TN-s if it really is TN-s will be fine.
The problem is that a lot of services that present as TN-s are really TNC-s, or at least have some of the risks of it, as additions and repairs to the DNO network occur , and often in the new wiring neutral and earth are combined.

TNC derived supplies can be more problematic, as there can be a voltage offset between the true ground (the stuff you stand on) and the earth in the sense of the CPC of the appliance.

Wet concrete is not a great insulator, better but not a lot better than bare earth, but equally, the fact this is a class 2 device means you wont be grabbing the CPC and standing well connected to the terra-firma earth at the same time, which is the real risk of tingles.

So  to the SELV lights - if they are SELV, they should be fine, as SELV implies Extra Low Voltage  ( 50VAC/120VDC or 50VAC 75VDC depending on your choice of international standards. ) and double or reinforced insulation between the Extra low voltage output and the Low voltage (Mains input). That said, the term is falling out of favour as it does not align well with how modern electronic supplies are made and then graded.
The regs also allow AC lights at 12V  that are not really SELV as an alternative, but the preferred lighting  for Zone 1 is indeed SELV. The switches that operate it need to be further into the dry  zone though if carrying mains.

EDIT having read other answers, and re-read this one, and apologies my initial reading of the regs was not correct - ignore the above. As it is potentially misleading, I have now struck it through.

I assume the IET guidance you refer to is similar to this wiring matters 2021 article, but if you have not seen it as well, then  this older but more wordy article is perhaps more comprehensive.

Mike.

mapj1 said:

The regs also allow AC lights at 12V  that are not really SELV as an alternative,

I don't believe that's the case for Zones 0, 1 and 2 for a Section 702 location (702.414.3 prohibits "non-SELV/PELV" conformant electronic sources). Recommend to check with the manufacturer that the supply is actually SELV (at the appropriate voltage limits, see below).

Similar prohibition on sources to 413.3(iv) apply in Sections 701, 703, 706, and 710.

The reason for this prohibition, is that some product standards do not conform to BS EN 61140, and permit, in some fault conditions, the touch-voltage to rise above that above the relevant voltage limit for the location (or 50 V AC/120 V DC in general), and the touch-current to exceed that available from a suitable isolating transformer. Some product standards base this on the risk of the product being in dry locations (which, for example, the PSU may be) but don't consider the power from the product may be used in a 'special location'.

mapj1 said:

So  to the SELV lights - if they are SELV, they should be fine, as SELV implies Extra Low Voltage  ( 50VAC/120VDC or 50VAC 75VDC depending on your choice of international standards. )

And again, in Zone 0 of a Section 702 is limited to 12 V AC/30 V DC, and Zone 1 limited to 25 V AC/60 V DC (702.410.3.4.1).

Zone 1 extends 2 m from the edge of Zone 0, so in that case appears as the lights may be in Zone 1, so the 70 V DC is a problem.

There are ground mounted floor lights within 2 meters of the tub on the steps above the tub level.

Playing devil's advocate, the definition of zones seems to be limited to 'the floor or surface expected to be occupied by persons' - so (by the letter if not the spirit of the regs) would these lights be below zone 1 rather than in it?

   - Andy.

Good point, but you could exit the tub and stand on a light fitting within 2 meters of the tub, so surely I'd say its in zone 1.

However, the fittings are mounted in a aluminium canister, with IP rated connectors beneath the ground in the bottom of the canister.

The actually fittings are made of solid copper, and are Im presuming Class 2.

I guess the main question is:  Should a plug and play type hot tub which is being permanently connected comply to section 702?

By your helpful and appreciated replies Id say yes.

Ive since realised the fittings are available in a 24v DC constant voltage format. So hopefully I can get these changed, before they are shipped.

Regarding the earthing:

702.410.3.4 Application of protective measures against electric shock

NOTE: Where a PME earthing facility is used as the means of earthing for the electrical installation of a swimming pool or other basin, it is recommended that an earth mat or earth electrode of suitably low resistance, e.g. 20 ohms or less, be installed and connected to the supplementary protective equipotential bonding.

The chances of an earth stake getting lower than 20 ohms on the rocky ground are very slim.The tub is fed off a local distribution board in a shed, which in turn is fed via a 30m x 10mm swa run back to the main plant room. Near the main plant room in a service void there are structural steel rsj's embedded in the foundations with a sub 20 ohm reading.

I'm thinking of connecting the earth core of the SWA via a separate earth bar in the plant room to this steel work as an earth electrode. I could keep the armour connected to the TN-S system and isolate it via a plastic gland at the shed and use the TT connected 10mm core as the earth. The above reg doesn't specify where the electrode or earth mat is located.

Or am I interpreting the reg wrong : earth electrode of suitably low resistance, e.g. 20 ohms or less, be installed and connected to the supplementary protective equipotential bonding.

Should the structural steel work as an earth electrode be connected to the main earth terminal (TN-S) as a supplementary protective equipotential bonding?

If so Ill have a TN-S which could be PME, supplemented by a TT system. At this rate my head is hurting more than a perceived electric shock!

AMC said:

The actually fittings are made of solid copper, and are Im presuming Class 2.

Can we assume Class II, if they are supplied at ELV intended to be SELV?

That makes far more sense. Ignore my previous answer as complete bull based on misreading the advice.

M

I'm thinking of connecting the earth core of the SWA via a separate earth bar in the plant room to this steel work as an earth electrode. I could keep the armour connected to the TN-S system and isolate it via a plastic gland at the shed and use the TT connected 10mm core as the earth. The above reg doesn't specify where the electrode or earth mat is located.

If I'm reading this right, the SWA would basically be on the TN system, but the 3rd core would be the earthing conductor for the TT'd shed? And the 3rd core would only be separated from the live conductors of the TN system by basic insulation.... so a fault (say impact damage to the buried SWA) could connect TN L to TT PE ... make the TT system live but not necessarily trigger ADS.

   - Andy.

AMC said:

I guess the main question is:  Should a plug and play type hot tub which is being permanently connected comply to section 702?

By your helpful and appreciated replies Id say yes.

This is not really an engineering question. The real one is whether a judge would include a hot tub within the definition of, "Swimming pools and other basins". 702.11 (scope) includes e.g. paddling pools, but excludes natural features.

I do not think that there is any doubt: the common feature is that nearly naked people can stand within the structure. I would say that the size and depth are irrelevant.

AJJewsbury said:

If I'm reading this right, the SWA would basically be on the TN system, but the 3rd core would be the earthing conductor for the TT'd shed? And the 3rd core would only be separated from the live conductors of the TN system by basic insulation.... so a fault (say impact damage to the buried SWA) could connect TN L to TT PE ... make the TT system live but not necessarily trigger ADS.

That would be very unlucky for a shovel to impact the cable through the earthed (TN) armour into the phase conductor and (TT) earth conductor. without operating the protective device.

So if I was to ditch the separating of the earthing systems and follow the recommendation on 411.4.2 regarding TN System 

411.4.2

It is recommended that an additional connection to Earth, by means of an earth electrode in accordance with Chapter 54, is made to the main earthing terminal. This recommendation does not apply to outbuildings of dwellings served by the installation.

I could introduce the Structural steel work near the Main DB in the plant room as additional earth electrode.

Id then satisfy this reg with using the TN-S earthing that's been provided?

AMC said:

702.410.3.4 Application of protective measures against electric shock

NOTE: Where a PME earthing facility is used as the means of earthing for the electrical installation of a swimming pool or other basin, it is recommended that an earth mat or earth electrode of suitably low resistance, e.g. 20 ohms or less, be installed and connected to the supplementary protective equipotential bonding.

Found this video interesting from efixx regarding 411.4.2 https://youtu.be/_owGBQia29U?si=WzwImtxOvKo8BtHU