Another Hot tub install

[Gazthesparky]

I have my tin hat on because I know this has been done to death and have read through a lot of the old posts.

I just wanted to run my proposal past a few of you and see if there is anything I have missed as its always good to get someone else's view.

Customer is needing a 32amp supply and hot tub company has said they require a 6mm supply with an isolator and flex left for them to connect.

My plan is to run a 10mm 3 core swa to a 40amp isolator from a new 2 way consumer unit Henley blocked from the tails. There is a Isolator on the current tails so will keep this and it will mean that there is one form of complete isolation. I have done the calculations and a 6mm will suffice with a volt drop of 11.2v so its on the borderline so hence going a larger size cable. It also allows for future proofing.

Now this is where it gets a bit messy. The supply to the property is PME. I intend to use the PME for the hot tub supply supplying it from a 32amp RCBO in the house. I am also intending to install an earth rod am I right in thinking that this gets installed at the hot tub end of the SWA? the idea is that should there be a PEN failure there is an earth path and the potential between the floating PEN earth and true earth is reduced as far as possible.

am I right in thinking that the rod does go at the hot tub end and not by the intake position of the property?

I am going to mount the isolator onto some uni strut installed into the ground. There is a fence panel 1 meter from the hot tub so too close to the tub. Also the fence doesn't seem too safe so I want to install my own support.

Finally they have also asked for a double socket by the tub again, I'm going to keep this 2 meters away from the tub. I am thinking of running a separate cable back to the house so this can be isolated as the customer is wanting to plug fairy lights in so the customer can switch them from the house. Im just wondering if i'm over complicating it though and should I just spur off the hot tub supply and install a fused spur and double socket

Thanks to any replys

 

[Mike Johnson]

Except the Wylex/Crabtree/Fusebox units do not show a physical disconnection of the N as they do for the line on their data sheets, so I am dubious that the N is switched, as I have said before, the physical size of the compact units may not be able to accommodate two switch units, it is supposition on my part as you say without taking a Hager unit apart it's difficult to say categorically, but I do think the diagram may be indicative of the units parameters.

All seems a bit strange to me, if these units are 2p switched why don't they just say so instead of 1P+N?
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Thinking about the previous question, I wonder if the difference is that the N does not have it's own fault circuit and is just dragged along with the line if a fault develops?

I think that is what N switched concurrently is supposed to mean, Hmmm

 

[pc1966]

Except the Wylex/Crabtree/Fusebox units do not show a physical disconnection of the N as they do for the line on their data sheets, so I am dubious that the N is switched,

But they do:

All seems a bit strange to me, if these units are 2p switched why don't they just say so instead of 1P+N?

Thinking about the previous question, I wonder if the difference is that the N does not have it's own fault circuit and is just dragged along with the line if a fault develops?

Exactly. N is switched, it is not tripping on an over-current fault alone. There is only 1 pole that is capable of sensing and fully interrupting the fault current.

In the diagram from Wylex you see the 'X' for the thermal trip is only on the L path ("LOAD 2" as they call it).
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Of course, as an RCBO if you do have a fault leading to N-E current it will trip.

But it would not be pretty if you had reversed polarity fault AND then a 'N' to E fault in the kA range!

 

[Mike Johnson]

So they do, thanks for that, it's been an interesting discussion, with I think a conclusion.

One must assume that the compact single module units are just a space saving exercise.

 

[HappyHippyDad]

that is a good idea providing both types of earthing but the written paperwork that has been provided to me by the customer from the hot tub company states that they do not provide any wiring and they require 5meters of flex left connected to the isolater. They just connect up the flex and if it’s not as requested then they charge for a return visit. So I get the feeling they are not interested what they have just as long as it looks like they normally have.

with regards to the DP RCBO I am going to go down that route but the customer is wanting another DB installing so they can fit a hob in the future as there current one is full so I was going to install a second consumer unit and use RCBOs.
I’m just not sure how these DP RCBOs fit do they need a separate enclosure or could I fit a consumer unit and cut the bus bar back and fit the DP RCBO at the end with a link from the neutral bar ?

The isolator will be at least 2 meters away from the tub and the socket 3 meters away.

I am still thinking of going down the supplementary rod road just incase the loss of a PEN conductor

I have just started a thread which has some similarities to this thread with regards the earthing. See in particular post 3 Understanding the relationship between TNCS, TNS and TT - https://www.electriciansforums.net/threads/understanding-the-relationship-between-tncs-tns-and-tt.188980/

It would seem that if you are going to extend the PME AND add an earth rod for the Hot tub you could well be making it more dangerous in the event of PEN failure. Unless of course you did get the Ra down to around 2 ohm (which isn't going to happen).

I have to admit I find it a bit confusing but if you Just made the new CU a TT and did not extend the PME then the new earth rod would not raise to 230V in the event of a PEN failure which seems like a better option!

If you TT the new install and don't extend the PME you rely solely on the RCD for fault protection.

If you extend the PME you have ADS and additional protection with an RCD but in the event of PEN it could become dangerous.

I think I would go for option one, TT for the new install. I'd probably run a 3 core 10mm SWA (and isolate the armour and 3rd core) in case I changed my mind!

 

[Midwest]

I thought I understood this subject some time back, but this thread has got me confused again.

What is the point of using a DP or SP & Switched Neutral RCBO for this hot tub supply & PEN conductor faults, if it was TT'd? Other than manufactures (i.e. Schneider) or disconnecting neutral for fault finding etc.

Where is this hot tub being installed, on what surface is it being placed? I recall reading a thread about faulty Lazy Spa hot tub, with voltages on the water, but all of these hot tubs are insulated, no metal surfaces. Not as if its like a ev car. If the bathers not stepping on bare Earth, would it not be better to rely on the PME, and not to be too concerned about a lost PEN conductor?

 

[stevethesparks]

pme is acceptable as is a SP rcbo, but nothing wrong with a belt and braces approach. Lost PEN may be quite rare but it still happens.
Iv'e always adopted the idea of treating a hot tub as a swimming pool.

Drifting slightly off the subject, it is strange that if you have a shower in a bedroom that a socket outlet requires to be no less than 3 metres away. However, with a swimming pool where it is not physically possible to locate a socket outside of zone 1, then it will be allowed 1.25mts horizontally from zone 0???
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I missed part of your post. If a PEN fault occurs, then with a SP rcbo, this will still allow a path through the hot tub element/pump via the faulty neutral will it not?

 

[Midwest]

pme is acceptable as is a SP rcbo, but nothing wrong with a belt and braces approach. Lost PEN may be quite rare but it still happens.
Iv'e always adopted the idea of treating a hot tub as a swimming pool.

Drifting slightly off the subject, it is strange that if you have a shower in a bedroom that a socket outlet requires to be no less than 3 metres away. However, with a swimming pool where it is not physically possible to locate a socket outside of zone 1, then it will be allowed 1.25mts horizontally from zone 0???
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I missed part of your post. If a PEN fault occurs, then with a SP rcbo, this will still allow a path through the hot tub element/pump via the faulty neutral will it not?

I‘m not to familiar with hot tub designs, I would seek guidance from the manufacturer.

However, the point of lost pen conductor, and using a SP or DP device, would not make much difference surly, as both would still be energised during this situation. In this situation in is the still connected exposed conductive parts, extraneous conductive parts and true Earth, that’s cause for concern, not the broken neutral.

Section 702 does not preclude the use of pme for swimming pools.

Most hot tubs I’ve seen have no exposed or extraneous conductive parts. As said, I’m not sure of the internal components design,.

 

[stevethesparks]

However, the point of lost pen conductor, and using a SP or DP device, would not make much difference surly, as both would still be energised during this situation. In this situation in is the still connected exposed conductive parts, extraneous conductive parts and true Earth, that’s cause for concern, not the broken neutral.
.

I agree. there is no guarantee that an rcd/rcbo would trip in such a circumstance, but my ideology is just to be able to provide total isolation, hence TT to isolate from the pme and DP to ensure total isolation of the supply.


You are correct also about pme not being precluded with swimming pools, but again that's something I would not do.

 

[pc1966]

In the TN case from PME then a DP breaker is not really an advantage here - if your PEN goes live your hot tub earth, etc, is still live with L & N open anyway.

But in the case of going TT to avoid the PEN fault risk you really must be DP breaking otherwise a 'live N' style of fault arising from the PME fault would not be isolated by the breaker opening.
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Just to add the RCBO would not open on a PME fault alone, but if you had another fault leading to earth leakage current at the same time (e.g. rise in L-true E volts causing breakdown, etc) you would still expect it to be safely disconnected.

 

[Mike Johnson]

In real terms how often does a PME fault occur? I have never experienced or seen one.

 

[pc1966]

In real terms how often does a PME fault occur? I have never experienced or seen one.

I have heard around 300-350 per year.

 

[Mike Johnson]

Heard about is not experiencing them, how many in your time as an Electrician have you actually had to deal with one?

 

[ipf]

Somehow, I think, if it were a seriously regular occurrence, the supply authorities might be under a great deal of pressure to 'get it sorted'.
It is, after all, their duty.
In all fairness, every supply earthing problem I've reported has resulted in a same day visit.

 

[Mike Johnson]

All true, but I was trying to get to the bottom of how many Electricians on this Forum have had first hand experience of a PME fault, personally I have never found or had to deal with one!

 

[HappyHippyDad]

All true, but I was trying to get to the bottom of how many Electricians on this Forum have had first hand experience of a PME fault, personally I have never found or had to deal with one!

I think even if @pc1966 had said he had never come across a PEN break that would not really mean they do not exist. It would neither suggest they were common or rare. You would need to ask perhaps 1000 electricians across varying locations, then perhaps you could form a statistical judgement.... but not on a population of 1.
Why not start a poll, it would be interesting.