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Spectric

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The video is quiet informative, but I always used to run the earth as a core within the armoured cable rather than the armour itself because the armour is not only steel but it is also zinc plated. The issue with the TN-C-S system for providing the means of earthing is that if for any reason the Neutral becomes open circuit then you lose your earth and are now relying on supplementary bonding but also any connected loads will cause the Neutral to rise towards 230V and therefore any extraneous metal parts like the steel frame of the building if used in it's construction. This can cause touch and step voltage hazzards. Now with industrial steel framed buildings the frame will be earthed at multiple points for lightning protection that is also connected to the main earthing bar, plus the supply will be a TN-S system so no issues.

For most domestic instalations it is easier to just run a 10mm 3 core SWA cable if the DNO will allow this otherwise just use a 10mm 2 core SWA cable and suitable earthing rods / mat to ensure the earth impedance is low ..

Again remember this type of work is notifiable and should be undertaken by a qualified electrician.
 

Bodger7

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Hi Paul
Thanks for your further input. My workshop is a concrete block building with timber cladding built on a solid concrete base and there are no extraneous conductive parts. The only conduit coming into the building is the electricity cable. I have watched the John Ward video and the explanation contained in it has clarified the position for me. Unless John Ward is wrong then there seems to be nothing to prevent the use of 3 core 10 mm2 armoured cable connected to the existing 3 core 10 mm2 pvc cable already wired into my CU by an electrician. I see that you are in hospital and I wish you a speedy recovery to full health.

Thank you GF. What an informative video that explains the position in simple language. As mentioned above my workshop is built from concrete blocks on a concrete base with no extraneous conductive parts ie no water pipes or any other pipes coming into the structure. I can't see any potential danger from using a 3 core 10 mm2 armoured cable and connecting all 3 wires to the pvc cable already wired into my CU.

Hi Roy
Yes, I found the video very informative. As you suggest, I propose to run a 3 core 10 mm2 SWA, connected to the 10 mm2 pvc cable, to the workshop and use the earth within the SWA. I see that John Ward makes the same point as you about the armoured part of the cable being steel. You will see from the above that there are no extraneous conductive parts so no bonding is necessary (or possible). I tried to contact my DNO, UK Power Network, today but the people who deal with questions such as these are not available at the moment due, I suspect, to a combination of Covid and weather. I will contact them before finalising my plans and will get an electrician to check my installation before going live. Thank you again for your advice.
 

NetBlindPaul

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Since John did that video other hazards have come to light with TN-C-S systems that are currently being reported on prior to this report being presented to the IET, DNO’s, HSE & the ORR.
Once the report is complete both myself and JW will have the final copy for peer review, we were (myself and JW & a few other #e5 guys) on a video call with the guy that commissioned the research last night.
One of the things that has come to light is that the network operation can easily result in the N conductor rising well above the potential of true earth and thus the earth will also rise. Both may also carry fortuitous current and therefore pose an unexpected hazard.
I cannot recommend strongly enough, and I know that John would now agree that to export the TN-C-S earth is not recommended on safety grounds.
 

Bodger7

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Hi Paul, Roy, GF21 and others.
For obvious reasons I was concerned at the suggestion that my proposed installation could prove fatal, especially as I have young grandchildren who like to "make something" (albeit that they can't at the moment). As a result I have done some more research, sent an email to John Ward (that I have not yet received an answer to) and written to my DNO, Power Networks UK. I haven't been able to reach any firm conclusions but it seems to me that:-
1. The debate about exporting an earth from a TN-C-S system to an outbuilding is not new and has been rumbling around for 10 years at least.
2. There seems to be no definitive right or wrong answer and seemingly very competent electricians will disagree about the dangers inherent in exporting the earth from a TN-C-S system and the need to consult/seek permission from the DNO. For example, John Ward's video was produced during the period when this matter has been the subject of debate when others were advocating a TT system for all outbuildings. As far as I know that video remains on Youtube. Examples of totally opposed views can be found at, for example, Exporting the earth?
3. The DNO, or at least UK Power Networks, do not have to be asked for permission to export the earth. Their answer to my question on this and other points raised was that "I do not know of any process that we have in place to provide written permission for works such as this to take place as our responsibility is up to the main head and for any equipment we have installed and maintain, or any alterations that have been carried out by ourselves." and "I have had a look at the various forums online and this seems to be a common debate in terms of whether to use a spike, export the incoming earth etc and the question or permission from the Dno seems to be one brought up on occasion but again shot down by others in the forums without a definitive answer."
4. I had asked the DNO to confirm "that there is no known danger from exporting a TN-C-S earth to an outbuilding (subject to there being no extraneous conductive parts)." To me this is a fairly simple question to which I would have expected a comment at least, even if the answer was "it depends.....". The question was not answered so I wonder whether opinion is also divided at the DNO? I am surprised that there is no guidance given for such an important safety issue.

Although there are arguments for both exporting the TN-C-S earth and creating a separate TT system for a workshop there doesn't seem to be any safety reason for not creating a separate TT system, so that is now my preferred option. I understand that the armoured cable should be connected to the TH-C-S system at the supply end of the cable but insulated from the circuits in my workshop to ensure that in the event of any accidental damage to the SWA there will still be an earth. The SWA will be cheaper as I will be using 2 core but will need ground spike(s) for the separate earthing system. Advice as to the size for the earth cable would be appreciated. At the moment the power circuit in my workshop is in 2.5mm and the lighting circuit in 1.5mm. The earthing circuits are therefore in 2.5 and 1.5 mm cable.

When I asked the initial question I didn't realise what a can of worms I was opening but thank you to you all for the advice offered (even if it was bit worrying).
 

guineafowl21

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At the moment the power circuit in my workshop is in 2.5mm and the lighting circuit in 1.5mm. The earthing circuits are therefore in 2.5 and 1.5 mm cable.
If it’s twin and earth, the earth is smaller than the other two, e.g. for 2.5mm2 T&E, the earth is 1.5mm2.

Size of earth wire connecting TT rod to installation varies. For normal sheathed wire, it should be 16mm2 if buried, but 4mm2 if not. In either case, it can be just 2.5mm2 if mechanically protected. For the cost of a short piece, you might as well err on the large side.

John Ward also did a series of vids on earth rod installation (search “John Ward earth electrode”).

You must ensure your earth loop is below 200 ohm, ideally less than 100 ohm, and of course all circuits must be RCD protected.
 

Spectric

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Unless John Ward is wrong then there seems to be nothing to prevent the use of 3 core 10 mm2 armoured cable connected to the existing 3 core 10 mm2 pvc cable already wired into my CU by an electrician.
You have stated an obvious potential problem, if that existing 10 mm2 pvc is say T&E then the conductors may be 10mm but the CPC will not. Now without extranous parts the bonding issue goes away but changing the size of the CPC is not good practice. Solution would be to run just a two core live & neutral and use earth rod at the workshop which covers everything, even if you do have extranous parts. Make sure the armour is earthed but only at the workshop end. As I have mentioned in another post the dangers stem not from yourself but at the supply with a PME system. If the Neutral fails at the supply side then you lose Neutral and Earth but still have live. Now if there is a load connected, it obviously will not work but it will mean the Neutral is now also LIVE, ( look at Ohms law) and with extranous conductive parts that are connected to Earth they will also become live, so you have a hazard. With an Earth rod this will prevent conductive parts having a higher potential than ground/Earth and so reduces the hazard of touch/step voltages.
 

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