hot tub again

[5544]

well its been a while since I've done one of these. 7.5KW load wiring request from company.
my intention
1. to split the consumer unit in the garage and provide its own RCD and type C MCB
2. SWA two core to the Hot tub location (50mtrs away)(will work out cable size to match Zs)
3. Earth rod at load end where there will be a two pole lockable isolation switch 2 mtrs from tub.
4. requested a 4mtr length of cable to connect to the tub for the installers

Q. would anyone do it differently ( there has been no mention of other lights or sockets so i only have the isolator at the tub end)
Q. what cable would you use from the isolator to the hot tub.

many thanks

 

[Octopus]

Why the earth rod?

 

[5544]

told be Elecsa that due to distance and PME main supply, best to keep earthing separate due to risk of dropping a neutral under fault conditions.

 

[Octopus]

What CSA are you using for the SWA? and MCB rating?

 

[5544]

not worked that out yet. volt drop says 6mm on my app. but will check the Zs on that calc. will use 32amp type c as stipulated by manufacturer.

 

[davesparks]

Try reading the regs rather than trusting ELECSAs advice about the earth rod!
The recommendation in BS7671 is for an electrode of under 20ohms connected to the main bonding of the installation.

 

[5544]

in all honesty i phoned them because i was unclear on the answer. so your saying to rod the supply end as well, by the garage where the consumer unit is. I take it this is to avoid the danger of the neutral dropping out? and then could i still use the MET as the point of earth for the hot tub in the garden through the armoured sleeving and third core for earthing.

 

[D Skelton]

Rodding a hot tub on a PME has got nothing to do with protection from loss of supply neutral, but everything to do with equalising the potential between earth and 'true' earth. That's why the Ra has to be so low.

It's standard practice to rod outdoor hot tubs when fed by TN-C-S.

The way I tend to go about it is to feed a local IP DB with an s-type up front and a 10mA RCD for fault protection, a TT system created from here on with a rod (or however many is needed) local to the tub itself.

I've done it this way a few times but it would be overkill to do it this way on a TN-S for example.

 

[davesparks]

in all honesty i phoned them because i was unclear on the answer. so your saying to rod the supply end as well, by the garage where the consumer unit is. I take it this is to avoid the danger of the neutral dropping out? and then could i still use the MET as the point of earth for the hot tub in the garden through the armoured sleeving and third core for earthing.

Not quite, I am not saying how you should do it, merely advising you of what the regulations have to say on the matter.

There is no requirement, only a recommendation, in bs7671. However bs7671 is a minimum standard only (all too often forgotten)

If I was doing it I would probably install at least 2x 8' rods connected to the MET with the supply to the hot tub run in 2core SWA protected by a SPSN 30mA RCBO.

 

[edexlab]

I looked into this recently and found the regs to be (as usual)a bit ambiguous in the wording

I read numerous threads and posts which seemed to suggest in their wording to use the Pme earth but install a rod at the tub and main bond back to the Met as the reg does seem to suggest ,
I might be wrong in some of this but I think the general interpretation of this is wrong and it should actually say isolate the Pme earth and bond the rod back to the Met
ie the Met of your new TT installation not the TN Met as it doesn't clarify this

After listening to advice from several engineers inc Eng 54 who was very helpful and a couple of independant engineers , Also the Niceic helpline who were actually very good for once and doing my own research which included calls to manufacturers what I decided to do in this case as the tub was well away from the house was;

Isolate the pme earth adjacent to the tub install at the local rotary isolator
(no more than 2 m away from the tub and used a local Rcd protected db and TT 'd it I eventually got the Ra down to 6 ohms which I was happy with due to the location


I used a 30mA Rcd although I was thinking as D Skelton said above to use a 10mA but it turned out the tub also had a 30mA installed internally, so I left both in place as discrimination isn't an issue here versus safety
ie I'd prefer to have an 2nd Rcd as a back up, than receive a phone call saying someone's been fried especially as this tub seat's 6 people.

One other thing to note is the manufacture of this hot tub said max loading was 35 A and to use a "D type Mcb" due to the combination of pumps, heating elements etc which then leads to trying to achieve the Zs required, and which type Mcb would be suitable for the submain .

Nothing wrong with seeking advice from someone more qualified but also remember who's signing for it!

 

[sparks1234]

I was called to a hot tub several years ago with complaints that the owners got a shock every time they got in it.
It wasn't a bad one by any means.
I arrived and put my hand in, nothing, felt fine to me.
So, I took my shoes and socks off and hey presto, got a nice tingle off the water.
Changed the supply a bit so that the hot tub saw on its own TT system with a local RCD and earth rod and all was fine
The supply length saw about 60 mfrs I guess from memory so enough length to get a difference in potential