Adding 32A Underground cable supply

[RickH]

I have an existing install in my house and an extension supply to an outbuilding. It has a lighting cct and a power radial (no permanent items, rarely used – so happy with load & discrimination)
I wish to add a 32A supply to a Hot Tub, via 6mm2 SWA cable.


Thinking the easiest way is to add the Hot Tub off the Outbulding supply (closer to Hot Tub than house), this is a 40A RCD protected supply, any reason not to do this ?

Only requirement stated by the Hot Tub manufacturer is that it is a 32A suply an isolator must be fitted to supply for servicing, but no closer than 2m, are there any Regs requirements on location of the isolator wrt Outbuilding?

I could fit IP65 rotary switch isolator on outside wall of the Outbuilding and terminate underground cable on that. I can’t think of a reason why it would need to be inside, unless there is a need for it to be within a certain distance of MCB ?

Or simpler I could put a 32A switch fuse inside & alongside the existing Outbuilding install (TT supply) and terminate the undergound cable on that, avoiding need for MCB, and any external isolator ..... drawing attached for both options in case my description not clear.

Also attach a schematic of the existing install and what I want to add (shown in red)

Welcome advice.

 

[MadMonkey]

Either option is fine. The outbuilding supply cable and circuits are protected via the MCB with additional protection from the RCD at the origin. How you split the load then and protect individual circuits in the outbuilding is up to you and the equipment loading. Providing your sw fuse can be locked off for maintenance and is adequate for 32A, I would say that would be adequate. Might be worth checking with the hot tub people as they might want a separate isolator installed, some say distribution equipment is not an appropriate means of isolation, which is rubbish.

If your outbuilding is TT then you’re good. Seen too many done with exported TNCS or TNS.

 

[RickH]

I did ask them, they just need an isolator no closer than 2m. Thinking I will put a switch fuse in the outbuilding .... and for convenience terminate the SWA cable feed on an isolator at Hot Tub end (2m away) Then use NYY-J Cable for the 'tail from the isolator to the tub, at that point its all above and undercover ground.

 

[aesmith]

Is exported TNS bad practice, is that something specific to hot tubs?

 

[MadMonkey]

I did ask them, they just need an isolator no closer than 2m. Thinking I will put a switch fuse in the outbuilding .... and for convenience terminate the SWA cable feed on an isolator at Hot Tub end (2m away) Then use NYY-J Cable for the 'tail from the isolator to the tub, at that point its all above and undercover ground.

That’s sound.

Is exported TNS bad practice, is that something specific to hot tubs?

If you ask the DNO they would never usually allow you to do export their earth. Most sparkies don’t ask and do it anyway. Certain installations should be treated as TNCS even when they are TNS, such as EV charging and hot tubs. The DNO could upgrade a TNS to TNCS which can cause issues.

 

[EFLImpudence]

If you ask the DNO they would never usually allow you to do export their earth.

Why not? Do they care?

Certain installations should be treated as TNCS even when they are TNS,

True, PME because of the inadequate and poor maintenance by the DNO who just do as they please.

 

[MadMonkey]

The DNO are under no obligation to supply an earth. If we ever have earthing issues from the DNO it’s always best to be overly nice and avoid them telling you to fit an earth rod. I’m not 100% sure of the reason why as DNOs take a different stance. My best guess would be because exporting to an outbuilding comes with an increase risk of problems with TNCS, it’s not just limited to EV chargers and hot tubs, issues arise with metal buildings, caravans, sometimes even just concrete floors. This element of risk is avoided by not exporting the earth. The DNO has most likely accepted/mitigated the risks in which properties have TNCS arrangements in the event of a broken neutral. This is only my thoughts though.

Can’t say I’ve experienced any poor maintenance from our DNO and their engineers are pretty sound, the call centre staff can be a pain sometimes, but no different from, say, the gas or water supplier. The on the road engineers are very helpful and knowledgeable.

 

[RickH]

Learnt some good things here guys .. appreciate it.
In case anybody wants to know …I will do all the physical install, but I have an Electrician who will do an inspection & Part P certificate on this.
He is not too keen to dig a cable trench

This is prior to final connection to Hot Tub, which will be done by Hot Tub company when they install it.

 

[EFLImpudence]

The DNO are under no obligation to supply an earth.

True, so if they don't then they don't care.

If we ever have earthing issues from the DNO it’s always best to be overly nice and avoid them telling you to fit an earth rod. I’m not 100% sure of the reason why as DNOs take a different stance.

Not sure what that means.

My best guess would be because exporting to an outbuilding comes with an increase risk of problems with TNCS, it’s not just limited to EV chargers and hot tubs, issues arise with metal buildings, caravans, sometimes even just concrete floors. This element of risk is avoided by not exporting the earth.

True, but what has that to do with the DNO?

The DNO has most likely accepted/mitigated the risks in which properties have TNCS arrangements in the event of a broken neutral. This is only my thoughts though.

Really?

Can’t say I’ve experienced any poor maintenance from our DNO and their engineers are pretty sound, the call centre staff can be a pain sometimes, but no different from, say, the gas or water supplier. The on the road engineers are very helpful and knowledgeable.

They failed to maintain the TN-S supply cables and leave it to the customer to mitigate whatever they do.

 

[MadMonkey]

True, so if they don't then they don't care.

If your providing a TT then they won’t care but you specifically asked about my comment of the DNO not allowing the export of their earth and why do they care. If you're providing a TT you're not exporting their earth for them to care.

Not sure what that means.

Sorry that part was poorly written and the last sentence was in relation to why the DNO's earth can not be exported. They all seem to have a different stance on why their earth can not be exported.

True, but what has that to do with the DNO?

It was in relation to exporting the TNCS, which the DNO supplies.

Really?

As I said its just my best guess.

They failed to maintain the TN-S supply cables and leave it to the customer to mitigate whatever they do.

I have no experience of this. the DNO has corrected any issues I've ever had.

 

[bernardgreen]

My view of the situation is that an "Earth" derived from the incoming Neutral is fine for "earthing" all the exposed metal inside the equipotential zone of a domestic building provided that no one can ever form a bridge between "Earth" inside the building and another "Earth" outside the building such as the ground around the building.

Provided that such a bridge cannot ever be created then the "Earth" and the CPC ( from the Neutral ) can drift tens of volts above the ground potential without any hazard for any person or equipment inside the building.

It is impossible to ensure that such a bridge between two different "Earth" will never be created, A gas meter reader I knew wore insulating gloves when wiping clean the window of an external gas meter after he received a significant shock from a correctly bonded meter,

Loss of the incoming Neutral while the Live is still connected to the building can result in the "Earth" and CPC inside the building being pulled up to the Live potential via any load in the house between the Live and the Neutral conductors The occupiers will not beware of this. Nothing electrical in the house will work and this will suggest to the occupier that there is a power cut, ..

 

[ericmark]

Caravans, boats, and petrol stations are not permitted PME/TN-C-S it has to be TN-S or TT, the problem is the DNO can turn a TN-S supply to TN-C-S without telling the user, and the M in PME stands for multiple, but often when a DNO cable is replaced there are not multiple earth points.

If every home in a street has gas and water pipes which are metal bonded to the TN-C-S earth then any current caused by the loss of the PEN is shared between the homes, and no bonding wire has an excessive current. The problem is when only one property in the street has grounded metal work connected to the TN-C-S earth, then excessive currents can flow which in the case of a gas supply can cause fires.

But there is a secondary problem with earthed metalwork, it can cause a voltage gradient in the ground, so an earth rod close to a metal water or gas pipe with the loss of the PEN may not be at true ground potential as it is picking up the voltage from some near by metal work.

Since this only happens with the loss of the PEN, it could never cause a problem, and any transfer is near impossible to detect, so the electrician has to try and work out likely routes of metal services, simply making the supply TT may not work, it is not really a problem with caravans and boats, with a supply to a caravan in the middle of a field, there is unlikely to be metal services under the ground near to the supply outlet.

Having the hot tub electrics remote to hot tub, with plastic feed pipes is favourite, the pumps and heaters are some safe distance away, or using class II electrical equipment which has no earth, still not worked out why EV's are not class II.

However the dangers are complex, and to DIY hot tub, or EV charging points supplies is asking for problems. In the main a 70 volt shock from an EV charging point hurts but does not kill, the same is not true for a hot tub.

 

[EFLImpudence]

I have no experience of this. the DNO has corrected any issues I've ever had.

You said:

The DNO could upgrade a TNS to TNCS which can cause issues.

It is not an upgrade; it is a botch to save them repairing/maintaining the earth conductor.

 

[MadMonkey]

It is not an upgrade; it is a botch to save them repairing/maintaining the earth conductor.

It might not be an upgrade, maybe change is better language. We will have to agree to disagree on the last part.

 

[RickH]

However the dangers are complex, and to DIY hot tub, or EV charging points supplies is asking for problems. In the main a 70 volt shock from an EV charging point hurts but does not kill, the same is not true for a hot tub.

In this case the work will be fully inspected and PART P certified by an electrician, I am DIY'ing the physical installation. My NICEIC electrician is happy to work this way.