Exported PME to a summerhouse (shed...) with a hot tub
- Thread starter Bluepeppo
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How far from house?
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Is the metal frame definitely extraneous? Is the swa 2 core?
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No. Extraneous - being an extraneous-conductive-part, one liable to introduce a potential - most likely, by being in contact with the ground.
What about the floor when it gets wet? Concrete on the mud?
What about the floor when it gets wet? Concrete on the mud?
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So i see what you're saying, theoretically you can bond any extraneous parts with the 10mm. The frame would be easy to bond but as efli says the floor might be the tricky part, especially with wet people around and in and out.
Are you saying that, unless someone screws something into it (in the wall) no part of the metal frame (nor anything which was in electrical continuity with it) would be 'touchable'?
Mind you, as has been said, even if the metal frame did not qualify as an "extraneous-conductive-part", the floor could well be an issue/problem.
Kind Regards, John
Exactly. Not exposed until someone makes it so will be bonded. The structures is sat on a concrete raft on a plastic membrane. Floor is lino on timber so I don't see that being an issue.
However... 702.410.3.4.3 States - Where a PME earthing facility is used as the means of earthing for the electrical installation of a swimming pool or other basin, it is recommended that an earth mat or earth electrode of suitably low resistance, e.g. 20 ohms or less, be installed and connected to the protective equipotential bonding.
After reading many posts on many forums about hot tub, which should be treated as a pool, supplies I've never seen this.
So now with this in mind and should I add a local earth electrode connected to the exported TN-C-S earth or just go for a TT? What do others do?
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That could be the reinforcement fabric contained in the concrete slab, which will be far better than a single electrode.
Unfortunately if the slab is already done, accessing that will be next to impossible.
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Is the metal frame accessible from outside the building? It starts to get like an electricity substation with an earth mat around the perimeter of the site before you know it!
I'm not a sparky but I'd be erring on the side of TT, then you only have to worry about disconnecting the live conductors if there's an actual fault rather than what the earth is doing.
I'm not a sparky but I'd be erring on the side of TT, then you only have to worry about disconnecting the live conductors if there's an actual fault rather than what the earth is doing.
Quoting from THIS to save typing it.
TN-C-S
This is the most common configuration used in the UK. It can also be known as protective multiple earthing (PME) and provides low voltage supply with reliable and safe earthing. This system allows multiple users to utilise one supply cable. The resultant rise in current flow creates a voltage rise in the protective earthed neutral (PEN) which needs multiple connection to earth along the supply route. The neutral is earthed close to the source of supply, at the intake of the installation and at necessary points throughout the distribution system. As the DNO uses a combined neutral and PEN return path the maximum external earth fault loop impedance is 0.35 Ω.
Despite its popularity, the TN-C-S arrangement could prove hazardous if the PEN conductor becomes an open circuit in the supply because the current would not have an immediate path back to sub-station level. Because of this, there are certain installations where it is not allowed to be used – including petrol stations, building sites, caravan parks and some outbuildings.
My preference for earthing a hot tub in a summer house would be TT with one or more ground rods. The armouring of the SWA supply being earthed at the supply end ( house ) and isolated from the CPC in the summer house. A 30 mA trip RCD on the supply to the summer house.
Neutral bounces are not common but can happen and when they do the CPC of a TN-C-S installation can be at a potential very different from true ground. This creates a shock hazard for anyone able to touch ground and the CPC at the same time.
Just a thoughtvthat wandered through my mind ...
A PME earth is usually "stronger" and more reliable than a bit copper bar poked into ground of variable conductivity. So it generally makes sense to use it where available.
But loss of the PEN can cause problems ...
So are we back to a breaker that disconnects everything (including PME earth now) from the supply if there's excessive voltage/current between PME earth and a local earth rod ?
Or just lay a ground mat/grid around and under the whole house and garden connected with thicker cable than the DNO PEN conductor
But then we could create voltage gradients at our boundary, so best extend the grid across the street and join it up all those from the neighbours
BTW, earth grids at substations aren't always reliable - I'm told that a few have been ripped out by theiving b'stards. I guess some thieves aren't too clever - like I'd even want to be in the vicinity while they're ripping out the ground grid at the end of a 132kV feeder
A PME earth is usually "stronger" and more reliable than a bit copper bar poked into ground of variable conductivity. So it generally makes sense to use it where available.
But loss of the PEN can cause problems ...
So are we back to a breaker that disconnects everything (including PME earth now) from the supply if there's excessive voltage/current between PME earth and a local earth rod ?
Or just lay a ground mat/grid around and under the whole house and garden connected with thicker cable than the DNO PEN conductor
But then we could create voltage gradients at our boundary, so best extend the grid across the street and join it up all those from the neighbours
BTW, earth grids at substations aren't always reliable - I'm told that a few have been ripped out by theiving b'stards. I guess some thieves aren't too clever - like I'd even want to be in the vicinity while they're ripping out the ground grid at the end of a 132kV feeder
Especially where a person can have one foot in water with a PME "Earth" and the other foot on real Ground ( or wet decking with a conductive path to Ground )
Or when a person in the water holds hands with someone standing on the wet ground.
TT with a reliable Ground electode and a reliable RCD are as safe as an experted PME "earth" when the PME is in full working order. But hazardous when the PME is compromised and Neutral is not at ground potential.
That would be a 'solution', but it would be reliant on most/all installations having such an arrangement - since if just a few of the installations affected by a 'lost TN-C-S neutral' disconnected the PEN, currents through bonding in the remaining affected properties could rise.
I presume that current regulations would not allow disconnection of the PEN (and/or a PEN-derived 'earth').
Kind Regards, John
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