Discuss Earthing on TNCS in the Electrical Forum area at ElectriciansForums.net

Slimmct

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I realise this overlaps with the Earth Rod install post a little, but I am attempting to plan part one of an upgrade. Currently I am mostly confident I have TNCS supply (there is a query with the DNO to confirm) but I am a little confused by the additional strap to the lead sheaf in a TNS kind of way.

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Currently, there is a gas meter about an inch away from the old fuse box, that is supplied by a plastic main, and a lead water main. Neither of these are bonded. There is a 6mm earth cable from the existing fuse box to the DNO earth point. The fuse board has a 40A cooker circuit, a single 32A breaker for the entire 4 bed house and a single 5A breaker for all the lighting.
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The meter was recently replaced, and the chap put an isolator switch in for me.

The long term plan is to replace it all, but I am splitting into 2 building regs applications.

The first will put in a consumer unit with a 63a 30ma RCD protecting a supply to some outside sockets 32a B, outside lighting on a 6a B, a hot tub supply on a 32a C, and a 32a B radial circuit to supply a laundry room. There will also be a 80a fused switch off a Henley block supplying a 37m run to a garden office. This will have a smaller consumer unit in with a 80a 30ma RCD and 40a B for a 10kw instantaneous water heater, a 32a B for sockets in the office and workshop, a 6a B lighting circuit, and 20a D for a 16a 2PE commando for a welder.

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Part of the work will also replace the lead water main with MDPE. My question relates to the earthing and the possibility of PEN failure. I had first considered bonding the new system to the gas and water, but reading around it, there seems to be the suggestion that with plastic supplies on both, it is not necessary, and may introduce risk. Also, the braid from the earth point to the lead sheaf. It looks like ancient aluminium braid, or could be tinned copper, either way, I am genuinely concerned that it may burst into flames if asked to carry any level of current.

With the talk of PEN fault taking an alternative path to earth, I am left considering whether it should have a 16mm bond added, or whether the braid should be removed.

I dare not ask the question about adding an earth rod to the TNCS system. I cannot find a copy of the apocryphal document by Paul Cook of the IEE on the benefits of adding a stake to TNCS. I have seen some discussion that adding a stake to TNCS is effectively the same as PME, but also the counters of it being the neutral for all your neighbours and the stake not having a good enough earth to realistically reduce touch voltages below 50v in a PEN failure. It appears that most installs of a hot tub within reach of a house use the available earth rather than TT, because it is better.

The rest of the ring mains would be replaced over winter. The plan is to use the garden office and hot tub during summer, then power off and isolate the new install, submit another application for the house ring mains, and do those over winter by installing new sockets and switches, in parallel, to the old ring mains and lighting, then once inspected and if passed, rip out the old and plaster up the holes.

I am understanding that the old stuff doesn’t have to be brought up to regs if not touched. Would the addition of a Henley block in its supply and upgrading the earth to 16mm change that, or can it remain as is?
 
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Maybe the DNO has converted it from TN-S to TN-C-S perhaps in the past the sheath of the supply cable has gone very high resistance so was converted and then the sheath was earthed as it is an extraneous conductive part, although this is only speculation

It appears as if you are doing a fair amount of building work etc, would it be possible at the same time to either install an earth mat underground connected to the TN-C-S earth for where the hot tub will be or install some decking or other insulated outdoor flooring which the hot tube will sit on, in turn reducing PEN fault risks

You can install an earth rod and connect it to the TN-C-S earth although in most cases a single rod will have little effect, as for it to be effective at mitigating PEN faults it needs to be very very low resistance
 

Slimmct

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That was my first thought. I had a read through guidance note 8 and it seemed straight forward. There is a 100mm concrete pad going under the patio surface where the hot tub will be. That I weld an upstand on to the reinforcing, as per 8.9 and bonding the metallic grid, and join that to earth.

Then I started reading forums. The proposed risk of the earth being used as not just your neutral, but all your neighbours neutrals, in the event of a PEN failure and you having the best earth. My assumption was that the DNO fuse only protects the live feed, so the only limit to the back feed being the maximum capability of the earth. It seemed so straight forward before a world of possible failures cropped up. To earth the grid under the hot tub, and drive an earthing rod down into the ground within the same earthing pit, and connect the whole lot back to the TNCS earthing block on 16mm in conduit.
 

timhoward

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I’ve not fully read GN8. But is there anything to be said for bonding the concrete rebar to a decent earth electrode and use same electrode for TT Earth to hot tub?
The impedance would be easily low enough to trip an RCD if a fault occurred.

It’s true the PME Earth would be better for carrying current but the reality seems to be that it would only carry current for a fraction of a second before an RCD goes pop.

And as you’ve said the PME Earth can carry current in both directions.
 

Rockingit

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That head arrangement looks ghastly to me and it's putting neutral current down the earth sheathing on the incomer as well as/instead of the incoming CEN - I doubt you'll get the same Ze twice and it doesn't conform to ESQCR as it's neither one thing nor the other. I can't quite make out the clamp either but it looks like a standard bonding clamp which is also non-compliant. Job for a DNO fitter to sort out.
 

Slimmct

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Would a PEN fault trip the RCD? Surely the circuits would still be outputting to neutral through the RCD, wouldn't it be only at the cutout where this would divert to earth with loss of the PEN? The ringmains and radials still remaining balanced.

I guess knowing it is not PME would help some of the decision. The thinking was, that with plastic gas and water mains, and not bonding the services, if the braid was detached from the lead outer, and the hot tub delivered as TT with grid and rod, in the event of a PEN, there would be no path to earth at the cut out, and any attempt to use earth as a neutral through equipment connected to an RCD protected circuit would unbalance the circuit and trip the RCD. But surely that just leaves all the class 1 stuff live to touch through earth still.
 

Rockingit

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Would a PEN fault trip the RCD? Surely the circuits would still be outputting to neutral through the RCD, wouldn't it be only at the cutout where this would divert to earth with loss of the PEN? The ringmains and radials still remaining balanced.

I guess knowing it is not PME would help some of the decision. The thinking was, that with plastic gas and water mains, and not bonding the services, if the braid was detached from the lead outer, and the hot tub delivered as TT with grid and rod, in the event of a PEN, there would be no path to earth at the cut out, and any attempt to use earth as a neutral through equipment connected to an RCD protected circuit would unbalance the circuit and trip the RCD. But surely that just leaves all the class 1 stuff live to touch through earth still.
The problem that you have is that it's basically not anything. Yes, there's a return path in the event of a fault so ADS should work so it's not necessarily 'dangerous' it's just non-compliant and not your responsibilty to sort out as you don't own that equipment. However, until you DO get it resolved you can't do any future works.
 
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That was my first thought. I had a read through guidance note 8 and it seemed straight forward. There is a 100mm concrete pad going under the patio surface where the hot tub will be. That I weld an upstand on to the reinforcing, as per 8.9 and bonding the metallic grid, and join that to earth.

Then I started reading forums. The proposed risk of the earth being used as not just your neutral, but all your neighbours neutrals, in the event of a PEN failure and you having the best earth. My assumption was that the DNO fuse only protects the live feed, so the only limit to the back feed being the maximum capability of the earth. It seemed so straight forward before a world of possible failures cropped up. To earth the grid under the hot tub, and drive an earthing rod down into the ground within the same earthing pit, and connect the whole lot back to the TNCS earthing block on 16mm in conduit.

I cant see why this can't be done. Any extraneous conductive parts connected to the MET have the same effect and it is required to bond extraneous conductive parts. The current would be limited by the resistance of the electrode (mat and rod)

By doing this you are rasing the ground around the hot tub in potential during a fault and then inturn reducing the difference between the water or exsposed metallic parts and what your standing on
 

timhoward

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Would a PEN fault trip the RCD? Surely the circuits would still be outputting to neutral through the RCD, wouldn't it be only at the cutout where this would divert to earth with loss of the PEN? The ringmains and radials still remaining balanced.
Agree. (My mention of tripping RCD was in relation to fault conditions with the hot tub, not a broken PEN scenario, sorry for any confusion)
with plastic gas and water mains, and not bonding the services, if the braid was detached from the lead outer, and the hot tub delivered as TT with grid and rod, in the event of a PEN, there would be no path to earth at the cut out, and any attempt to use earth as a neutral through equipment connected to an RCD protected circuit would unbalance the circuit and trip the RCD. But surely that just leaves all the class 1 stuff live to touch through earth still.
Also agree.
Broken PEN's are rare, around 400-500 reported per year which means the chance of any given property having one is rather small.

In either case, TT or PME, a hot tub fault would trip the RCD before the size or impedance of the earthing conductor became material.
TT possibly provides better isolation for the general area, removes tingle problem.
PME also works as @marcuswareham points out by keeping potentials similar.

So everything else being equal, does it actually matter?!

In my mind it's a completely separate question regarding how much help an electrode would be on a PME install in a broken PEN scenario. It certainly wouldn't blow any fuse. I don't think I'd want the electrode near a hot tub really.
(I can't quickly work out the maths for touch voltage in that scenario. If we said 50 ohms rod, fault current would be 4.6A which then needs multiplying by (I think) the resistance of the total cable path from supply transformer to earth rod , and there I come unstuck....!)
 

Slimmct

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Thank you. And yes, I understand on the RCD bit now. A TT implementation would trip the RCD in the event of fault to earth before current became an issue. I think I had just got concerns about the idea of neighbours supplies trying to use my earth by flowing back up the neutral/earth, and how on earth you would calculate or plan for such an occurrence. That and discussion about issues with TT too close to a PME.

I think I am settled now that the best approach would be grid and rod TT for the hot tub and disconnect the braid to make it just TNCS without bonding to the other services. Once the work is approved to start, I'll be hiring a multifunction tester to help in testing the earth rods for the office and hot tub. I'll test the TNCS earth with and without the braid.

If I stick to the no installation within 2m of the hot tub perimeter, I am assuming that the outside sockets and lights will be okay using the TNCS earth.
 

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