Testing Ze on Separate earth rod

[DonClif]

Hi lads,

We turned down a job today that has got me thinking.
Customer wanted a new single socket for a hot tub, we wanted to take a spur from the interior ring, into a rotary isolator, then into a single socket for the tub.
Pic attached.

Earthing is currently PME, so I wanted to add an earth rod for the new bits outside.
But the question is, hoe do you test Ze/Zs with the earthing so far away from the origin of the installation?
Could I just Zs at the end of the circuit?
Wander lead from the rod to the main board?

Am I even close?

I have a good understanding of testing but I has little experience with TT and earth rods, and I'd rather ask a silly question here than make a silly mistake later.

 

[Fleeting]

Your TT becomes a separate system with your earth fault loop path using the electrode. It no longer relies on the main installation earth so that is not part of your TT Zs.

 

[SPECIAL LOCATION]

Earthing is currently PME..

I have a good understanding of testing but I has little experience with TT and earth rods, and I'd rather ask a silly question here than make a silly mistake later.

Hi DonClif...

(1) Nit-Picking-Friday-Night-Beer-Hat-On...
I think the earthing is currently "TN-C-S" NOT "PME" ???
BS7671 model forms do not actually have any tick-box for "PME" as the supply characteristics and/or earthing arrangements..
{for some of the older members here I think I am correct in suggesting that one of our past members "Steptoe" would have raised this point??? but I may be wrong.. as I have just opened another bottle!!! }

(2) Agree 100% far better to ask a well described question with an attached sketch wiring diagram of what you think you understand, to try and clarify any grey-areas you are still a bit unsure about..

(3) So here goes.. (Still with my Friday night beer head on.. ).. A few thoughts & points you may want to consider..?

How far away is this hot tub?

What is it mounted on, Insulated wooden decking, Slabs, Good earthy conductive lawn?

Is it fed via a SWA to a socket mounted on a separate building / shed / outdoor structure?

Or is it just screwed on the external wall a short distance from where you are spurring off the ring?

What is your reasoning for thinking an earth rod is needed for this arrangement?
i.e. what hazardous potential earth-fault paths have you identified?

How much different is the proposed hot-tub single socket alteration, to just a weather-proof outdoor socket for a lawn mower & Christmas lights on the outside wall of a property? (Do these need an earth rod???).

If there are concerns about exporting earth to external buildings, (especially those with exposed extraneous metalwork), from TN-C-S/PME installations.. and you consider it requires splitting the installation earthing arrangements..

You would generally also be adding an additional small CU with maybe 2 or 3 circuits to supply the remote building and this is where the new rod connects onto..

So all Ze & Zs related readings refer to this new additional small CU and its earth rod.. Not the original installation earth arrangements you have taken your supply from..

Hope some of that helps you get your mind thinking about what are the actual real dangers to people, property & livestock..
What are the essential requirements of BS7671..
What are some of the myths and/or misunderstandings about TN-C-S/PME...
Is there a wiring regulation stating that an outdoor socket supplied from a TN-C-S/PME installation must have an additional earth rod?

And do I need another beer now???

 

[DonClif]

Your TT becomes a separate system with your earth fault loop path using the electrode. It no longer relies on the main installation earth so that is not part of your TT Zs.

But my understanding would be that you could have the RCD protecting the ring as the protection for the TT'd socket

Hi DonClif...

(1) Nit-Picking-Friday-Night-Beer-Hat-On...
I think the earthing is currently "TN-C-S" NOT "PME" ???
BS7671 model forms do not actually have any tick-box for "PME" as the supply characteristics and/or earthing arrangements..
{for some of the older members here I think I am correct in suggesting that one of our past members "Steptoe" would have raised this point??? but I may be wrong.. as I have just opened another bottle!!! }

Fair enough, but I state PME due to TNS & TNCS being assumed PME in reference to PEN faults, so I look at it in the same vein as old EV installations.

(2) Agree 100% far better to ask a well described question with an attached sketch wiring diagram of what you think you understand, to try and clarify any grey-areas you are still a bit unsure about..

(3) So here goes.. (Still with my Friday night beer head on.. ).. A few thoughts & points you may want to consider..?

How far away is this hot tub?

Customer was told under no uncertain terms that a minimum of 2m would be acceptable to comply with section 702, I considered swimming pools as close as for guidance.

What is it mounted on, Insulated wooden decking, Slabs, Good earthy conductive lawn?

Slabs at the moment, but they did have plans to fit a purpose built deck. But I wouldn't necessarily believe that without seeing it

Is it fed via a SWA to a socket mounted on a separate building / shed / outdoor structure?

Or is it just screwed on the external wall a short distance from where you are spurring off the ring?

Plan was to have SWA spurred from an indoor socket in the configuration shown

What is your reasoning for thinking an earth rod is needed for this arrangement?
i.e. what hazardous potential earth-fault paths have you identified?

Manufacturer instructions, and 702.410.3.4
No extraneous parts in the location, if that's what your asking?
There's a tap, but PVC piping throughout.

How much different is the proposed hot-tub single socket alteration, to just a weather-proof outdoor socket for a lawn mower & Christmas lights on the outside wall of a property? (Do these need an earth rod???).

If there was no mention of a tub I wouldn't have even thought of a spike, as I have done in the past.

If there are concerns about exporting earth to external buildings, (especially those with exposed extraneous metalwork), from TN-C-S/PME installations.. and you consider it requires splitting the installation earthing arrangements..

You would generally also be adding an additional small CU with maybe 2 or 3 circuits to supply the remote building and this is where the new rod connects onto..

So all Ze & Zs related readings refer to this new additional small CU and its earth rod.. Not the original installation earth arrangements you have taken your supply from..

Hope some of that helps you get your mind thinking about what are the actual real dangers to people, property & livestock..
What are the essential requirements of BS7671..
What are some of the myths and/or misunderstandings about TN-C-S/PME...
Is there a wiring regulation stating that an outdoor socket supplied from a TN-C-S/PME installation must have an additional earth rod?

And do I need another beer now???

I definitely need another beer, so I'd say yes, you do to

 

[SPECIAL LOCATION]

702.410.3.4

Another day...
Consumption of some more brain lubricating beverages..
And a few more thoughts to ponder.....

Section 702 in my regs books refers to the particular requirements applicable to:-
Basins of swimming pools..
Basins of fountains..
Basins of paddling pools..
No mention of hot tubs?

702.410.3.4.1, 702.410.3.4.2, 702.410.3.4.3, detail what is permitted in the various Zones around the basin of a swimming pool, fountain or paddling pool.

The note at the start of 702.410.3.4 refers to installing a Rod or Mat connected to the supplementary protective equipotential bonding where PME conditions apply... Does this hot tub have any requirements for supplementary protective equipotential bonding?

Because this item can connect via a 13A socket , I got the impression it was a typical plug-and-play type,
customer assembled hot-tub?

Which to my understanding is not a permanent fixed item in a constructed basin, as per the illustrations on,
Fig 702.1, Fig 702.2, Fig 702.3 or Fig 702.4?

I am not convinced that section 702 has any relevance...??
any more than section 704 would if they are going to construct a decking!

If this outdoor socket is fixed to the external wall of the property..
I would think all you need is a circuit with appropriate load capacity,
Overload protection,
30ma RCD protection,
within volt-drop & earth loop impedance permitted values etc..

Otherwise we would have to put earth-rods onto all TN-C-S supplied outdoor sockets.....
just in case the customer plugs in a hot tub later on that we were not told about!

I think you may have been overthinking the design requirements of this minor-works alteration..?

(But obviously I am open to correction as I may have had a few too many brain lubrications so far....?
However in my defence.. it is a Saturday night on a bank-holiday weekend..
so technically it may be impossible to have too much brain lubrication!!! )

 

[DonClif]

Another day...
Consumption of some more brain lubricating beverages..
And a few more thoughts to ponder.....

Section 702 in my regs books refers to the particular requirements applicable to:-
Basins of swimming pools..
Basins of fountains..
Basins of paddling pools..
No mention of hot tubs?

I thought the same, but I couldn't think of a better comparison. Maybe 702 is being a bit over cautious. But rather over than under.

702.410.3.4.1, 702.410.3.4.2, 702.410.3.4.3, detail what is permitted in the various Zones around the basin of a swimming pool, fountain or paddling pool.

The note at the start of 702.410.3.4 refers to installing a Rod or Mat connected to the supplementary protective equipotential bonding where PME conditions apply... Does this hot tub have any requirements for supplementary protective equipotential bonding?

The specs for the tub did include recommendations for a rod...

Because this item can connect via a 13A socket , I got the impression it was a typical plug-and-play type,
customer assembled hot-tub?

Which to my understanding is not a permanent fixed item in a constructed basin, as per the illustrations on,
Fig 702.1, Fig 702.2, Fig 702.3 or Fig 702.4?

I am not convinced that section 702 has any relevance...??
any more than section 704 would if they are going to construct a decking!

If this outdoor socket is fixed to the external wall of the property..
I would think all you need is a circuit with appropriate load capacity,
Overload protection,
30ma RCD protection,
within volt-drop & earth loop impedance permitted values etc..

Otherwise we would have to put earth-rods onto all TN-C-S supplied outdoor sockets.....
just in case the customer plugs in a hot tub later on that we were not told about!

That's all making sense, if the customer had just asked for a socket, then I wouldn't have thought twice, but as he mentioned a tub and that he plans to keep it there indefinitely, I felt like it was irresponsible of ne to just treat it like a socket for general gardening purposes.

I think you may have been overthinking the design requirements of this minor-works alteration..?

Again, totally possible. I always tend to go on the overdone side, trying to get too clever.

(But obviously I am open to correction as I may have had a few too many brain lubrications so far....?
However in my defence.. it is a Saturday night on a bank-holiday weekend..
so technically it may be impossible to have too much brain lubrication!!! )

This is all good food for thought, if the job comes back round, or something similar, then I'll have a better idea of what's needed, or not.

Cheers

 

[SPECIAL LOCATION]

I thought the same, but I couldn't think of a better comparison. Maybe 702 is being a bit over cautious. But rather over than under.

The specs for the tub did include recommendations for a rod...


That's all making sense, if the customer had just asked for a socket, then I wouldn't have thought twice, but as he mentioned a tub and that he plans to keep it there indefinitely, I felt like it was irresponsible of ne to just treat it like a socket for general gardening purposes.

Again, totally possible. I always tend to go on the overdone side, trying to get too clever.

This is all good food for thought, if the job comes back round, or something similar, then I'll have a better idea of what's needed, or not.

Cheers

At the end of the day in this industry it is probably best to be over-cautious and lose a job..
Rather than be overconfident and leave a dangerous, non-compliant installation....
With your name on the paperwork!!

 

[DonClif]

At the end of the day in this industry it is probably best to be over-cautious and lose a job..
Rather than be overconfident and leave a dangerous, non-compliant installation....
With your name on the paperwork!!

Glad to hear my thoughts shared.
No matter how positive I am that I've done a perfect job, the thought of F-ing up through overlooking something scares the **** out of me still.

 

[binky]

The instructions were probably written in China. Most have a get out clause about complying with local regulations. I would want to know a lot more about the construction of the hot tub itself, such as does it claim to be class II ?

However, our regs really only apply to the hard wired circuit, not what is plugged into any circuit. So I would concurr that your responsibility and legal liability ends at the outdoor socket. The legal responsibility for the safety if any item plugged into a compliant circuit rests with the supplier/ manufacturer. If it didn't we could be held responsible for every lawn mower or hedge trimmer that cuts it's own cable...

 

[Murdoch]

Not sure I’d be using any of these nasty Chinese made so called plug in hot tubs