(Thought it easier to start a new thread)

So, following on from my various disaster DIY decking threads/ hot tub threads ..

I’m planning to install an outdoor socket 25m from my house, using arctic cable and some conduit feeding from a plug socket in the conservatory.

I have two questions!

– presumably I need 2.5mm three core artic cable ie same diameter as sockets would usually have?

– should I use an additional RCD plug on the indoor end? My house has RCBO’s and the mobile filth pond has an RCD inside I think. I’ve been advised that an additional RCD on the plug going into the indoor socket could interfere with the house RCBO’s. I obviously want it to be safe but not tripping every 5 mins!

Many thanks

a mobile filth pond runs off a 13amp plug ?

My first thought is that the sex pond should have a comprehensive installation guide with it? I mean, what could go wrong with badly installed electrickery and water?

The filth pond does indeed seem to require a 13A supply.

It’s not arrived yet so I can’t check instructions. However, it seems all mobile filth pond manufactures advise against using normal extension leads, hence my hoping to put something a bit more substantial in place.

You would normally try to ‘TT’ earth any hot tub that is remote from the house.
That means you shouldnt really use the house earthing to supply an earth for the hot tub, but use an earth from the hot tub area.

I really think you should be getting an electrician who has competently installed one of them before.
A separate supply, non RCD protected, would be my choice, ran in SWA cable, to a small, RCD protected consumer unit.
What you are proposing is a little bit risky, and (without looking it up in my Regs book) probably against the electrical regs.

Thanks alanl

Re earthing – well it’s an inflatable hot tub so I guess wouldn’t normally be earthed on its own (they’re meant to be plugged straight into a normal socket).

I don’t want to do anything silly obviously, but it seems a little confusing when (legally at least) you could just plug an inflatable hot tub into an all weather extension lead?!

I’d hoped that an RCD/socket would at least be preferable to that?

I guess my thinking was that if i did it myself the only difference between it and a kit like below, is the cable length?

Presumably, these are safe for DIYers to install, hence being sold by Wickes

Outdoor socket kit

OK, I’ll bite.
You don’t need the 2nd RCD. It is unlikely to cause interference with the RCD in the consumer unit, but it will be hit or miss, if there was a fault, as to which one will trip. They will both be rated as 30mA RCD’s, there is a little tolerance, so one may trip at 23mA, the other at 28mA.Either way, if there is a fault (the fault may be 500mA) either or both may trip.
Local earthing. The earth you have in the house comes from the cable in the street, or in older houses, direct from the local transformer. The earth voltage, within the house, will be constant, so when you touch a metal, earthed object, you will not get a tingle (slight shock).
Now, where this doesnt work is when you step outside onto wet ground, as the earth (actual ground) may then be a better conductor than the one supplied by the electric company. So any electrical appliances with a metal casing (or heating elements / pumps in a hot tub) that you use outside could give you a tingling feeling/slight shock, as, most items have a small amount of earth leakage, that means that some of the return current that usually goes down the neutral, does not, some of it gets diverted by electronics in the equipment, water and other things into the earthed casing of the appliance.Computers used to be notorious for this, so much so that there was a limit of how many computers could be installed on one circuit.

So now, you have your hot tub pump and heating elements buzzing away, and putting around 15mA into the earth. And guess what has got a really good connection to that 15mA of current – you when you are sitting in the hot tub. Inside the hot tub, there is no problem, as you are not earthed, and the current will just buzz around and go through the cables earth. But then you step out of the tub, one foot in the water, one on the floor.
That 15mA current can then go through your body to the grass, as that grass is a better conductor than the cable from the house. If you use a TT system, the earth around the hot tub will be at the same potential as you when you get out, so you will not get a shock.

This tingly shock is quite common when using an outside tap that is connected to the house earthing. So much so that it is now recommended to put an insulating section in outside taps to stop this shock risk. It is a definite risk, anyone with a weak heart can be killed by such low currents.

Cable length. Each thing in your house should trip in less than 0.4 seconds. Testing by Nazi Germany has shown that the vast majority of people can live after a 230volt shock of 0.4 seconds. To ensure the circuit breaker trips at less than 0.4 s, the cable length needs to be kept within specified guidelines. To check if a 2.5mm 3 core cable will comply, you will need to know the earth fault loop impedance at the socket you intend to take the feed from. Cable calculations will then be able to see if that cable would comply. I’d say a 25M 2.5mm cable will be very close to the acceptable limit.
Add in extra impedance from the plug socket pin connections, then it may not be suitable.

I would definitely not recommend plugging a hot tub into a socket 25M from the house.

Thanks Alan, for the very comprehensive reply.

So, excuse my ignorance but how does a hot tub plugged into a socket in a house or an outside socket resolve the earth issue you’ve described?

I’m genuinely not meaning to sound flippant, Just interested.

If I were to ask an electrician to install something would be able to figure a way round the excessive cable length and trip timings or would he just have to test it and see? Thanks again

It doesnt. You need the remote end, near the tub to have an earth spike fitted, so that the local area around the tub has the same earth potential as that in the tub.
I’ve just looked online as some instructions for 13A plug in tubs. They dont mention anything about the earthing. I think thats a bit reckless.

Larger cable would make the Earth Fault Loop Impedance lower., That is a larger cross section area. 4mm would probably comply with the Regs if spurred of an existing house supply.