Another Hot Tub Thread Part 2

[Midwest]

Following on from the last two more technically debated threads:

32A supply for Hot Tub of a PME supply
Another Hot Tub install

I posted a question, in the last one, but didn't get a reply, so starting another one. Hope the Mods don't mind.

So posting my question again, slightly edited.

I’ve been reading lots of threads, about hot tubs & the like, using a PME supply.

I’m thinking of buying one of those inflatable hot tubs Lazy Spa, primarily as a glorified paddling pool for my grandson. I was quite happy to plug into my outdoor socket on my composite (plastic) decking; I have a PME supply.

But since then, I've been reading all sorts things about extending pme (to an outside environment), and I've been contemplating whether to risk it. Manufacturer recommends connecting to equipotential bonding terminal

My options I know, are to either TT just the supply to the tub socket, use my pme supply and try and supplement that with a rod of <20. My sub soil is littered with rocks. Or just use the pme supply without a rod, and don't worry cause its on my plastic decking.

Or just forget it, and keep filling the paddling pool!

Your thoughts?

 

[Midwest]

Why not use existing way in existing cu to rotary isolator and TT from this location?

Are you saying use the pme, and add a local earth rod?

 

[bigspark17]

Are you saying use the pme, and add a local earth rod?

no isolate the pme in rotary then continue with the electrode.

 

[pc1966]

One though I had was to use a 4-pole RCD after the usual 2-pole RCD/RCBO to allow the supply to trip including CPC if there was a stray current in the CPC. But that has several issues in practice, such as needing enough volts to trip the RCD and no normal earth currents to confuse it.

Also the PME-derived earth for a hot tub also has the issue that it could be 5-10V above true Earth without any fault which is enough to make a wet person notice it.

 

[Midwest]

no isolate the pme in rotary then continue with the electrode.

I'm confused as to how that would work. Upstream RCD might notice imbalance, but earth fault down that earth rod? Suppose it would make no difference where the RCD was, compared to where the earth rod is.

 

[pc1966]

I think @bigspark17 suggestion is you break the CPC at the rotary isolator, so your CU has a DP RCD and the SWA out has the PME-derived CPC, but there is a different earth system from the isolator to the tub (and any related sockets).

Of course you need to make sure there is no metalwork, etc, in touchable range that is on a different earth system (e.g railings with a PME-derived earthed floodlight, CCTV camera, etc, mounted on them etc).

Stand alone metalwork in contact with the Earth is fine, as that is your TT rod's system anyway. No harm in bonding to the rod just in case though.

 

[Midwest]

I think @bigspark17 suggestion is you break the CPC at the rotary isolator, so your CU has a DP RCD and the SWA out has the PME-derived CPC, but there is a different earth system from the isolator to the tub (and any related sockets).

Of course you need to make sure there is no metalwork, etc, in touchable range that is on a different earth system (e.g railings with a PME-derived earthed floodlight, CCTV camera, etc, mounted on them etc).

Stand alone metalwork in contact with the Earth is fine, as that is your TT rod's system anyway. No harm in bonding to the rod just in case though.

Yep, think I was over engineering things. Don't even need the rotary, just dedicated outdoor socket for the Lay-z-spa. Thanks @bigspark17

Just gotta get an earth rod in somehow. That might prove interesting. Might have to drill a hole, to get through the top layer of stones, below that its clay.

 

[pc1966]

Recently we had to put in some rods through a top layer of tarmac-like stuff on hardcore, with clay below. The hardcore was very hard, chunks of granite which is not typical for here (but is up in Aberdeen, presumably it came from rubble there 70 years ago).

Used an 20mm Armeg SDS drill 1m long to get through the lumpy bits and down far enough the rock could be screwed in to the clay.

Use 2m long M16 stainless threaded rod, angle grinder to cut the tip like a drill, and drove in with the SDS drill adaptor without needing hammer action!

Overkill for your case, but might be something you could look at with a smaller rod, etc.

 

[Midwest]

Would it be desirable to locate the rod away from the likely position of said hot tub?

Still might use a isolator of some sort inside, so supply can be locked off when tub is not in use, to prevent accidental use near other pme fed devices, in the garden.

 

[pc1966]

Main worry with rod location is not to hit buried cables or pipes!

In our case we knew where the local 11kV cable ran and nothing else to worry about

Having an indoor DP switch is a good idea generally so you can stop the outdoor socket from being used. I don't see any risk with the socket being used by you for garden equipment, etc, so don't understand the "to prevent accidental use with other pme fed devices" comment.

I would be more worried by local scroats doing something dumb with it...
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If it is just a double socket or so outside then a 20A RCBO and a 20A DP normal wall mount switch are easy options to use. Or even 16A RCBO if cable size an issue, you are unlikely to trip that by overload even with hot tub and a strimmer being used.

Not in the tub, obviously!

 

[richy3333]

The tub we have to do is in a summer house so they can avoid the rain and midges!

watching this thread with interest but I guess I know we’re gonna have to work like bast@&£s to get rods in.

 

[Midwest]

Main worry with rod location is not to hit buried cables or pipes!

In our case we knew where the local 11kV cable ran and nothing else to worry about

Having an indoor DP switch is a good idea generally so you can stop the outdoor socket from being used. I don't see any risk with the socket being used by you for garden equipment, etc, so don't understand the "to prevent accidental use with other pme fed devices" comment.

I’ve got other outdoor sockets on pme. I’ve a plan for services, so shouldn’t be hitting anything hopefully.

 

[bigspark17]

Yep, think I was over engineering things. Don't even need the rotary, just dedicated outdoor socket for the Lay-z-spa. Thanks @bigspark17

Just gotta get an earth rod in somehow. That might prove interesting. Might have to drill a hole, to get through the top layer of stones, below that its clay.

Yes. Maybe the use of a Rcbo if you have a high integrity cu is ideal but use of an dp rcd would be ok too, yes it could affect or be affected by multiple circuits using same rcd but that is a common problem on dual rcd cu’s. Youre giving rcd protection and isolating pme earth at isolator if required.

 

[Midwest]

The tub we have to do is in a summer house so they can avoid the rain and midges!

watching this thread with interest but I guess I know we’re gonna have to work like bast@&£s to get rods in.

Don’t hold your breath, I’ve another project just about to get going. Only thing I’ll do with this one is get the cable in.

 

[pc1966]

I’ve got other outdoor sockets on pme.

Might it be an idea to put all of them on TT so you don't have to concern yourself about any class 1 stuff plugged in to extension leads, etc, as well?

 

[Midwest]

Yes. Maybe the use of a Rcbo if you have a high integrity cu is ideal but use of an dp rcd would be ok too, yes it could affect or be affected by multiple circuits using same rcd but that is a common problem on dual rcd cu’s. Youre giving rcd protection and isolating pme earth at isolator if required.

Doing my over engineering thing, I’m gonna have two CU’s. Existing one is going to be replaced, and remain in my cinema room, don’t wanna go outside on wet windy nights to the garage to reset things.

I’ll have a small storage area left in the remaining garage, putting another CU in there, as the other one won’t be really accessible to add to. Going for a Wylex CU in the storage are, as they seem to do SP&SwN RCBO’s.
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Might it be an idea to put all of them on TT so you don't have to concern yourself about any class 1 stuff plugged in to extension leads, etc, as well?

I’m comfortable with the use of these sockets. Only used for mainly Class 2 stuff by me.