just love John Ward Videos,, no silly speeded up nonsense and stupid muzak,, just clear explanations and live drawings and illustrations ,,,,,,, so calming relaxing also takes all the days tensions away
I have a degree in electrical engineering and I still manage to get confused by this. Your explanation is masterful! And this is exactly what my late father-in-law did in his 18th Century farmhouse, because there were other properties close by. Glad I’m in software instead…
In South Africa it is common to have a earth electrode on a TNCS system, to improve the earth from the electrical supply ( the neutral is periodically spiked to earth on regular intervals on the supply side as well) So the more people spiking to earth the better the neutral connects to earth.( Less ground lift under load) RCDs were also invented in South Africa, so we have had them since the early 70s. So you don't have to rely on the earth tripping a circuit breaker, but rather the RCD tripping.
Excellent explanation. I have a Tesla point in an outbuilding and the electrician installed a TT rod solely for that unit. The sockets and lights in the building use the TN-C-S ....and now I know why! Thanks.
So with the density of modern housing its pretty much impossible to do a TT system. So I have noted that some vehicle charging point manufacturers have tackled this with PEN monitoring therefore disconnecting the supply if the CNE cable is faulty. On inverter systems it is important to be certain that in island mode these do recombine the neutral & earth connection as well as not being capable of back feeding into the network.
Boat house in Nottingham had a TT earth connected to the incoming water pipe which was plastic, so there was no earth for the building. But the telephone line was connected to the building earth, and earthed at the exchange building 600 yards away on the other side of the river. When an earth fault occurred in the boat house, the telephone line card caught fire in the exchange building.
The telephone line shouldn't have been connected to earth in the building. Since shared service was done away with some 30 years ago there has been no requirement for any local earth on an exchange line
@@davidfaraday3085 Used to have at the office a TELEX line, which, due to the crappy cable, was running on a single core of the incoming cable, and earth ground as return.Yes was noisy, but this was the only wire available, as the rest were a combination of split pairs for incoming phone lines, and a lot of dead pairs from old cable that was damaged. Took 5 years of weekly fault calls logged before the incumbent Telco finally approved laying a new cable from the street manhole (dating back to pre 1900) to replace the old lead sheathed paper cable into the building. Had to run it via another route as well, because that 50 year old cable had absolutely no intention to come out of the conduit it had rusted into under the building. Thus new PVC drain pipe (disguise the cable as being copper and stealable) up the side and into the building, to another demarc point. Later on I tidied up the demarc points, and got 3 garbage bags full of copper wire out from there, all old wire and unused wire that had forever been added to and overlaid with newer cable instead of removing the old. Still had a lot of the original wiring in there from 1950 when it was built, long disused but left there, and often still connected and introducing noise.
@@SeanBZA Telex required two wires, one for transmit and the other for receive. In the UK at least you could not have a standard exchange telephone line run over one wire with earth return. At least not if you wanted to receive calls as the earth would have caused immediate ring-trip.
This is the exact issue I have on a current property, house has 100A supply and TNCS, Garage which contains the boiler/water main etc. All metallic pipes supplying the house which has a 10mm bond coming from it to bond the boiler plant. However the garage (metal framed with a reinforced concrete floor which is all bonded) has a separate 100A supply which is on TT and also linked into the boiler plant because of the oil tank. Inherited system and is going to be a complete headache to sort out. Great video as always.
This ia where JW advise is to TT the whole thing, plastic incomers on service ie water gas. Then the hardest part.. Keep the rod well away from anything metallic.
@@nmajid1984 Problem being its not usually possible to plastic all the incoming services on older properties, unless you go right back to the main service heads of the various supplier's, there is also another issue these service pipes are on many older properties the main earth plus its not unusual to have properties daisy chained together . My supply and the neighbours are linked , My supply is TNC-S converted because of a fault six months ago neighbour who we feed off underground is TNS , our water supplies are also lead . Its a right mish mash
In Ireland, a TN system comes into our house, and also an earth electrode is linked to the dis board, via a 16 csa cable, the 16 csa t/e that connects the dis board to the suppliers equipment has the cpc also connected to the neutral of the suppliers service fuse, thus TT and TN system combined
In the last scenario you described John if the neutral conductor was broken somewhere up the street surely the neighbours houses return path would then be through your TT system and potentially make all your extraneous conductive equipment live as the current rose?
I Agree. Also even if it was just 1 house where the pen failed, i bet it wouldn't take long for the electrode to bake under load and go high resistance
The ridiculous thing about the continual water supply from the house to the outbuilding is that the cross-sectional area of 15mm copper piping with a 0.7mm wall thickness is 31.4 sq mm, or three times that required for earth bonding. Why the authorities do not allow a continuous copper feed to an outhouse to count as a bonding connection, I've no idea. After all, for a TT outhouse connection, then we would have to know the converse is true; that is it is not metallic throughout its length. Where it is continuous, it's likely to have a lower impedance than any bonding cable that is put in (something readily measured as well). I think I would also argue that if you have a TN-C-S system and are bonded to metal water and gas supply pipes, they are going to provide a much lower impedance earth return path than any rod you are likely to stick in the ground as there will simply be vastly more metal surface area exposed to the ground. Of course, the regulations don't allow a TT system to use those water and gas pipes as the earth, but I suspect that where it's a TN-C-S supply in such circumstances, the reduced earth return impedance provided by an ground spike is going to be insignificant.
I'd be interested to see a video on how radio frequency grounds fit in with all this if that's something you've had any experience of. For a lot of radio transmitter systems you want a very low impedance ground. Some antennas like 1/4 wave verticals need a very good ground to drive against. This can mean burying lots of metalwork in the ground like a large mesh mat or hundreds of radials coming from the antennas base. When combined with a tn-c-s system this good ground can present a very low impedance to the rest of the street and if you're not careful you can end up sinking ground current through your radio equipment for the rest of the street. Grounding can be a nightmare for radio amateurs like myself as it can also provide a path for interference and you can end up with RF in places it shouldn't be. Then there's also the risk from lighting strikes to consider. Then there's the distance between the equipment and the ground point because at high frequencies where that distance can easily be significant compared to the wavelength of the frequency of operation and it's impedance can vary. My station isn't grounded and I use a DC power supply that's isolated from the mains. I've never had any issues but because of this I steer clear from antennas that require a good ground to operate but I should probably ground my station to protect against lighting but I'm not sure that's even realistically possible so I just disconnect my antennas
I believe there are some solutions to this, my "ground mat" is not engineered for low impedance at 50hz. but of course in a fault condition significant current could flow through it. The mat is bonded to the house ground (arguably by an under sized conductor). Commercial radio sites have massive ground bonding conductors and contrary to myths generally survive lightning strikes unscathed. I think the more correct engineering approach to mine will involve lifting the patio (again) and using an appropriately larger conductor. I dare not post some of the other solutions.
So when I find a bad earth connection on the supply side and the DNO tells me that an earth connection is my responsibility, but the gas supply is metallic and I live in an urban situation with all sorts of buried metallic services and variety of neighbouring earth connections, how prey do I make a compliant system for my vehicle charger or outside power point?
Thanks JW very informative always love your videos. I have TN-S, my Ze is 0.97 called uk power. They tested and was same unsatisfactory reading. They said its not their responsibility as it is pre 1988 and I should install an earth rod or pay them to rectify. What options do I have? Thanks in advance.
Late coming to the party, but this shows the advantage of TN-C-S off a PME system. Ze would be around 0,3 ohms. However, 0.97 is not too bad for a sheath earth. If you had TT it would be higher. Suggest if you have RCBOs fitted - residual current combined with overcurrent circuit breakers then - provided Ze is stable - you should be okay.
Few things to consider, if it’s in an out building and there is no extraneous conductive parts comming in ie plastics service pipes and you can get your rod 10 meters away so you don’t induce any voltage from a fault then yes , mind you I’m very weak on swimming pools so could be totally wrong
Hi John, so basically TT a hot tub in a standard house is probably a waste of time. I was always taught putting a rod in on a TNCS system would stop any tingles if you were in contact with true earth & the water. Although I do seem to remember that it was if you some distance away from the tncs earthing. Also the tech helplines still state that hot tubs should be TT’d.
Why do I get the impression, that in the future, there will be an investigation into why so many people are being zapped due to the incorrect(ish) installation of car chargers.
This reminds me of a dangerous situation that I once encountered when someone I once knew had a small camper van with a mains shore supply connection and he had it plugged in outside his house on the local public road with an extension that was plugged in to the live mains via a dodgy adapter that wasn't earthed! Oh dear indeed! Needless to say I quickly sorted it out and warned him of the deadly serious danger of his vehicle body becoming seriously LIVE!
Metal shipping container used as a shed with power in it, TN-C-S supply. Do you bond the container? You wouldn't bond cable tray (I don't think?), none of the electrics are directly connected to the container itself and were you to bond it and get a PEN fault the whole thing would become sizzly. I'm currently taking the view that it would be more of a risk to bond it than not, but I'm not all that sure! The workbench is probably more important to bond IMO.
I supplied power to a garage with exposed metalwork going into the ground recently from a TNCS supply, I contacted the local DNO for plans of exactly where the PME earth electrodes are and how many there is (to get a sense of the likelihood of a PEN fault (underground cable)), I got the plans and there was a surprisingly small amount of electrodes and based on this I made it TT as class 1 equipment was also expected to be used outside of the garage. Shortly after I got an email from the DNO saying do NOT use the PME earth it must be done as TT, The email did also state that each PME joint has a "pigtail" electrode so there is more than can be seen on the plans, but they didn't sound too confident on using PME around true earth
10:35 - all well and good while the PME is intact but when the PME is bringing 70+V into the MET that water pipe will act as an excellent earth between those two installations. Ordinarily, the TNCS will give the lower resistance reading but when the PME issue arises, I can't see how that pipe is ever going to deliver anything like the 50V to earth (safe touch voltage limit). I'm still inclined to go for the TT arrangement.
CAUTION! This video has wiring colours to two versions of BS 7671. Great care should be taken before undertaking extension, alteration or repair that all conductors are correctly identified.
street lighting uk distribution is mostly tnc-s but they are now shifting to TT to try to eliminate a situation of neutral loss that can raise the columns to a dangerous voltage or where a neutral to earth fault occurs in the consumers installation along with a high circuit loop impedance which can also occur resulting in a situation that raises protective device operation to greater than 5 secs. However from what i have just heard John explain if i understand correctly then voltages may well be transmitted to columns due to possible voltages on underground services in close proximity caused by faults to earth on other electrical systems
Great presentation thank you. Re exporting TNCS to an outbuilding in garden 10-20m away from house, if a totally wooden structure with a water supply would it be best to put it on a TT system and make sure water pipe is plastic and use RCBO s at CU? Or fully import TNCS and use additional earth rod and not worry if water pipes are metal or plastic?
4:10 but then it can't clear faults. If you get a fault to earth in the spa then it the breaker won't trip because there too much resistance in the mass of earth for it to draw enough current to clear a fault.
Extraneous conductive parts require bonding, as they may not be at the same potential (voltage) as other parts. If it's bonded, then the outbuilding cannot be TT, as it's connected to the main house which is a different earthing system. Only other option would be to replace the pipe with plastic so it's non-conductive.
Hi John, I am currently excavating a 6m x 4m x 1.8m deep extension to my TN-C-S supplied house. I am adding a 393 rebar matrix before concert slab pour. After the pour I will add 100mm of insulation before adding 100m concrete. I am then building a pool that will sit 275mm away from the rebar matrix. I am sure I will get a reading of less than 4 Ohms. I plan to bond all connections to this and keep it TN-C-S
Great video JW. A question for you, if I may. If you were going to install an hot tub or EV charger in your home, based upon your power supply characteristics, how would you install them, to give you complete peace of mind
So most tt earthing systems aren’t actually tt?? I’ve tested a lot of tt systems and it is surprising how many of these systems have an unexpected low earth fault loop impedance (Zs) at the extremities of the final circuits (as good as you would expect on a tn-c-s system), due to the water and gas parallel paths that exist, obviously neighbouring properties that have had a tn-c-s conversion at some point. In all honesty I think the “convert the incoming extraneous parts” to plastic is unrealistic solution. So that leaves the question what is the real practical solution 🤷♂️ Great video JW👍
John, would this same low value impedance theory apply to SPD’ s with say only one house has an SPD and a Surge occurs . I.e Surge would seek out the low value impedance of the only SPD in the street?
Just add a decent local earth to the TNCS (and I mean decent, not a foot long stick hammered in the ground) and it's safer than any TT system. But it looks like the island has to do their own thing again....
@@stefantrethan You will need that earth electrode to be less than around 1.5ohms to keep the touch voltage under 70V in a PEN fault (Calculated with a house having MAX loads of 60A (using the calculation from A722.3). I have put in quite a few electrodes (8ft long) and never got less than around 30ohm
@@marcuswareham1 even with putting more than one electrode say 2 or 3? The problem is as John explains putting in a small UK properties even one is difficult putting 2/3 is near enough impossible at the requisite distance to ensure low earth impedance.
I found this channel a couple of months ago and have been really enjoying it from over here in the USA! Thanks for all these videos. I'm planning to install an EV charging point sometime soon here at my home here, and your posts have raised a question for me. We have TN-C-S with neutral and earth joined at the service entrance, grounded on the property with two separate rods, and also by bonded to a metallic gas line and a buried copper main water supply that no doubt both supply their own grounding. I wonder if that difference -- having grounding present on each property -- is related to why we do not have special requirements for EV chargers to use TT grounding or to be otherwise protected in any special way. (I believe that the charging points themselves already have built in GFCI/RCD protection, but that's all as far as I know.) I wonder if our installations are inherently more dangerous than those going in in the UK, or if the better grounding, coupled with split phase 240V giving a maximum difference of 120V to ground, somehow adequately mitigates the dangers in a way that I do not understand. Maybe something to do with charging relying on the two hot wires to get 240V and not really using the neutral? It seems to me like 120V between the body of the vehicle and the ground would still be a possible failure mode, and plenty dangerous. But I'm not an electrician, and I may be misunderstanding something. Any thoughts on whether this difference in requirements is based on some mitigating factor present here in the US, or maybe just represents a different tolerance for risk or lagging standards?
Late reply but I want to clarify this for future readers. The USA doesn't really use TN-C-S or TT (I had never even heard these terms before watching UK electricians on youtube). We have our own system which is similar to the TN-C-S variant that JW describes at 20:07. The neutral coming in from the utility is grounded/earthed at the transformer but we never treat it as a ground, we always have at least one ground rod on each property. At the main panel (consumer unit), the neutral is bonded to ground. There isn't really a single point failure which could occur that would put a dangerous voltage on the ground, so we don't need any special rules or protection equipment.
Interesting! I have an odd situation in my house a previous electrician has cut of the cpc from what i believe is a TNS supply as its only something like a 2.5mm. (Cpc disappears behind the DNO wooden backboard i think the DNO head is in my neighbours house as im mid terrace and the house was built in 1950s. All i have is VIR into the meter on my side). Anyway iv done a Ze on the cut cable and its 0.28ohms but a earth rod has been installed instead of using the 2.5 as i assume it was decided to be undersized. (60A supply) iv been tempted to connect the DNO cpc into the Db to improve the current Ze and leave the earth rod in. However i figured any fault would still travel down the 2.5 as the resistance is so much lower than the rod.
Why not extend the earth cable at 12:03? Let's assume we install an external WIFI router instead of an A/C condenser. That means an ethernet cable probably also runs inside and connects to other electrical equipment. Wouldn't it be a good idea to bond the earth spike to the house earth in order to prevent differences in ground potential difference? I'm asking this because I personally had wifi gear on a pole outside my house, and almost every electrical storm without fail blew ethernet ports on the routers (I was only using an earth spike for the gear and pole outside). I'm guessing I could have alleviated or prevented some of that damage if I bonded the earths, or relied exclusively on the house earthing rather than a separate earth spike?
Stuff like wifi routers are all isolated low voltage dc made by switchmode power supplies. There should never be a path to earth from the isolated secondary side. So long as your router has a power brick and only a dc input you just need some form of optoisolation on the ethernet cable as soon as it enteres the property to prevent a potential path to earth via your ethernet cable. Also providing a preferntial path to earth for lightning strikes near your pole may be benificial.
Many years ago I was part of a team that created a "pole top" concentrator for low calling-rate telephone lines. It linked 14 telephone lines to the exchange using only 4 pairs of wires, the idea being to be able to offer service to new customers when there were no spare pairs in the existing cable. The prototype worked beautifully linking our office phones to the site PABX. However when we put the pre-production units into the field, actually on the tops of poles and linked to the exchange via overhead wiring, the failures started. Unit after unit was returned as faulty and in every case the fault was lightning damage. Some of these units had been miles from the lightning strikes but the induced current in the phone cables had been enough to blow the semiconductor switches used in the unit. It proved impractical to adequately protect the unit from such surges and the project was abandoned.
Not really as many haven't got a clue what they are doing and do things by parrot fashion because someone else has done it. Use an isolated supply or understand what the RAE teaches not just enough to pass the exam.
Non electrician here. This may be a daft question but how do you measure the resistance of a TT earth? I live in France and Im pretty sure I have a copper rod in the ground. The house is 30 years old now and I'm wondering if the copper stake degrades over time, thus, degrading my path to earth. Presumably I need a Megger type tester (which I don't have)? Great vid again John, as per usual. Can't thank you enough for your clear and concise content.
If there is a long metal pipe between houses, what's the chance that its resistance to real earth would be high enough that it would rise to a significant voltage?
@@lordjaashin Remember your voltage drop calculations. The ground dissipates voltage effectively due to it's relatively high resistance but a metal pipe is just a large hollow wire.
Interesting video John. There is one thing though; at the end of the video you suggest that connecting an electrode that is outside of the property to the PME connection is a good idea. If the electrode cannot by accessed by persons then this is OK, but if the electrode can be assessed and electrode impedance is not low enough, then this will possibly export a raised voltage outside of the ‘faraday cage’ bonding of the property.
I've been aware of this for a long time but recently TT systems have sort of become a protective measure in their own right for some external applications and this can only be the case if there are no metallic services to be bonded and the electrode is clear of anything connected to the network neutral. Should we have new earth type possibly TT-N ? and TT just for a clean earth (Island Earth) If the IET does this I thought of it first.
You have mentioned about removing metal water pipes and preplacing with plastic. What about the water within the pipe is this not conductive and will be at earth potential ?
Not sure the massive low resistance earth electrode would help, it might even make it worse. Your neighbours really mess everything up. If you did have shared bonded metal pipes, in the event of a fault on your neutral alone, there would still be a path to neutral via your bonding, through the shared pipes, out through their bonding and Back to the transformer through their neutral. To make a difference, the impedance of the earth electrode would have to be lower than that of the path joining to your neighbour, potentially very short if in a semi detached. On the other hand if the neutral fault occurred upstream of everybody connected to the transformer, seem like it would do more harm than good. You'd become the prime return path for the current for everybody on the street- through their bonding and pipework to your earth electrode, and back to the transformer. But then an entire street's worth of current is flowing through pipes and wires designed for one house, something they may not be able to handle. Many amps flowing through the gas main to your house continuously seems a little worrying, even the 16mm^2 conductor probably isn't rated for hundreds of amps contunuously. Worse, the connection could be good enough to mask the break, allowing it to persist for an extended period, where without, the earth impedance would be high enough to cause disruption, and a call to fix it.
John what would you do in this situation? Power to a carport for some lights and sockets. The carport has a metallic frame and is part buried in the ground, the ground inside the carport has good conductivity with true earth. It is also known that class 1 equipment is to be used inside the carport. The carport is also 2m away from buried PME metalwork The house supply is TN-C-S, the 2 problems I see are: - Use the TN-C-S earth and bond the metallic frame (extraneous conductive part) using 10mm The problem being this makes using class 1 equipment inside the carport potentially dangerous due to associated risks of PEN fault. - Make the carport TT, Although due to the location and presence of buried PME metal work electrode separation and separation of the metallic frame will be under 5m therefore rendering the point of making it TT pointless?
Where you say you should have 10mm2 earth if you're using the house's earth for the outbuilding - what equivalent is that to the armoured sheath of an SWA cable? Say I have a 4mm2 three conductor SWA (15mm overall diameter) - is using the steel sheath good enough along with one conductor?
For bonding of 10mm², the armour needs to have the equivalent conductance, which for steel is about 8x less, so steel armour of 80mm² would be required. For a 3 core cable, the cores would have to be 70mm² or above to have armour of that size, something that would never be used on a domestic installation. 4mm² 3 core SWA has armour of about 20mm², which is equivalent to about 2.5mm² copper. if bonding is required to the outbuilding, then the only viable option is 10mm² 3 core. If bonding is not required due to no extraneous conductive parts in the outbuilding, then the cable can be smaller, as the requirements for a protective conductor are different to those of a bonding conductor.
In the TT system as you describe it, how are you assured that you get enough current from a fault to trip the breaker? US systems ground the neutral at the transformer, but also at the main panel. Past the main panel, everything has an earth wire that goes back to the main panel and the bonding point. Sharing a ground and neutral has been against code for probably 50 years in the US?
@@sidwainhouse Ah. US installations normally only put GFCIs on circuits subject to moisture, such as outdoor outlets, bathrooms, hot tubs, and the like. I think the newer codes have a requirement for AFCIs on circuits going to bedrooms.
@@PaulSteMarie In domestic we pretty much have everything on RCDs now. These are done at the source of the circuit so the whole installation is protected. With the new trend of electric vehicle charging points taking off we're having to change the type of RCD as we have seen DC currents generated by the EV point blinding the RCD and stopping it from operating when required. Customers are more than happy to show off their brand new Tesla but begrudge paying more money out for something that may well save their life... 🤣 AFDDs arc fault detection devices, (AFCIs) are coming in slowly but we've never really had any major issues with arc faults causing problems, probably because we use 230 VAC single phase which tends to burn things out a lot quicker than 110 VAC.
Hi John, excellent informative video thank you very much. I have a question you may be able to advise, our supply in a TN earth but I have two containers sitting on the ground and a metal garage alk of which need to be earthed. Will the surface area of the structures sitting on the ground introduce a TT effect or would this be negligible. Many thanks👍
I assume that if you just added an insulated section on the pipe that feeds the outbuilding in your first example then that would be fine and you could avoid exporting the earthing system?
Thanks for tackling this tricky & poorly understood subject John. It would have been useful to have some practical illustration of isolating the TNC-S earth from an armoured cable at an outbuilding. With all the Electrician videos we have on RUclips, I haven't seen a single one of them demonstrate this, I haven't even managed to find a photo... I gather the most common approach is to terminate the armoured cable in a plastic box rather than directly to the "garage" consumer unit. Perhaps worthy of another video please. Another suggestion I have seen is to use a plastic step-down adapter from say 25mm to 20mm at the consumer unit. I have seen a straight through 20mm, but only as a component of a gland used in explosive environments, so too expensive to be practical. An important point that seemed to be missed in the video is that 10mm earth cable to an outbuilding (for bonding requirements) as part of an armoured cable isn't really a very feasible size as it would make for a huge armoured cable! Also worth mentioning that exporting TNC-S is more of an issue where you have a metal outbuilding, particularly if damp is a problem. Plastic vs metal accessories in the outbuilding might also be worth mentioning. Also concrete floor vs wooden floor.
Normally when the armoured leaves the house it would be connected into a plastic box on the wall as you wouldn't want a SWA run through the house so this is where the disconnection would take place.
13:34 - The "voltage" you are talking of isn't really clear. It sounds like you are talking of some kind of point voltage rather than a difference in potential between two conductive parts. I love your videos and really appreciate your replies, so please don't think I'm having a go. Just curious and trying to get my own mind fully around the topic.
I thought most modern water pipes were plastic ? Some examples / pics of incoming supply TNCS TNS and TT system would be awesome / the actual boxes. Comparing a 1930 house to a 1980 house (new build estate) 30 meters away.
Would it be fair say that in a garage/outbuilding with no extraneous conductive parts it’s fine to use SWA and connect to the house TN system? Thank you for the videos and sharing your knowledge.
Yes. If there are extraneous conductive parts, the TN earth can still be used, those parts just need to be bonded to the main earth terminal in the same way those in the main building are.
But what about Exposed Conductive Parts - such as the metal chassis of a freezer? Especially if one could simultaneously touch the freezer and be in contact with the ground, True Earth, upon entering/exiting the wooden outbuilding? Seems like there isn't a perfect answer and the designer has to make a judgement based on all things considered. Then explain in detail to anybody questioning his/her judgement as to the suitability of the design.
Hi John, i enjoy your videos and explanations very much, thank you. I have a swim spa in my garden, (large hot tub), it's on it's own TT. The earth rod is 13m away from my incoming supply which is a PME. Water and gas are plastic. so hopefully my TT is good. The rod tested to 53 ohm's, ground very dry. My question refers to bonding. I have introduced a set of portable aluminium steps to access the spa. They sit on "true ground" and it would be normal to be in contact with them and the water at the same time. Should these be bonded to the TT. Thanks.
TT not really an option in places where there is a dry season, we don't get any rain for up 5 months of the year and the soil totally dries out, conductivity just about zero.
really good video, But how do we stop earth loops with all the copper pipes in the group? lightning strikes in the area and current can flow from earth electrode through the house out the water pipes.
My water pipe was lead up until last year when I put a plastic one in. I suspect most of the houses around my 1930s estate (of which there are many many in the country, as I'm sure you know) probably haven't been converted. Most houses around here tend to be TN-S (at least house presentation wise anyway). For extra confusion mine is TN-S, looped from next door in split concentric cable, so looks a lot like, and is always mistaken for, TN-C-S. Substation is around 350m away, Ze at DB last measured as 0.24 ohms.
Am I being a bit simple here or are you saying that, roughly, all EV charging installations, past, present and future, are deficient? I've watched this and also your previous video on EV chargers and you've convinced me. I'm not an electrician but I do have experience of unrealistic requirements and the worry they cause the conscientious (but not the cowboys).
Thank you for this video. Can I ask a question please - with a TT earth system, how does an RCD work? My understanding is that they need the reference to neutral to be able to trip? Many thanks in advance
John, very interesting, thank you. I carry out Quinquennial surveys on Churches and see it becoming common to bond the Lightning conductor to the earth on the electrical system with 10mm earth wire. In the case of many older churches where there may be only one lightning electrode on a tower or spire the ground termination of the tape is within a metre or two of the external wall. I have been told by an electrician that this is correct but it appeared to me that in the case of a strike the entire earth circuitry in the church will become live. Could you comment please and also on why TT earths on Churches always seem to be within a metre of the building?
If you have plastic water pipe, surely the water it carries still provides a fairly high earth link between other houses so even if you have all supplies coming in using plastic pipes, you still cannot a fully effective TT earth system?
So work in the caravan industry, I vist caravan touring sites, storage sites and customers homes. Every single home i visit will have the caravan or motorhome pretty much continually connected to the mains supply, many, but not all will have metal body's, all will have a metal chassis and copper gas pipes bonded to this supply via the 230 volt hook-up cable, most of these homes will be TNCS arrangements Your video indicates that officially the out let to the caravan or motor home should really be converted to a TT system which in a street is actually next to impossible 🤔
Just further to that, with the caravans and motorhomes having a 12 volt DC system and charger for this, what is the potential for a DC current making its way back to the RCD and blinding it to a fault?
Excellent explanation, thanks very much, that’s all the things you probably can’t do, what is the answer though when someone wants a hot tub on a TN-C-S system?
I have an earth electrode outside my property to mitigate against pen faults as at the time the DNO cabling was over 50 years old. Thankfully all this old cabling has since been replaced with insulated 3 phase and pen.
If the outbuilding is a Metal structure on concrete floor with incoming water in plastic pipe, what earthing arangement should be used If house has TN-c-s? And shall you bond metal structure? Thank you JW
I believe it would be as simple as making a insulated coupling at the furthest entry point of water / gas lines. Example just after the meter when installed outside the home of course.
That's ok for water, but gas services cannot have any plastic components after the meter. Newer gas services are often plastic outside underground, so having an older steel supply converted to plastic may be an option if the network operator offers it.
I would be interested to see how I might connect a small suitcase type standby generator to a TN - C -- S system. Particularly the earthing requirements.
There isn't one, but to have any real effect, that earth rod needs to have a very low resistance. A single rod with 10s of ohms or more will do nothing.
@@jwflame Thanks for the reply, I agree the earth rod must be effective to benefit or several. Some idiot stole the copper bonding from our local sub station station. The power company came and installed temporary earth rods as were on a TNCS supply. Since then we I felt safer with our own earth. Thanks.
I am not an electrician.. but I am involved a lot in Cambodia where skills are minimal and safety is not thought about. On our small farm, there is no earthing system - this was going to cost more money, so they don't do it. I have a shed where I have a lot of tools, lathe and particularly welders - all without Earth connections. I am thinking, can I at least, make things a little safer by putting an Earth rod in where the shed is.. it is remote from the main house... and then connect this Earth rod to the earth outlet connections in the shed. Would this provide a level of protection, and should this Earth connection then also be connected to the Neutral ? which of course then would connect all the way back to the sub board in one of the properties and then to the main board where the power enters the properties... I know the answer is to put a proper earthing system in all the way through but at the very least, I want to improve the safety of welding where there can be a lot of water also.. ugggg.. :-( ...
I think most of the chargers are class 2, however, the car plugged into to charger I think in most cases is seen as class 1, although David Savery speaks some interesting things about if EVs would be class 1 or not, here is a link to the part in the video I am talking about ruclips.net/video/qQ4OSUusPOk/видео.html
So how would you argue your point to the DNO when the TNS earth connection has failed and they try an get you to install a rod, or they would be happy to charge for a new PME connection? I suppose we could argue that they took on the DNO system so they need to maintain their systems earth path that was installed so many years ago?
Would it make sense to run the last meter or two of metal pipe in plastic instead? (ref 10:55) then you would not need the bonding when it enters the out building?
So if you connect a remote building to a TNCS house, you should take the earth over from the house and join it to an additional ground rod. If the remote building has a steel structure (RSJ into the ground) could that be used instead of a ground rod?
Help please! I had an electrician install an all metal construction, step lift in the garden. Few years on, after watching JW, i realise the armoured cable was never properly installed via a glad to the CU, nor is it possible now because the cable entry into the house is buried in the wall. It might well be a 3 core cable, not checked yet. If this appliance isn’t bonded properly, would a TT setup be useful?
Shouldn't the RCD to your car charger or shed trip even if the ground stake is more than 10 ohms? Is an isolation transformer to likely to fail for safety?
A broken combined N&E conductor on the supply will not cause any RCD to trip, the break is on the supply side of the RCD. EV chargers can be supplied from an isolating transformer - the problem is that the transformer will cost more than the charge point.
@@jwflame OK thanks, I never thought about it before but this is probably why we use to get a shock on the kitchen sink when running high current stuff like the feed mill as the mains line is always going to have some resistance.
Yes they are. An isolation transformer is mentioned in BS7671 as an option for EV charge points. Cost is the major problem, £100s for even a small one.
For a 3phase and a neutral supply, which system of earthing will now be the safest to adopt ? The neutral is at a zero potential thus has not current under a balance load condition . I am thinking that a TN-C will be safer than a TT . What do you think JW ?
Hi John, great video however if you say that outhouses need to be TT due to extraneous metalwork and the need for 10mm bonding how does that relate to a hottub ? As i understand it hottubs are mainly plastic or fibre glass. Appreciate your reply. Thanks Tony
if there are extraneous conductive parts, they either need to be bonded to the TN-C-S supply with a minimum of 10mm², or if it's configured as a TT install those ECPs cannot be connected to anything with a TN-C-S supply, which in most cases is impossible as it's very likely those parts will be shared with the main building or other buildings nearby. Hot tubs don't need bonding as they are not extraneous conductive parts. They will have an earth connection to the heating element and possibly other components, and that is sized based on the cable that supplies it. Making a hot tub TT is an option but can be difficult or impossible as the electrode for it has to be far away from anything connected to a TN earth. Using TN-C-S for a hot tub can have other problems, those are covered in another video which will be published later today.
I have a question-why must one conductor be earthed at all? With the electrocution hazard involved, would it not be better if neither was referenced to earth?
If a system could remain isolated, it would be safer. It's used in a few specialist applications. The problem is keeping it that way - any single fault to earth anywhere would make it into an earth referenced system but with no control over which conductor was earth referenced and no way to detect that fault until a second fault occurred.
@@jwflame The man himself answered me! Thanks John. I use an isolation transformer when I work on old radios and every time I use it I wonder why the residential service has reference to ground. By the way, could I make a suggestion for a future video? The SWER system used in very remote Australia and other parts of the world? I find it fascinating and I know your analysis would be spot-on. Take care, Ian.
We use exactly this IT based system on the railway. However copious use of insulation monitors mitigates the chance of a single earth fault causing issues... For installations with no insulation monitors then every time you open the cabinet you have to do an earth test (unless it's vern done in the last couple of days) thankfully they are quick and easily done but you do need to understand the method and readings you get.... It gets even more complicated when 25kV overhead wires are involved because those are earth referenced (whether you want it that way or not, rail lines even when on insulating pads make a fairly good earth over many miles!} www.bender-uk.com/products/insulation-monitoring-overview
If I make an earth connection to the rebar in the driveway concrete along with the TNCS Supply more than 10m away from my incomer or services connecting both with 16mm do I not limit any PD over the whole area?
As the water pipe comes from the main building, and as you suppose it is already bonded. Would you have to bond it again? Or could you just instead use the main earth from the cable to do bonding only?
Well, I know the water supply to me is supplied via asbestos cement pipes, the HDPE stopped 50m away, leaving the old AFC pipes instead. The feed though off the fibre pipe is a corroded steel pipe, and half the underground pipe is copper or plastic, along with big chunks of the inside pie as well. Still have boned the copper sections together and to the water heater, as the inside pipes are a lot better off in copper, as it lasts so much better than the plastic, and definitely is likely to last better than the galvanised steel pipes did. Still is some steel pipe around, but lucky for me not to me, all long gone, feel almost sorry for those with the steel, as it will be expensive to replace. Still have cast iron waste pipes though in the stacks, they will all eventually be replaced with PVC when they fail, but till then they are generally in good condition for the vertical parts. the horizontal parts in the ground are vitreous clay pipes, when they eventually fail the new part of them will be PVC, except for the one section, where the replacement part will be expensive, involving hydrocracking and pulling through a HDPE replacement pipe, as this one runs under the foundations of the building. Already know which company will do this, they are good at it, and have done a lot around here.
May I ask, what your views are on the A2:2022 regulation 411.4.2 (p68 of the BBB - BS7671:2018+A2:2022) please? Whereby, it is now recommended to (i.e. you SHOULD) install an additional connection to earth by means of an earth electrode (as a TT backup to the PEN)? And, with respect to outbuildings, would you still divorce the PME from the house and install a local TT, or does this now mean that you can export the PME/TT to it? Thanks.
Thanks for the video, always appreciated. Does someone not make an RCD type device which looks for a current >"x" in the conductor between the MET and the earth rod - and when tripped will disconnect Line and Neutral conductors?.
Interesting idea. In order to keep the integrity of the cable between the MET and the earth rod I envisage a current transformer around the existing unaltered cable. Hypothetically, ? could a very small amplified voltage generated by the sensing coil around the cable be applied to something like a modified RCD deign (100 Amp). One MET = the whole supply disconnected.
just love John Ward Videos,, no silly speeded up nonsense and stupid muzak,, just clear explanations and live drawings and illustrations ,,,,,,, so calming relaxing also takes all the days tensions away
I have a degree in electrical engineering and I still manage to get confused by this. Your explanation is masterful! And this is exactly what my late father-in-law did in his 18th Century farmhouse, because there were other properties close by. Glad I’m in software instead…
In South Africa it is common to have a earth electrode on a TNCS system, to improve the earth from the electrical supply ( the neutral is periodically spiked to earth on regular intervals on the supply side as well) So the more people spiking to earth the better the neutral connects to earth.( Less ground lift under load) RCDs were also invented in South Africa, so we have had them since the early 70s. So you don't have to rely on the earth tripping a circuit breaker, but rather the RCD tripping.
Very clear and concise explanation. Enjoyed it JW.
Excellent explanation. I have a Tesla point in an outbuilding and the electrician installed a TT rod solely for that unit. The sockets and lights in the building use the TN-C-S ....and now I know why! Thanks.
So with the density of modern housing its pretty much impossible to do a TT system. So I have noted that some vehicle charging point manufacturers have tackled this with PEN monitoring therefore disconnecting the supply if the CNE cable is faulty. On inverter systems it is important to be certain that in island mode these do recombine the neutral & earth connection as well as not being capable of back feeding into the network.
Boat house in Nottingham had a TT earth connected to the incoming water pipe which was plastic, so there was no earth for the building. But the telephone line was connected to the building earth, and earthed at the exchange building 600 yards away on the other side of the river. When an earth fault occurred in the boat house, the telephone line card caught fire in the exchange building.
The telephone line shouldn't have been connected to earth in the building. Since shared service was done away with some 30 years ago there has been no requirement for any local earth on an exchange line
@@davidfaraday3085 Used to have at the office a TELEX line, which, due to the crappy cable, was running on a single core of the incoming cable, and earth ground as return.Yes was noisy, but this was the only wire available, as the rest were a combination of split pairs for incoming phone lines, and a lot of dead pairs from old cable that was damaged. Took 5 years of weekly fault calls logged before the incumbent Telco finally approved laying a new cable from the street manhole (dating back to pre 1900) to replace the old lead sheathed paper cable into the building. Had to run it via another route as well, because that 50 year old cable had absolutely no intention to come out of the conduit it had rusted into under the building. Thus new PVC drain pipe (disguise the cable as being copper and stealable) up the side and into the building, to another demarc point. Later on I tidied up the demarc points, and got 3 garbage bags full of copper wire out from there, all old wire and unused wire that had forever been added to and overlaid with newer cable instead of removing the old. Still had a lot of the original wiring in there from 1950 when it was built, long disused but left there, and often still connected and introducing noise.
exactly
@@SeanBZA Telex required two wires, one for transmit and the other for receive. In the UK at least you could not have a standard exchange telephone line run over one wire with earth return. At least not if you wanted to receive calls as the earth would have caused immediate ring-trip.
This is the exact issue I have on a current property, house has 100A supply and TNCS, Garage which contains the boiler/water main etc. All metallic pipes supplying the house which has a 10mm bond coming from it to bond the boiler plant. However the garage (metal framed with a reinforced concrete floor which is all bonded) has a separate 100A supply which is on TT and also linked into the boiler plant because of the oil tank. Inherited system and is going to be a complete headache to sort out. Great video as always.
This ia where JW advise is to TT the whole thing, plastic incomers on service ie water gas. Then the hardest part.. Keep the rod well away from anything metallic.
@@nmajid1984 Metal framed building and metal supply pipes. Basically impossible.
@@nmajid1984 Problem being its not usually possible to plastic all the incoming services on older properties, unless you go right back to the main service heads of the various supplier's, there is also another issue these service pipes are on many older properties the main earth plus its not unusual to have properties daisy chained together . My supply and the neighbours are linked , My supply is TNC-S converted because of a fault six months ago neighbour who we feed off underground is TNS , our water supplies are also lead . Its a right mish mash
In Ireland, a TN system comes into our house, and also an earth electrode is linked to the dis board, via a 16 csa cable, the 16 csa t/e that connects the dis board to the suppliers equipment has the cpc also connected to the neutral of the suppliers service fuse, thus TT and TN system combined
And another very informative video. Absolutely brilliant JW.
Do enjoy a Saturday afternoon lecture from JW. 😁
In the last scenario you described John if the neutral conductor was broken somewhere up the street surely the neighbours houses return path would then be through your TT system and potentially make all your extraneous conductive equipment live as the current rose?
I Agree. Also even if it was just 1 house where the pen failed, i bet it wouldn't take long for the electrode to bake under load and go high resistance
Why? It's not connected to N.
I can see the TTs on the statue in the background at the start of the video
The ridiculous thing about the continual water supply from the house to the outbuilding is that the cross-sectional area of 15mm copper piping with a 0.7mm wall thickness is 31.4 sq mm, or three times that required for earth bonding. Why the authorities do not allow a continuous copper feed to an outhouse to count as a bonding connection, I've no idea. After all, for a TT outhouse connection, then we would have to know the converse is true; that is it is not metallic throughout its length. Where it is continuous, it's likely to have a lower impedance than any bonding cable that is put in (something readily measured as well).
I think I would also argue that if you have a TN-C-S system and are bonded to metal water and gas supply pipes, they are going to provide a much lower impedance earth return path than any rod you are likely to stick in the ground as there will simply be vastly more metal surface area exposed to the ground. Of course, the regulations don't allow a TT system to use those water and gas pipes as the earth, but I suspect that where it's a TN-C-S supply in such circumstances, the reduced earth return impedance provided by an ground spike is going to be insignificant.
I'd be interested to see a video on how radio frequency grounds fit in with all this if that's something you've had any experience of. For a lot of radio transmitter systems you want a very low impedance ground. Some antennas like 1/4 wave verticals need a very good ground to drive against. This can mean burying lots of metalwork in the ground like a large mesh mat or hundreds of radials coming from the antennas base. When combined with a tn-c-s system this good ground can present a very low impedance to the rest of the street and if you're not careful you can end up sinking ground current through your radio equipment for the rest of the street. Grounding can be a nightmare for radio amateurs like myself as it can also provide a path for interference and you can end up with RF in places it shouldn't be. Then there's also the risk from lighting strikes to consider. Then there's the distance between the equipment and the ground point because at high frequencies where that distance can easily be significant compared to the wavelength of the frequency of operation and it's impedance can vary. My station isn't grounded and I use a DC power supply that's isolated from the mains. I've never had any issues but because of this I steer clear from antennas that require a good ground to operate but I should probably ground my station to protect against lighting but I'm not sure that's even realistically possible so I just disconnect my antennas
I believe there are some solutions to this, my "ground mat" is not engineered for low impedance at 50hz. but of course in a fault condition significant current could flow through it. The mat is bonded to the house ground (arguably by an under sized conductor). Commercial radio sites have massive ground bonding conductors and contrary to myths generally survive lightning strikes unscathed. I think the more correct engineering approach to mine will involve lifting the patio (again) and using an appropriately larger conductor. I dare not post some of the other solutions.
@@g0fvt When you say ''arguably by an under sized conductor'', I'm curious what size is this?
@@Orgakoyd certainly not 10mm² cross section...
Sounded like an advert for the latest crazy regulations - foundation earthing.
So when I find a bad earth connection on the supply side and the DNO tells me that an earth connection is my responsibility, but the gas supply is metallic and I live in an urban situation with all sorts of buried metallic services and variety of neighbouring earth connections, how prey do I make a compliant system for my vehicle charger or outside power point?
Travel back in time and prevent PME ever existing.
Thanks JW very informative always love your videos. I have TN-S, my Ze is 0.97 called uk power. They tested and was same unsatisfactory reading. They said its not their responsibility as it is pre 1988 and I should install an earth rod or pay them to rectify. What options do I have? Thanks in advance.
Late coming to the party, but this shows the advantage of TN-C-S off a PME system. Ze would be around 0,3 ohms. However, 0.97 is not too bad for a sheath earth. If you had TT it would be higher. Suggest if you have RCBOs fitted - residual current combined with overcurrent circuit breakers then - provided Ze is stable - you should be okay.
@@jackmu7409 For a TN-S supply, the Max Ze is 0.8 Ohms...
Few things to consider, if it’s in an out building and there is no extraneous conductive parts comming in ie plastics service pipes and you can get your rod 10 meters away so you don’t induce any voltage from a fault then yes , mind you I’m very weak on swimming pools so could be totally wrong
Hi John, so basically TT a hot tub in a standard house is probably a waste of time. I was always taught putting a rod in on a TNCS system would stop any tingles if you were in contact with true earth & the water. Although I do seem to remember that it was if you some distance away from the tncs earthing. Also the tech helplines still state that hot tubs should be TT’d.
Thanks JW for the input 👍
Why do I get the impression, that in the future, there will be an investigation into why so many people are being zapped due to the incorrect(ish) installation of car chargers.
Especially passers by when street charging outside your house!
No ones been zapped sofar and evs have been around 10 years
@@MatthewEng2593 I take it that Risk Assessment isn't one of your strong points!
@@MatthewEng2593 there aren't really THAT many of them though? Not in the grand scheme of how many people/houses there are?
This reminds me of a dangerous situation that I once encountered when someone I once knew had a small camper van with a mains shore supply connection and he had it plugged in outside his house on the local public road with an extension that was plugged in to the live mains via a dodgy adapter that wasn't earthed! Oh dear indeed! Needless to say I quickly sorted it out and warned him of the deadly serious danger of his vehicle body becoming seriously LIVE!
Thanks
This is why it is so important to look after your TTs
Pay attention to that ladies
Metal shipping container used as a shed with power in it, TN-C-S supply.
Do you bond the container?
You wouldn't bond cable tray (I don't think?), none of the electrics are directly connected to the container itself and were you to bond it and get a PEN fault the whole thing would become sizzly.
I'm currently taking the view that it would be more of a risk to bond it than not, but I'm not all that sure! The workbench is probably more important to bond IMO.
I believe you'd isolate it from the TNCS supply and earth rod it.
I supplied power to a garage with exposed metalwork going into the ground recently from a TNCS supply, I contacted the local DNO for plans of exactly where the PME earth electrodes are and how many there is (to get a sense of the likelihood of a PEN fault (underground cable)), I got the plans and there was a surprisingly small amount of electrodes and based on this I made it TT as class 1 equipment was also expected to be used outside of the garage.
Shortly after I got an email from the DNO saying do NOT use the PME earth it must be done as TT, The email did also state that each PME joint has a "pigtail" electrode so there is more than can be seen on the plans, but they didn't sound too confident on using PME around true earth
@@marcuswareham1 wow, seriously detailed reply, thank you.
So easy to assume that the CPC should be at earth potential.
10:35 - all well and good while the PME is intact but when the PME is bringing 70+V into the MET that water pipe will act as an excellent earth between those two installations.
Ordinarily, the TNCS will give the lower resistance reading but when the PME issue arises, I can't see how that pipe is ever going to deliver anything like the 50V to earth (safe touch voltage limit).
I'm still inclined to go for the TT arrangement.
CAUTION! This video has wiring colours to two versions of BS 7671. Great care should be taken before undertaking extension, alteration or repair that all conductors are correctly identified.
😂
where does it stop
street lighting uk distribution is mostly tnc-s but they are now shifting to TT to try to eliminate a situation of neutral loss that can raise the columns to a dangerous voltage or where a neutral to earth fault occurs in the consumers installation along with a high circuit loop impedance which can also occur resulting in a situation that raises protective device operation to greater than 5 secs.
However from what i have just heard John explain if i understand correctly then voltages may well be transmitted to columns due to possible voltages on underground services in close proximity caused by faults to earth on other electrical systems
Great presentation thank you. Re exporting TNCS to an outbuilding in garden 10-20m away from house, if a totally wooden structure with a water supply would it be best to put it on a TT system and make sure water pipe is plastic and use RCBO s at CU? Or fully import TNCS and use additional earth rod and not worry if water pipes are metal or plastic?
4:10 but then it can't clear faults. If you get a fault to earth in the spa then it the breaker won't trip because there too much resistance in the mass of earth for it to draw enough current to clear a fault.
11:10 why can't you just do the TT and not bond the pipe in the out building? It's already bonded house-side so what's the point in bonding it again?
Extraneous conductive parts require bonding, as they may not be at the same potential (voltage) as other parts.
If it's bonded, then the outbuilding cannot be TT, as it's connected to the main house which is a different earthing system.
Only other option would be to replace the pipe with plastic so it's non-conductive.
Hi John, I am currently excavating a 6m x 4m x 1.8m deep extension to my TN-C-S supplied house. I am adding a 393 rebar matrix before concert slab pour. After the pour I will add 100mm of insulation before adding 100m concrete. I am then building a pool that will sit 275mm away from the rebar matrix. I am sure I will get a reading of less than 4 Ohms. I plan to bond all connections to this and keep it TN-C-S
Sounds good. Suggest the bonding is at least 25mm², rather than the commonly used 10mm²
Great video JW. A question for you, if I may. If you were going to install an hot tub or EV charger in your home, based upon your power supply characteristics, how would you install them, to give you complete peace of mind
So most tt earthing systems aren’t actually tt??
I’ve tested a lot of tt systems and it is surprising how many of these systems have an unexpected low earth fault loop impedance (Zs) at the extremities of the final circuits (as good as you would expect on a tn-c-s system), due to the water and gas parallel paths that exist, obviously neighbouring properties that have had a tn-c-s conversion at some point.
In all honesty I think the “convert the incoming extraneous parts” to plastic is unrealistic solution.
So that leaves the question what is the real practical solution 🤷♂️
Great video JW👍
John, would this same low value impedance theory apply to SPD’ s with say only one house has an SPD and a Surge occurs . I.e Surge would seek out the low value impedance of the only SPD in the street?
Just add a decent local earth to the TNCS (and I mean decent, not a foot long stick hammered in the ground) and it's safer than any TT system.
But it looks like the island has to do their own thing again....
@@stefantrethan You will need that earth electrode to be less than around 1.5ohms to keep the touch voltage under 70V in a PEN fault (Calculated with a house having MAX loads of 60A (using the calculation from A722.3). I have put in quite a few electrodes (8ft long) and never got less than around 30ohm
@@marcuswareham1 even with putting more than one electrode say 2 or 3? The problem is as John explains putting in a small UK properties even one is difficult putting 2/3 is near enough impossible at the requisite distance to ensure low earth impedance.
I found this channel a couple of months ago and have been really enjoying it from over here in the USA! Thanks for all these videos.
I'm planning to install an EV charging point sometime soon here at my home here, and your posts have raised a question for me. We have TN-C-S with neutral and earth joined at the service entrance, grounded on the property with two separate rods, and also by bonded to a metallic gas line and a buried copper main water supply that no doubt both supply their own grounding. I wonder if that difference -- having grounding present on each property -- is related to why we do not have special requirements for EV chargers to use TT grounding or to be otherwise protected in any special way. (I believe that the charging points themselves already have built in GFCI/RCD protection, but that's all as far as I know.) I wonder if our installations are inherently more dangerous than those going in in the UK, or if the better grounding, coupled with split phase 240V giving a maximum difference of 120V to ground, somehow adequately mitigates the dangers in a way that I do not understand. Maybe something to do with charging relying on the two hot wires to get 240V and not really using the neutral? It seems to me like 120V between the body of the vehicle and the ground would still be a possible failure mode, and plenty dangerous. But I'm not an electrician, and I may be misunderstanding something. Any thoughts on whether this difference in requirements is based on some mitigating factor present here in the US, or maybe just represents a different tolerance for risk or lagging standards?
Late reply but I want to clarify this for future readers.
The USA doesn't really use TN-C-S or TT (I had never even heard these terms before watching UK electricians on youtube). We have our own system which is similar to the TN-C-S variant that JW describes at 20:07.
The neutral coming in from the utility is grounded/earthed at the transformer but we never treat it as a ground, we always have at least one ground rod on each property. At the main panel (consumer unit), the neutral is bonded to ground. There isn't really a single point failure which could occur that would put a dangerous voltage on the ground, so we don't need any special rules or protection equipment.
Can you please do a video on IT earthing system.
Yes please!
👍
Interesting! I have an odd situation in my house a previous electrician has cut of the cpc from what i believe is a TNS supply as its only something like a 2.5mm. (Cpc disappears behind the DNO wooden backboard i think the DNO head is in my neighbours house as im mid terrace and the house was built in 1950s. All i have is VIR into the meter on my side). Anyway iv done a Ze on the cut cable and its 0.28ohms but a earth rod has been installed instead of using the 2.5 as i assume it was decided to be undersized. (60A supply)
iv been tempted to connect the DNO cpc into the Db to improve the current Ze and leave the earth rod in. However i figured any fault would still travel down the 2.5 as the resistance is so much lower than the rod.
Why not extend the earth cable at 12:03? Let's assume we install an external WIFI router instead of an A/C condenser. That means an ethernet cable probably also runs inside and connects to other electrical equipment. Wouldn't it be a good idea to bond the earth spike to the house earth in order to prevent differences in ground potential difference? I'm asking this because I personally had wifi gear on a pole outside my house, and almost every electrical storm without fail blew ethernet ports on the routers (I was only using an earth spike for the gear and pole outside). I'm guessing I could have alleviated or prevented some of that damage if I bonded the earths, or relied exclusively on the house earthing rather than a separate earth spike?
Stuff like wifi routers are all isolated low voltage dc made by switchmode power supplies. There should never be a path to earth from the isolated secondary side. So long as your router has a power brick and only a dc input you just need some form of optoisolation on the ethernet cable as soon as it enteres the property to prevent a potential path to earth via your ethernet cable. Also providing a preferntial path to earth for lightning strikes near your pole may be benificial.
Many years ago I was part of a team that created a "pole top" concentrator for low calling-rate telephone lines. It linked 14 telephone lines to the exchange using only 4 pairs of wires, the idea being to be able to offer service to new customers when there were no spare pairs in the existing cable.
The prototype worked beautifully linking our office phones to the site PABX. However when we put the pre-production units into the field, actually on the tops of poles and linked to the exchange via overhead wiring, the failures started. Unit after unit was returned as faulty and in every case the fault was lightning damage. Some of these units had been miles from the lightning strikes but the induced current in the phone cables had been enough to blow the semiconductor switches used in the unit. It proved impractical to adequately protect the unit from such surges and the project was abandoned.
Ham radio adds even more complexity to earthing of antennas and having ‘rf’ grounds etc
Not really as many haven't got a clue what they are doing and do things by parrot fashion because someone else has done it. Use an isolated supply or understand what the RAE teaches not just enough to pass the exam.
Great video as usual John 👍🏻
Non electrician here. This may be a daft question but how do you measure the resistance of a TT earth? I live in France and Im pretty sure I have a copper rod in the ground. The house is 30 years old now and I'm wondering if the copper stake degrades over time, thus, degrading my path to earth.
Presumably I need a Megger type tester (which I don't have)?
Great vid again John, as per usual. Can't thank you enough for your clear and concise content.
If there is a long metal pipe between houses, what's the chance that its resistance to real earth would be high enough that it would rise to a significant voltage?
in about 10 meters of ground any stray voltage is dissipated but I'm confused how stray volrage don't dissipate in 10 meters of pipe
@@lordjaashin Remember your voltage drop calculations. The ground dissipates voltage effectively due to it's relatively high resistance but a metal pipe is just a large hollow wire.
@@Mattja1 Good point
Great videos John. The best!
Interesting video John. There is one thing though; at the end of the video you suggest that connecting an electrode that is outside of the property to the PME connection is a good idea. If the electrode cannot by accessed by persons then this is OK, but if the electrode can be assessed and electrode impedance is not low enough, then this will possibly export a raised voltage outside of the ‘faraday cage’ bonding of the property.
I've been aware of this for a long time but recently TT systems have sort of become a protective measure in their own right for some external applications and this can only be the case if there are no metallic services to be bonded and the electrode is clear of anything connected to the network neutral. Should we have new earth type possibly TT-N ? and TT just for a clean earth (Island Earth) If the IET does this I thought of it first.
Thanks a lot for the nicest explanation
You have mentioned about removing metal water pipes and preplacing with plastic. What about the water within the pipe is this not conductive and will be at earth potential ?
Clean water is a very poor conductor.
ruclips.net/video/MF9DCNkaE8I/видео.html
Yes that told me. So its okay to heat the bath water by throwing an electric fire into it. Happy New Year try smiling it doesn't hurt.
Not sure the massive low resistance earth electrode would help, it might even make it worse.
Your neighbours really mess everything up.
If you did have shared bonded metal pipes, in the event of a fault on your neutral alone, there would still be a path to neutral via your bonding, through the shared pipes, out through their bonding and Back to the transformer through their neutral.
To make a difference, the impedance of the earth electrode would have to be lower than that of the path joining to your neighbour, potentially very short if in a semi detached.
On the other hand if the neutral fault occurred upstream of everybody connected to the transformer, seem like it would do more harm than good.
You'd become the prime return path for the current for everybody on the street- through their bonding and pipework to your earth electrode, and back to the transformer. But then an entire street's worth of current is flowing through pipes and wires designed for one house, something they may not be able to handle.
Many amps flowing through the gas main to your house continuously seems a little worrying, even the 16mm^2 conductor probably isn't rated for hundreds of amps contunuously.
Worse, the connection could be good enough to mask the break, allowing it to persist for an extended period, where without, the earth impedance would be high enough to cause disruption, and a call to fix it.
Fantastic explanation as usual JW 👍
Thanks John Very interesting
John what would you do in this situation?
Power to a carport for some lights and sockets. The carport has a metallic frame and is part buried in the ground, the ground inside the carport has good conductivity with true earth. It is also known that class 1 equipment is to be used inside the carport. The carport is also 2m away from buried PME metalwork
The house supply is TN-C-S, the 2 problems I see are:
- Use the TN-C-S earth and bond the metallic frame (extraneous conductive part) using 10mm
The problem being this makes using class 1 equipment inside the carport potentially dangerous due to associated risks of PEN fault.
- Make the carport TT, Although due to the location and presence of buried PME metal work electrode separation and separation of the metallic frame will be under 5m therefore rendering the point of making it TT pointless?
Where you say you should have 10mm2 earth if you're using the house's earth for the outbuilding - what equivalent is that to the armoured sheath of an SWA cable? Say I have a 4mm2 three conductor SWA (15mm overall diameter) - is using the steel sheath good enough along with one conductor?
For bonding of 10mm², the armour needs to have the equivalent conductance, which for steel is about 8x less, so steel armour of 80mm² would be required. For a 3 core cable, the cores would have to be 70mm² or above to have armour of that size, something that would never be used on a domestic installation.
4mm² 3 core SWA has armour of about 20mm², which is equivalent to about 2.5mm² copper.
if bonding is required to the outbuilding, then the only viable option is 10mm² 3 core.
If bonding is not required due to no extraneous conductive parts in the outbuilding, then the cable can be smaller, as the requirements for a protective conductor are different to those of a bonding conductor.
@@jwflame Thank you John
In the TT system as you describe it, how are you assured that you get enough current from a fault to trip the breaker?
US systems ground the neutral at the transformer, but also at the main panel. Past the main panel, everything has an earth wire that goes back to the main panel and the bonding point. Sharing a ground and neutral has been against code for probably 50 years in the US?
Unless we can get a really low and stable resistance on the electrode we use RCDs (GFCI) to meet disconnection times for earth faults.
@@sidwainhouse Ah. US installations normally only put GFCIs on circuits subject to moisture, such as outdoor outlets, bathrooms, hot tubs, and the like. I think the newer codes have a requirement for AFCIs on circuits going to bedrooms.
@@PaulSteMarie In domestic we pretty much have everything on RCDs now. These are done at the source of the circuit so the whole installation is protected.
With the new trend of electric vehicle charging points taking off we're having to change the type of RCD as we have seen DC currents generated by the EV point blinding the RCD and stopping it from operating when required.
Customers are more than happy to show off their brand new Tesla but begrudge paying more money out for something that may well save their life... 🤣
AFDDs arc fault detection devices, (AFCIs) are coming in slowly but we've never really had any major issues with arc faults causing problems, probably because we use 230 VAC single phase which tends to burn things out a lot quicker than 110 VAC.
Hi John, excellent informative video thank you very much. I have a question you may be able to advise, our supply in a TN earth but I have two containers sitting on the ground and a metal garage alk of which need to be earthed. Will the surface area of the structures sitting on the ground introduce a TT effect or would this be negligible. Many thanks👍
I assume that if you just added an insulated section on the pipe that feeds the outbuilding in your first example then that would be fine and you could avoid exporting the earthing system?
That's an option, or ideally replace the whole pipe with plastic.
Pray tell me my good man, why does the old 1980's Tefal TV adverts spring to mind when ever I see you? Only joking big lad, keep up the good work.
Thanks for tackling this tricky & poorly understood subject John.
It would have been useful to have some practical illustration of isolating the TNC-S earth from an armoured cable at an outbuilding. With all the Electrician videos we have on RUclips, I haven't seen a single one of them demonstrate this, I haven't even managed to find a photo... I gather the most common approach is to terminate the armoured cable in a plastic box rather than directly to the "garage" consumer unit. Perhaps worthy of another video please.
Another suggestion I have seen is to use a plastic step-down adapter from say 25mm to 20mm at the consumer unit. I have seen a straight through 20mm, but only as a component of a gland used in explosive environments, so too expensive to be practical.
An important point that seemed to be missed in the video is that 10mm earth cable to an outbuilding (for bonding requirements) as part of an armoured cable isn't really a very feasible size as it would make for a huge armoured cable!
Also worth mentioning that exporting TNC-S is more of an issue where you have a metal outbuilding, particularly if damp is a problem. Plastic vs metal accessories in the outbuilding might also be worth mentioning. Also concrete floor vs wooden floor.
Normally when the armoured leaves the house it would be connected into a plastic box on the wall as you wouldn't want a SWA run through the house so this is where the disconnection would take place.
13:34 - The "voltage" you are talking of isn't really clear. It sounds like you are talking of some kind of point voltage rather than a difference in potential between two conductive parts.
I love your videos and really appreciate your replies, so please don't think I'm having a go. Just curious and trying to get my own mind fully around the topic.
I thought most modern water pipes were plastic ?
Some examples / pics of incoming supply TNCS TNS and TT system would be awesome / the actual boxes. Comparing a 1930 house to a 1980 house (new build estate) 30 meters away.
They are, but there are still plenty of older installations with steel or lead pipes. Many gas installations with steel supply pipes as well.
Would it be fair say that in a garage/outbuilding with no extraneous conductive parts it’s fine to use SWA and connect to the house TN system? Thank you for the videos and sharing your knowledge.
Yes.
If there are extraneous conductive parts, the TN earth can still be used, those parts just need to be bonded to the main earth terminal in the same way those in the main building are.
@@jwflame cheers JW 👍
But what about Exposed Conductive Parts - such as the metal chassis of a freezer? Especially if one could simultaneously touch the freezer and be in contact with the ground, True Earth, upon entering/exiting the wooden outbuilding? Seems like there isn't a perfect answer and the designer has to make a judgement based on all things considered. Then explain in detail to anybody questioning his/her judgement as to the suitability of the design.
You are awesome coming from Trinidad
Hi John, i enjoy your videos and explanations very much, thank you.
I have a swim spa in my garden, (large hot tub), it's on it's own TT. The earth rod is 13m away from my incoming supply which is a PME. Water and gas are plastic. so hopefully my TT is good. The rod tested to 53 ohm's, ground very dry.
My question refers to bonding. I have introduced a set of portable aluminium steps to access the spa. They sit on "true ground" and it would be normal to be in contact with them and the water at the same time. Should these be bonded to the TT.
Thanks.
Presumably JW will be marketing a 15m long self-drilling earth electrode, with a 14m insulating sleeve?
Yet another set of good reasons for not having water and gas in metal pipes. Works for me. But I rather wonder if my TT installation achieves
TT not really an option in places where there is a dry season, we don't get any rain for up 5 months of the year and the soil totally dries out, conductivity just about zero.
really good video, But how do we stop earth loops with all the copper pipes in the group? lightning strikes in the area and current can flow from earth electrode through the house out the water pipes.
Great video John, thank you. I am now wondering though, if you're getting a low resistance to earth via a gas/water pipe, i.e.
12:04 why does the earth cable stop there? Does it earth just the charger’s housing?
No connection between TN system and EV charger housing, the cpc stops before entering the charger.
How many water mains are metal these days ? Plastic pipes have been standard for decades
My water pipe was lead up until last year when I put a plastic one in. I suspect most of the houses around my 1930s estate (of which there are many many in the country, as I'm sure you know) probably haven't been converted. Most houses around here tend to be TN-S (at least house presentation wise anyway). For extra confusion mine is TN-S, looped from next door in split concentric cable, so looks a lot like, and is always mistaken for, TN-C-S. Substation is around 350m away, Ze at DB last measured as 0.24 ohms.
@@JonGTranceClassics Mine too. It sprang a leak and I replaced it with plastic. Left the old pipe in place to be melted down in the future.
Am I being a bit simple here or are you saying that, roughly, all EV charging installations, past, present and future, are deficient?
I've watched this and also your previous video on EV chargers and you've convinced me.
I'm not an electrician but I do have experience of unrealistic requirements and the worry they cause the conscientious (but not the cowboys).
Thank you for this video. Can I ask a question please - with a TT earth system, how does an RCD work? My understanding is that they need the reference to neutral to be able to trip? Many thanks in advance
I look forward to seeing CJR try to wrap his head round this 😂
John, very interesting, thank you. I carry out Quinquennial surveys on Churches and see it becoming common to bond the Lightning conductor to the earth on the electrical system with 10mm earth wire. In the case of many older churches where there may be only one lightning electrode on a tower or spire the ground termination of the tape is within a metre or two of the external wall. I have been told by an electrician that this is correct but it appeared to me that in the case of a strike the entire earth circuitry in the church will become live. Could you comment please and also on why TT earths on Churches always seem to be within a metre of the building?
If you have plastic water pipe, surely the water it carries still provides a fairly high earth link between other houses so even if you have all supplies coming in using plastic pipes, you still cannot a fully effective TT earth system?
So work in the caravan industry, I vist caravan touring sites, storage sites and customers homes. Every single home i visit will have the caravan or motorhome pretty much continually connected to the mains supply, many, but not all will have metal body's, all will have a metal chassis and copper gas pipes bonded to this supply via the 230 volt hook-up cable, most of these homes will be TNCS arrangements
Your video indicates that officially the out let to the caravan or motor home should really be converted to a TT system which in a street is actually next to impossible 🤔
Just further to that, with the caravans and motorhomes having a 12 volt DC system and charger for this, what is the potential for a DC current making its way back to the RCD and blinding it to a fault?
Excellent explanation, thanks very much, that’s all the things you probably can’t do, what is the answer though when someone wants a hot tub on a TN-C-S system?
I have an earth electrode outside my property to mitigate against pen faults as at the time the DNO cabling was over 50 years old. Thankfully all this old cabling has since been replaced with insulated 3 phase and pen.
From birth to Earth.... Another good JW production... As far as the TN thing goes... Nike make a good pair of those.
If the outbuilding is a Metal structure on concrete floor with incoming water in plastic pipe, what earthing arangement should be used If house has TN-c-s? And shall you bond metal structure? Thank you JW
I believe it would be as simple as making a insulated coupling at the furthest entry point of water / gas lines. Example just after the meter when installed outside the home of course.
That's ok for water, but gas services cannot have any plastic components after the meter.
Newer gas services are often plastic outside underground, so having an older steel supply converted to plastic may be an option if the network operator offers it.
Great discussion. Thank you
I would be interested to see how I might connect a small suitcase type standby generator to a TN - C -- S system. Particularly the earthing requirements.
Just what is the issue with bonding a TNCS supply to an earth rod. Surely you then have double protection and a lower protective earth resistance.
There isn't one, but to have any real effect, that earth rod needs to have a very low resistance. A single rod with 10s of ohms or more will do nothing.
@@jwflame Thanks for the reply, I agree the earth rod must be effective to benefit or several. Some idiot stole the copper bonding from our local sub station station. The power company came and installed temporary earth rods as were on a TNCS supply. Since then we I felt safer with our own earth. Thanks.
I am not an electrician.. but I am involved a lot in Cambodia where skills are minimal and safety is not thought about. On our small farm, there is no earthing system - this was going to cost more money, so they don't do it. I have a shed where I have a lot of tools, lathe and particularly welders - all without Earth connections. I am thinking, can I at least, make things a little safer by putting an Earth rod in where the shed is.. it is remote from the main house... and then connect this Earth rod to the earth outlet connections in the shed. Would this provide a level of protection, and should this Earth connection then also be connected to the Neutral ? which of course then would connect all the way back to the sub board in one of the properties and then to the main board where the power enters the properties... I know the answer is to put a proper earthing system in all the way through but at the very least, I want to improve the safety of welding where there can be a lot of water also.. ugggg.. :-( ...
great video. are ev chargers class 1 or class 2. I have never fitted one.
I think most of the chargers are class 2, however, the car plugged into to charger I think in most cases is seen as class 1, although David Savery speaks some interesting things about if EVs would be class 1 or not, here is a link to the part in the video I am talking about ruclips.net/video/qQ4OSUusPOk/видео.html
So how would you argue your point to the DNO when the TNS earth connection has failed and they try an get you to install a rod, or they would be happy to charge for a new PME connection? I suppose we could argue that they took on the DNO system so they need to maintain their systems earth path that was installed so many years ago?
Would it make sense to run the last meter or two of metal pipe in plastic instead? (ref 10:55) then you would not need the bonding when it enters the out building?
Yes, or do the whole supply in plastic pipe.
So if you connect a remote building to a TNCS house, you should take the earth over from the house and join it to an additional ground rod. If the remote building has a steel structure (RSJ into the ground) could that be used instead of a ground rod?
Help please! I had an electrician install an all metal construction, step lift in the garden. Few years on, after watching JW, i realise the armoured cable was never properly installed via a glad to the CU, nor is it possible now because the cable entry into the house is buried in the wall. It might well be a 3 core cable, not checked yet. If this appliance isn’t bonded properly, would a TT setup be useful?
Shouldn't the RCD to your car charger or shed trip even if the ground stake is more than 10 ohms? Is an isolation transformer to likely to fail for safety?
A broken combined N&E conductor on the supply will not cause any RCD to trip, the break is on the supply side of the RCD.
EV chargers can be supplied from an isolating transformer - the problem is that the transformer will cost more than the charge point.
@@jwflame OK thanks, I never thought about it before but this is probably why we use to get a shock on the kitchen sink when running high current stuff like the feed mill as the mains line is always going to have some resistance.
Hi John, I was wondering if large isolation transformers would be an option, although clearly would add Extra costs
Yes they are. An isolation transformer is mentioned in BS7671 as an option for EV charge points. Cost is the major problem, £100s for even a small one.
@@jwflame well isn't that the truth!
So are you going to take me on as an apprentice now or what?
For a 3phase and a neutral supply, which system of earthing will now be the safest to adopt ? The neutral is at a zero potential thus has not current under a balance load condition .
I am thinking that a TN-C will be safer than a TT . What do you think JW ?
Why not have lots of earth rods interconnected?
Hi John, great video however if you say that outhouses need to be TT due to extraneous metalwork and the need for 10mm bonding how does that relate to a hottub ? As i understand it hottubs are mainly plastic or fibre glass. Appreciate your reply. Thanks Tony
if there are extraneous conductive parts, they either need to be bonded to the TN-C-S supply with a minimum of 10mm², or if it's configured as a TT install those ECPs cannot be connected to anything with a TN-C-S supply, which in most cases is impossible as it's very likely those parts will be shared with the main building or other buildings nearby.
Hot tubs don't need bonding as they are not extraneous conductive parts. They will have an earth connection to the heating element and possibly other components, and that is sized based on the cable that supplies it.
Making a hot tub TT is an option but can be difficult or impossible as the electrode for it has to be far away from anything connected to a TN earth.
Using TN-C-S for a hot tub can have other problems, those are covered in another video which will be published later today.
I have a question-why must one conductor be earthed at all? With the electrocution hazard involved, would it not be better if neither was referenced to earth?
If a system could remain isolated, it would be safer. It's used in a few specialist applications.
The problem is keeping it that way - any single fault to earth anywhere would make it into an earth referenced system but with no control over which conductor was earth referenced and no way to detect that fault until a second fault occurred.
@@jwflame The man himself answered me! Thanks John. I use an isolation transformer when I work on old radios and every time I use it I wonder why the residential service has reference to ground. By the way, could I make a suggestion for a future video? The SWER system used in very remote Australia and other parts of the world? I find it fascinating and I know your analysis would be spot-on.
Take care, Ian.
We use exactly this IT based system on the railway.
However copious use of insulation monitors mitigates the chance of a single earth fault causing issues... For installations with no insulation monitors then every time you open the cabinet you have to do an earth test (unless it's vern done in the last couple of days) thankfully they are quick and easily done but you do need to understand the method and readings you get.... It gets even more complicated when 25kV overhead wires are involved because those are earth referenced (whether you want it that way or not, rail lines even when on insulating pads make a fairly good earth over many miles!} www.bender-uk.com/products/insulation-monitoring-overview
If I make an earth connection to the rebar in the driveway concrete along with the TNCS Supply more than 10m away from my incomer or services connecting both with 16mm do I not limit any PD over the whole area?
As the water pipe comes from the main building, and as you suppose it is already bonded. Would you have to bond it again? Or could you just instead use the main earth from the cable to do bonding only?
Well, I know the water supply to me is supplied via asbestos cement pipes, the HDPE stopped 50m away, leaving the old AFC pipes instead. The feed though off the fibre pipe is a corroded steel pipe, and half the underground pipe is copper or plastic, along with big chunks of the inside pie as well. Still have boned the copper sections together and to the water heater, as the inside pipes are a lot better off in copper, as it lasts so much better than the plastic, and definitely is likely to last better than the galvanised steel pipes did. Still is some steel pipe around, but lucky for me not to me, all long gone, feel almost sorry for those with the steel, as it will be expensive to replace.
Still have cast iron waste pipes though in the stacks, they will all eventually be replaced with PVC when they fail, but till then they are generally in good condition for the vertical parts. the horizontal parts in the ground are vitreous clay pipes, when they eventually fail the new part of them will be PVC, except for the one section, where the replacement part will be expensive, involving hydrocracking and pulling through a HDPE replacement pipe, as this one runs under the foundations of the building. Already know which company will do this, they are good at it, and have done a lot around here.
May I ask, what your views are on the A2:2022 regulation 411.4.2 (p68 of the BBB - BS7671:2018+A2:2022) please? Whereby, it is now recommended to (i.e. you SHOULD) install an additional connection to earth by means of an earth electrode (as a TT backup to the PEN)? And, with respect to outbuildings, would you still divorce the PME from the house and install a local TT, or does this now mean that you can export the PME/TT to it? Thanks.
Thanks for the video, always appreciated. Does someone not make an RCD type device which looks for a current >"x" in the conductor between the MET and the earth rod - and when tripped will disconnect Line and Neutral conductors?.
Interesting idea. In order to keep the integrity of the cable between the MET and the earth rod I envisage a current transformer around the existing unaltered cable. Hypothetically, ? could a very small amplified voltage generated by the sensing coil around the cable be applied to something like a modified RCD deign (100 Amp). One MET = the whole supply disconnected.