT island, and that RCD must be tested regularly.

Page 22 of this PDF has some info about electrodes and resistances, and the voltages near electrodes during faults that may be of interest.

Mike

Thanks Andy

Appreciate all your advice Mike. The plan was to place the air source heat pump on paving slabs given what you have said I will probably extend the slab area around the pump so it is not reasonable for anybody to touch it while on grass/bare ground.

The electrican has used SWA glands and all wires within the rotary switch are insulated. The rotary switch is protected by a 30ma RCD in the consumer unit with PME, the break in the earth for the rod is inside the plastic rotary switch housing. At the request of the hot tub supplier the heat pump is fed by a separate circuit from the consumer unit to a fused switch using the continous house earth, house PME, to pump. Should have said at the outset that the SWA cables x 2 (hot tub and heat pump) from consumer unit, rotary switch, fused switch connection unit and earth rod are all installed but not yet covered or final. So this chat is a little late but not too late that things cannot be modified to meet regs and be as safe as it can be.

Sounds like you have it under control, but do let us know how it goes. If the SWA armour is PME earth then the gland nearest the break ideally needs to be plastic or over sleeved, but in practice folk do not go around touching cable glands, and the risk compares to the average outside tap, also often PME earthed, and sometimes above wet ground.

Which raises another thought - how does water get to the hot tub - somewhere a plastic pipe may be needed to gap you if the house plumbing is all metal.

Mike.

whjohnson said:

All I can add to this is that the wiring regulations state that the manufacturer's instructions must be followed. So, if the manufacturer states that a TT install is required for their kit, then a TT supply you shall have.

But if you do that, note 411.3.1.1 contains a requirement for all systems where ADS is used: Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.

So, there IS a conflict here that ought to be addressed?