TN-C-S Danger - Broken PEN Conductor (Combined Earth & Neutral)
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- Опубликовано: 25 авг 2014
- Description of the dangerous situation which can occur when the combined earth & neutral supply wire is broken on a TN-C-S electricity supply, leaving the line conductor connected.
Any RCD fitted in the installation will NOT disconnect in the event of this type of fault, and dangerous voltages will appear on exposed metalwork within the installation.
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Finally someone did it brief, clean, easy understandable. Excellent work.
I agree. I’ve been wondering about earthing arrangements for an EV charger. Other videos have glossed over the problem and focused on impractical solutions. Thanks for posting
Basically you should split PEN to PE+N on your first distribution box and connect PE from PEN to your house PE which is mandatory in EU. Then, if N is disconnected from supplier, the L will flow to PE via the house PE connection. In my country the national company already splits the PEN before connecting the house so i have 3 phases + N + PE in the junction box and supplier PE si connected to house PE.
Radu, the UK was part of the EU when this video was made. You should explain you are talking about a local grounding system for each house, that must be provided by the home owner to the distribution operator...
Hi JW - I have watched this Video a couple of times, I really enjoy the way you explain things even though I thought I understood them you always seem to add another way of looking at things that I would not have thought of!
A girl in Australia got a bad shock from a similar scenario (TT/earth stake) a few years back from an outdoor tap. The report said the ground stake was correctly fitted however being the heat of summer it appears the girl unfortunately made a better connection to earth than the stake did. Poor girl survived but is badly disabled.
A number of people on here have mentioned about also fitting a local earth or connecting to pipework as well. The local network provider here (Western Power) strongly advised against this because in the rural hamlet I’m in on overhead cables, the break is likely to be some distance away and thus I would be suddenly providing the earth for all 100 homes. Dependent on what goes wrong elsewhere, I’d better be sure my earthing was capable of a good few hundred amps!
thank you very much, this is hands down the best video on earthing systems that I've found so far.
Just loving your work JW. Very helpful and clearly explained.
Hi John ,great explanation . Back in the late 70s as an apprentice electrician in Glasgow, i was nearly killed due to the above situation on what was Called then PME system .It was a factory unit and the SSEB were connecting mains supply ,3phase & N (earrth) + meter . I was at the other end of the factory unit connecting MK grid swtiches . The part i was working on was isolated .The SSEB Electrician had finished and left site ,and our Electricians began powering on finished/tested part of the installation .
I then received a severe shock through metal MK grid plate as this had earth attached.I fell to my knees and broke the circuit to earth .Further investigation from our electricians found the SSEB had screwed onto the Neutral insulation which had the same effect you described above . Im now retired after 44 years, but a lesson never forgot.
Enjoy your videos ,keep up the good work.
Hi John thanks for very clear and easy to follow and understand the lecture..these videos and your explanations are very useful ..
Had a hard time understanding this up until now, thanks for making it so simple
Another excellent video (I know I am late to it). This has quite serious implications for some radio hobbyists whereby equipment inside the house with it's normal safety ground is connected to structures outside the house via coaxial cable.
I know I co-wrote EMC 07 for the RSGB on the Subject, rsgb.org/main/files/2019/12/UK-Earthing-Systems-And-RF-Earthing_Rev1.4.pdf
Love the different arrangements of the board markers
Very good and easy to understand presentation, thanks John.
Good video! In addition, if the supply is three-phase (standard in for example Scandinavia) you get the additional problem that a lost neutral will render any voltage in between 0-400 V on the standard one-phase 230 V circuits that might destroy plugged-in equipment and cause a fire. This is due to one-phase loads with different impedances will back feed the neutral. In essence, two loads are connected in series between two phases (400 V) and if one of them is an immersion heater, the other a computer, essentially all 400 V will appear over the computer due to its higher impedance and the formula of voltage division. This combined with what's explained in the video makes a lost PEN a very nasty fault.
This happened in our home when I was young. It was a TT system, but the Neutral connection failed. As you said, all the light loads that were on a different phase to the heavy loads in the house blew up as the heavy loads presented a much lower voltage drop across them.
Hungary is the same electrical system. (3x400/230V)
PEN fault = neutral potential increases (N potential
same thing would probably happen here in the US, id like to see him explain it and draw it out though because i wonder if the earth/ground wire issue in the video happens on our split phase system. We have come close to losing the neutral as it was being chewed on by animals for 50 years and when you used the microwave the lights on the other phase got brighter (gained about 10-15v) and everything on the same phase lost the same.
also as far as i know we never have an earth/ground wire that comes from the pole here.
kibi15
The same thing once happened to me. I’m retired now, but used to work in a large college. One day I was seeing all sorts of strange adults, fluorescent lamps flickering or dim or over-bright, various other things not working properly or overheating etc. I reported it to security who told me not to worry about it, as EDF were working in the substation. They were indeed working in there, they had rollers laid out and were pulling through new HV cables, two bundles each of three single core cables, each about the size of a rubber fire hose. New cables not connected to anything, and old ones not disturbed in any way. Whatever the fault was it was not related to this work. Returned to security to to report it again. Was told that it was just the Voltage being a bit low, and not to worry about it. The installation was overloaded, hence the new HV cables and transformer being installed, so the Voltage did tend to be a bit on the low side.
At this point I grabbed an AVO, and measured the Voltage on a number of random sockets in various rooms, some were under Voltage and some over, by a considerable amount. I guessed that the different Voltages were on different phases, and thinking about it thought that it sounded like a neutral problem with the neutral point wandering about, closer or further from different phases depending on the load being drawn from each. Then measured between neutral and earth, and found various, considerable, Voltages. The installation was way out of balance at that time with L2 being the most heavily loaded, with L1 next and L3 least. I returned to security again, told them that something was seriously wrong, and it needed urgent attention. I received the incredible reply that they would give the keys to the LV intake room so I could go down to investigate. I told them in no uncertain terms that I was a humble I.T. technician, certainly not qualified in any to do anything about it, that they needed an electrician to deal with it, and I had no reason to think that the fault was in the intake room anyway. Unfortunately, the college no longer had an electrician, the one they used to have gad left and it was decided not to replace him to save money, but to just call in a local electrician required, which was most days in a building that size.
Local electrician was out on another job somewhere, but came in later in the day, came to see me, to ask why I had reported a problem; he’d just been told by security what the Voltage was a bit low, and there was nothing he could do about that.
When I came in the following morning the electrician was there. He told me to go with him and in a large panel in a corridor behind the library where the fault actually was.
6 years later... JW was ahead of his time 👍
Always! 👍🏼
Overhead supply, windy coastal location so subjected to plenty of stress. Had this occur on our mains supply just over a year ago, was not fully broken and started to get drops in voltage, able to diagnose the fault myself. SSE came out very quickly and sorted the fault. It brings to your attention the need for a properly designed and tested installation.
This is interesting, so the copper cable around the outside of the central aluminum cable started to deteriorate. I would like to see what that looks like
Picture situation at 3:55. Neutral Return cable is broken on Supply Side of the boundary, no earth connection shown. In New Zealand it is mandatory for the Neutral to be connected to Earth at or close to the point of entry to the property -- normally by a special suitably heavy cable running from the Earth Bus in the Fuse Box direct to the Earth Stake. The Neutral Bus is also connected in the Fuse Box to the Earth Bus by suitable heavy jumper. In domestic situations, the Earth Cable is an insulated (green and yellow, but may be green only) minimum 6 sq mm, stranded cable.
Thanks for another amazing video. These videos make one aware of how little knowledge we have.
Thank you John, love the 2 different marker pen towers 🙃
I very much enjoyed this video and thought it was beautifully explained
Brilliantly presented, thanks John.
This is basicly simple, but no one knows to explain it simple, thanks.
John, you have a very listenable voice. You should do voice overs and audio books. You'd make a bomb!
This guy is a good lad, thanks for helping others,
Blimey. Very interesting that earth wiring can actually contribute to a serious issue in this situation. Thanks.
I like your markers ! You made a nice building there!😃
Very well explained. Thanks mate
The combined neutral and earth to the service entrance (TN-C) is the standard utility practice in the USA. 2 hots with 120 to ground and 240 between them, one combined neutral and earth that is grounded at the distribution transformer and also at the service entrance to the property. Interestingly, this neutral on the secondary side of the distribution transformer is connected back to the neutral on the high voltage distribution (4-25 KV) side as well, and is often the literally same wire. This is possible because most utilities use a 4 wire wye connected distribution primary setup, with multiple ground points. This practice allows some utility primary neutral current to flow back to the substation via peoples electrical systems, ground rods, and water pipes. This can continue even if the main breaker to the property is off.
33:55 I was going to ask if this is a short circuit, but I realized the path is not complete. line->heating element->neutral->cpc->exposed metal part and then no connection back to line. thanks Mr john for the clear explanation.
could you explain the effects of using PME in TN-C-S systems ? how does it reduce the effects of electric shock in the case of a broken neutral. I read that PME is required in the case of a TN-C-S system
Had this occur on our 1970s TN-C-S (PME) service cable Gas board caught the PVC sheathing of the service in the pavement 10 yrs previous repairing a gas pipe next door and over time the Neutral aluminium outer conductor corroded until it eventually fell apart and failed under load (when i was in the shower). Western Power re-jointed with a new section of copper which is also less susceptible to corrosion
Excellent explanation John thank you.
Thanks John. I'm in NZ where I understand we use a MENS earthing system. We had a situation where a mouse bit through the supply wiring in the wall between the meter and switch panel causing a similar problem to what you describe here (earth was severed and some of the supply cable damaged). I think the house side of the earth connected to either the live or neutral cable at this point. All of the bonded items began carrying a charge (130V at the metal front doorknob discovered by my son as he exited for work). No fatalities (apart from the mouse) and an emergency call out to the electricians put things right. There may be an issue with our earth rod (on the MENS system) which theoretically should have carried the charge back but need to speak to speak to an electrician after the holidays.
I'd value any input from others to put my mind at ease in the meantime. I think we've been very lucky but until the electrician has a thorough assessment I remain twitchy. Thanks.
The problem is, the earth stake does not have low enough impedence to carry the full neutral current, so it, and everything bonded to it can carry hazardous touch potential. Near to the earth stake, you can also have a hazardous step potential.
Isn't the MENS the same TN-CS?
Hi J.W, you are the best teacher on the whole Internet. You explain everything so well, you make it easy to understand. You are funny sometimes too but I don't think you even know it. Thank you so much. 🙂
Thanks John
Thank you, this question bugged me and my mates for hours
Very interesting video John.
Hi J.W. Thank you for sharing your knowledge with us.
I was to install SC TN in my new home I will cancel because I learn from you this can be dangerous
regards from
Kosovo
From? Southeastern Serbia, you mean. Serbian code requires TT for residential since many years ago, have you been under a rock?
John In Austrialia which is TN-C-S and MEN means that the Neutral Link is bridged to the Earth Link. And every Installation has the Earth Link with an Earth Electrode
Hats off 4 this excellent video
I had a very narrow escape recently with this fault.
I’m a Plumber & Heating Engineer. While working on a new build & connecting the copper external gas supply, fortunately wearing rubber gloves, I got a very strong tinkle from the copper coming through the wall from the internal installation. Alarm bells rang & I knocked off the double pole incoming supply to the property & spoke to the Site Manager.
The Spark came & tested the new installation & found as John has described, the whole house was live.......everything metal connected to the electrical supply!!! But again as John states, I was standing outside & the conductor for the bonded gas pipe. RCBs or RCDs did nothing, never tripped on any circuit!! Very, very scary!!! Be very careful out there people!!!
What’s your view on the requirements for added protection John?
Tncs shouldn't be allowed
You plumbers don’t know what a real tinkle is 😝
@@mohammedkhan5344 we (Iceland) ground the PE wire here (after the breakup). So there would be a difference. The PE lead is connected to the plumbing as well as the rebar of the building. So you need 2 or more failures for this to become a deadly fault.
In Poland we have recommendation to ground the point of splitting PEN wire to PE and N. I cannot relate to your case, but I think it is worth carrying a voltage detector with you. It looks like thick pen or marker and is an equipment of every professional electrician.
@@Hammy135 your.proud that you a.shock??? You must be a terrible.electrician
Thank you very much.
Thanks for your sharing 👍
I just installed a shower with built in ELCB. When I bridged the neutral and earth the metalwork. on the shower went live and the ELCB, didn’t trip. Thankful it’s an old soviet apartment block and a few floors up there is little contact with earth. The unit did operate with live and neutral connected.
Claro las duchas electricas tienen una resistencia,esta calento la carcasa de la ducha
A simple solution would be for the RCD to drop out when its supply goes, like the 13A plug versions do. That would mean you would need to reset it after a power cut though, so could come home to a warm freezer.
A big contactor that only connected live and neutral when they have the right voltage between them would work. Should be incorporated into the smart meter but I read they only have single pole contactors.
Voltage monitoring only detects the broken PEN conductor in some situations, it doesn't work in all of them.
HI JW, This is quite an alarming scenario that you have taken the time to highlight, would the same go with supplying power to an outhouse such as a shed or sunhouse and would the only way to make such an installation safe be to turn the outside supply to the outhouse into a TT set-up. Thank you in anticipation of your answer.
The DNO should make it mandatory for an additional Earth Rod to be installed and connected to the supply intake for every TNCS system which would mitigate the risk of this unsafe condition.
That's how it is here in Ireland
That seems on first sight to be a good idea. As J.W also explains in a later video about earthing for E.V. charge points, such a supplemental earth is no where near adequate to ground currents that would otherwise make their way through low impedance paths. A TT. rod (at the cable head) has too high a Ze to handle large fault currents.
Additionally the neutral break may take in a number of properties all feeding to the now floating P.E system. The only way out is through that metal case.
Others mention 3 phase property. But we should remember that the DNO balances the phases by property via alternate single phase supplies. So you may be lucky and have a V drop rather than a V rise.
Do the regs protect us against conductor loss or live earth. Not yet...
@@alistercarmichael4990 you are right, maybe to provide condition/requirement with lowering TT impedance 20 ohm sth, it would be greatly safe and would be safer than TT only system.
You can also connect the metal case to a local earth electrode. So the current flows directly through the ground electrode and the earth back to the transformer. This is called TT (Terra-Terra) network. But you still need the RCD. The earth resistance should not be greater than 1666 ohms (50V : 30mA = 1666 ohms). You build up your own protective conductor over the ground. And the person is still fully protected. The neutral conductor must never be connected to the protective conductor.
Do you know what confused me most. How you got those pens and board wiper to stand on top of each other!!
I live in the sticks and my home used to have TT earthing, albeit totally inadequate. The earth wire can't have been more than 4mm and the earth rod was a rapidly corroding length of galvanised steel pipe. The property was fed using two overhead cables (L+N) from the DNO street pole to the gable end.
When I had some substantial extensions built I asked the DNO if they could replace the overhead cables with a buried feed. They jumped at the chance and did the work for far less than the cost of the parts. The new armoured cable runs up the same pole where there is now a junction box with new tails connecting to the L + N. The poles actually carry the 4 cables necessary for a 3-phase supply, with different houses tapping off different phases.
The DNO provided a TN-C-S earth so the sparky who fitted a new CU used that rather than replacing the earth rod and sticking with TT. That immediately struck me as potentially dangerous since the pole sure as hell doesn't have an earth. The argument is that the PEN conductor has multiple earths along its length, so even if there was a break between two poles, there would still be at least one earth on both sides of the break. But there are still two joins between the overhead line and the feed cable. If one of those failed then my PE would no longer be at earth potential. The water pipe is plastic, as is the oil pipe supplying the boiler, and there are no other services, so the TN-C-S earth is all there is.
Apparently a supplementary earth rod is not permitted since the type of PEN fault which is never supposed to happen could result in said earth rod providing the return path for my home and any of my neighbours who happen to be on the same isolated segment. Quite how an earth rod differs from metallic water and gas supply pipes, which must be earth bonded, is unclear.
A supplementary electrode is permitted and has been for decades. Most other countries have been installing them as a standard item for ever. It's now a recommendation in the UK that all installations have one.
@@jwflame
John do you mean earth rod when you say _supplementary electrode._
That's true it's a dangerous system when the nutral broken frome the power supply.
A proper TNCS earthing arrangement has a local ground electrode at each consumer unit. This provides a suitably low path of resistance to earth which ensure breakers trip if the PEN get broken.
I didn't know that. Do you have a reference number for it in BS7671?
hI John I have watched many of your vids .I am a reg niceic approved contractor.I was wondering about your thoughts on pen conductor faults and who should be really take the responsibility for them.At present the installer is required to provide protection for a fault which isnt there and may never be there.A scenario based fault.If the dno are to supply these systems knowing full well that if they fail ,the customer could be killed then isnt it time all meters were supplied simply with a 100 amp relay as standard.on all pme systems?This would cause any fault just before the property to disconnect that supply until the fault is rectified.This would mean having the earth bonding arrangement on the output side of the relay.I dont see how that would effect anything as the supply would be cut off anyway.The only issue would be that the next door property could still have your pipe work connected through to their installation.However if their property is ok then this wouldnt seem to be of any concern .Whats your thoughts?
Good video! Thanks
American 240v 3-prong clothes dryer plugs/receptacles prior to 1996 were able to get away with combined earth/ground and neutral, which means a break in said wire would present a potentially lethal shock hazard when touching the appliance. Afterwards, four prongs (two hot, one neutral, one ground) became the standard.
You could install a light bulb right before the RCD. When you notice the light is off, then there must be something wrong.
Or you could install something like under voltage detector. When the PEN is breaking, the under voltage detector will tripped.
Or how could you make this safe?
PEN fault protection device
TN-C-S:
The risk of electric shock is minimized if installed and connect the local earth electrode to the PEN => PE + N separator point. So the soil and PE's electrical potential will be nearly identical even if the PEN is damaged.
(hungarian electrical man)
In Poland for example you have to have a local electrode no matter what type of connection you have on the supplier side. That protects you against all possibilities. The earth electrode should be connected to building foundation metal reinforcement therefore the resistance is very low. Obviously this is a requirement for all the new buildings and there is thousands of private dwellings where there is no local point of earthing.
Exactly as we do in Israe
@ProfessionalAmateur
Unfortunately, everything is based on the distribution network and the "star point" of the transformer. (this is a fundamental problem in TN electrical networks).
The ground electrode is "only" in reserve)
@ProfessionalAmateur
Yes!
Unfortunately, our standards are not hard enough in this matter.
Building a low resistance earth electrode inside the house will be some help in such cases.
Am I right in thinking that the only way to detect such a break, as it is on the suppliers' side, would be with a ZE or Earth Fault Loop Impedance test? The reading would indicate a problem if it didn't meet that recommended for this type of system by the regulations?
Looking at the diagram a TT system eliminates this serious problem. But TT needs a foundation concrete earth rod (using the house foundations) or a soil earth rod (many rods around the house). A rod or coil of earthing wire can be set in concrete in say under the garden of a house. Then the TN-CS earth supplied by the supplier can be ignored - use your own. Of course RCD/RCBOs need to be on all circuits in a TT installation, and one _main_ whole house RCBO, as the French have.
John, water & gas supply pipes have been PE for decades, so they won't act to earth the neutral beyond the break. All bonded internal metallic pipework will just rise to live potential, so at least that contributes to a Faraday cage.
Good job
Hi thanks great information, just wondering ive got a project coming up witch involves rewiring some class one metal lamppost from a tncs supply, as the post have a sleeve that in the ground to about a metre depth, I would think this would act as a reasonable earth electrode?? Would reduce the risk? Any info greatly appreciated
In this part of the world in which I live, is required to have the ground electrode in each house. Here also all the electrical connections are 3-phase so the voltage in the neutral conductor does not rise high, even in a fault situation. (three-phase load)
+peto22 Which country is that? Norway? I know in Norway they have 3-wire Delta 220 Volts transformers in rural area's and at some places where they converted 3-wire Delta 220 Volts to 4-wire Wye 127/220 Volts the neutral is still not used because there are no mayor wiring standards in Europe for 127 Volts.
So
- In a 3-Wire Delta 220 Volts System everything s connected Phase to Phase because there is no neutral
- In a 4-Wire Wye 127/220 Volts System everything in Europe is still connected Phase to Phase because Europe does not have 127 Volt appliances in general and multiple voltages will lead to confusion among users.
- In a 4-Wire Wye 230/400 Volts System most appliances are connected Phase to Neutral and only 3-phase appliances and 400 Volt engines are connected Phase to Phase. 127 Volts appliances cannot be used without a transformer.
I live in Finland. We have 3x400/230v Wye. Delta isn't used under 1kv for electricity supply. 20/0,4kv transformers are Delta-Wye and larger ones Delta-Delta.
The same in France, TT-System grounding everywhere although residential houses without farming or other business are single phase 40A. They alternate between phases between the houses. But if you want more you always get triphase starting with 3x20A with increments until 3x80A. If you want more you need a business connection.
In the Netherlands old systems are the same (TT-System) however since 99% of the old houses has gas heating and cooking most people only need 1x25A. New houses are usually TN-C-S grounding 1x25A or 3x25A.
Almost everywhere 230/400 Volt Wye although some old streets in Amsterdam still had 127/220 Volt Delta which they have problems upgrading because if they forget one house it would cause a fire. That's one of the disadvantages of underground cables. Once you burried them, you cannot easily see what is connected.
Yeah very clear video well explained. but those felt pens on your desk were all standing on the table balancing i assume the white board eraser. then at the end of the video two of them are standing on each other with white board eraser on top... Very clever
Also, if the electric cooker and water heater were turned on, the pipes bonded to the ground point would become live until they disappear into the ground, 30A or more on the water pipe would make it hot and potemtially boil the water (seen steam coming from a cold tap when there was a live to earth fault on a cooker circuit, ground conductor was poor (old installation))
+Sparky Projects So you're telling me you can have hot water without a water heater? :)
Very helpful. I do have a question. I’m in Australia. You will see the Neutral bar has a connection to Earth in the switchboard. Also each premises has it’s own Earth spike in the ground.
Here your drawing should have a green earth connection to the right of the dotted line.
I’m guessing here the pour stick man doesn’t have to cop it before you know there’s a problem. Wouldn’t that be safer the way we have it?
The spike does reduce the risk but does not remove it.It still is high impedance so significant current can go through the person.
Is this something that an Earth Leakage circuit breaker would protect against? In that it would see current on the Earth terminal returning in lieu of the neutral and trip? There must be a way to protect against this, seems potentially lethal?
If U L to ground is less than U between L and N than you can use undervoltage realay to disconect L and N. But still its not safe because its stil in certain situations voltage could be same. What are your thoughts on that?
It seems to me that what's required is a breaker that would trip if neutral exceeds a set voltage above an independent earth. It would, of course, require a ground spike, and the right place to put such a device is where the supply enters the property. Maybe a nuisance to retro-install, but surely easy on new installations.
Such devices are mentioned in the new 18th edition regulations, specifically for electric vehicle charging. They do not appear to exist yet.
Ouch...!!! It might be a good idea to make a secondary earth connection from the consumer unit to either a gas or water pipe if you got TN-CS.
Or install an Earth Rod so that theres no chance of the water or gas pipes being seperated from earth if plumbers sk work. Just have an Earth rod driven far enough so that if the Neutral is broken then the Earth rod will direct the current flow via Earth.
Bonding other Extraneous Conductive Parts is already a requirement in any case, regardless of TN-C-S or not!.
@unknown 3226 yes thats why all housesin Australia have a earth rod because most houses are supllied via overhead cables. Can easily get a broken Neatral. I see it on a weekly basis while working. Its dangerous the methods used in Australia. Even with a earth rod the current will flow everywhere and all the Earthed appliances will get touch voltage. 1.3mtres is earth rod requirement in oz. But it needs to be driven 3 or 4 metres to get the best result when i test the rod
Thank you for this informative video. Please could you make a presentation about the yellow building site "isolation" transformers. When looking at the On-site guide to BS7671 (yellow) 3.4.3 SELV and PELV, it would seem to suggest that in effect these transformers are not true isolation transformers. 414.3 Suggests that to be considered isolatingThe transformer must "have no live part connected to earth or the protective conductor of another system."Since the centre tap of these transformers is in direct metallic contact with the earth of the flexible cord of the supply, the circuit protective conductor of the installation, and hence, quite possibly/probably the neutral conductor of a supply , especially if TNC(S), they appear not to be "isolating" in construction.Would these transformers more appropriately be described as Protective Extra Low Voltage Units?The transformer-fed shaver units do appear to meet the requirement of being truly Isolating.Many thanks for all your fascinating inputs.
Yellow site boxes are reduced low voltage RLV and are neither SELV nor PELV. The centre tapped transformer aims to reduce the potential so that line to earth is 55v or 63.5v three phase. reduced low voltage reduces the magnitude of the voltage if shocked. SELV and PELV means you cannot be shocked from only 1 pole
The main issue with these is miss location. Most are used at the Point of use and not at the supply point. So unprotected often un tested extension leads at a full unprotected state are laying around getting walked on and chopped. Builders will not bother in the race to get home each day.
We only use tncs here in Ireland but a separate earth electrode buried in the ground is mandatory
Good video!
Water pipe could be plastic and therefore provide no protection? Also, surely the same issue applies to a TNS system if the earth conductor is broken outside the property, so why aren’t PEN fault detector-like devices not also required in TNS systems ? Finally, is there still a need for a separate earth bar in the TN-C-S system consumer unit. If so, why (since they are both connected together?
Is there any other solution to that problem except for TC-S system ?
Cheers...
Hi JW you mentioned you can't feed an outhouse or shed if you have a tncs. Since most modern houses have this system what steps need to be taken in order to feed a shed?
steve quinsee, you need to supply your should with a TT system. Live and neutral, but don't connect to the MET at the source (house), you install an earth rod at the shed and use this as the earth for the supply in the shed.
But you forgot that the norm obliges you to bring a local equipotential to the first separation of the PEN conductor in this way even if we have an interruption of the PEN conductor from the electrical connection we have the local equipotential that takes over the PEN function, also the norm obliges the local PA to have a maximum resistance of 4 ohm (when it only has the function of the local equipotential , when it has a common 1 ohm discharge function) and it must have the ability to ground the phase conductor (upstream MCB protection
IEC 60364-5-54
EN 60364-5-54
End for UK
And in the BS 7671/2018 at the 48 page (fig. 3.9 ) , and 444.4.3.2 fig 44.2 , fig 705 , way not 16A5
What you said in this video is perfectly true, but this circuit-building model presented in the video is made in full contradiction with the norms and standards in force. And it presents a very dangerous situation when intentionally breaking the rules and standards
If the point at which the PEN became open circuit was further away from the installation, and between this point and the installation there was an earthed electrode, then what would happen in this case? Would the electrode then provide an alternative path back to the source earth? If so, the Line current that that returns from the appliance and back through the installation Neutral, would that then return to earth potential once it reaches the first connection to earth. If this happens, then this would prevent the current returning through the earthed parts of the system?
@@ianrainford8910
"If the point at which the PEN became open circuit was further away from the installation" = The norm said each electrical connection (branching distribution system) must be connected to the equipotential local networks air distirbutie if every 100m must be connected to a local earth connection. ( away ?? )
The next question I dont anderstand but I think you forget a thnik :
each earthing of an electric conductor increases the current that is controlled by the MCB so when a short circuit arises, MCB protection will interrupt the affected conductor within max. 5 sec. (in reality, this happens under 0.2 Seconds if the ground outlet was properly designed
i'm pricing a job and i don't deal with with a lot of TT supplies. the house is going to have a 3 phase supply installed into a meter cabinet i'm going to install 3 switch fuses for each phase to serve house, garage and cabin in garden, would i need to rod the 3 phase end and the cabin as there will be a hot tub within it?
John, would a normally open relay (requiring low impedance current path from L to N) that disconnects all the wiring protect from this type of fault? I mean, like your boiler and lights won't be working in broken PEN scenario, the relay would remain open too, and thus disconnect whole building from power. Even better, when broken PEN fault is corrected, it would automatically reconnect. But it would not reconnect when you touch the grounded boiler, as the impedance through you would be too high to trigger the relay. Or am I missing something?
In Ireland where we have TN-CS supply, we always have an earth electrode connected to earth bonding point on the consumer unit.
I assume it is to protect against a faulty PEN conductor.
Yes. Such things have always been permitted in the UK but unfortunately were not compulsory, so no one fitted them. They are now 'recommended' and need to become compulsory as soon as possible.
it was the most usefull video. Thanks for the effort. im renovating my electrical system and my house doesnt have its own grounding system but all the new plugs and wires are L, N and PE in the house. Now i know that u can get the ground from the supplier N (or PEN is the correct).
But what can i do to prevent that scenario if i dont have my own ground or supplier ground?
+Risto Q Nothing, you should at least supply your own ground. In case a neutral fault happens, everything rises to the same potential limiting electrocution risk. To prevent the Protective Earth from rising even with a broken neutral, you should switch to TT grounding. But this requires an earth rod with a very low impedance (aka more expensive).
At last! I've been looking for a decent explanation of this issue for ages. Thank you.
In your circuit diagram, the neutal return from the heater flows through the unfortunate man. It therefore flows through the rcd, which will trip.
With a break in the PEN conductor, current flows through the RCD, but the current is the same in both conductors so it will not trip, as described from 5:40 onwards.
There is no current imbalance, as 100% of the current is returning through the RCD and via whatever is connected to the main earth terminal of the installation, unlike in normal use where all of it should return via the PEN conductor and if a fault occurs or someone touches a conductor a proportion of the total current returns via the fault path and not through the RCD.
@@jwflame I stand corrected!!
Except BT's fibre to the cabinet, green metal street cabinets. TN-C-S DNO supplies and no RCD with only a class C breaker, all legal within the regs apparently
Heres one for you, if you have a SNE cable in your house can part of it be replaced if damaged before it enters with a 2 core cable ? bare in mind all mains cables are on a PME system from the substation or PT, what problems could happen ?
Why did they even consider the TN-C-S system to be a good idea? Seems to me that it just shits all over the protection provided by an RCD.
Are there any advantages of having this system over TNS or TT?
The advantage is the cable is significantly cheaper, as it only has 2 conductors. Usually a single centre core for line with the combined neutral/earth as separate wires surrounding it.
An RCD is used to protect against faults that happen after it. It cannot protect against faults that happen before it.
I am from a country where the TN-C-S system is quite common so let me provide some insight on why they might use this system while a better alternative, the TN-S system, is available.
My country used to be a part of the Soviet Union, and during the Soviet era residential buildings weren’t as electrified as they are today so they didn’t even bother to provide a proper earthing system. Hell, in some older buildings that didn’t undergo the modernization of electrical systems one can find socket outlets without PE contacts.
Now, when people use a lot more electrical equipment at their homes than before, the need in proper earthing arose, and the newer editions of our national electrical codes require to use the TN-S system when possible. They do provide the TN-S system in new buildings but due to poor financing they don’t do so in older buildings as installing a five wire all the way from the buildings to their respective transformer substations is quite costly. Instead, they split the PEN wire in building’s main distribution board into the N and PE wires (the point of split is also earthed to prevent dangerous touch voltages in case of the PEN wire failure) thus providing the TN-C-S system. It is worse than the TN-S system for sure but at least better than the TN-C system.
RCDs weren't as available and inexpensive as they are now and live to a grounded chassis fault on TN is a dead short so normal breakers react.
Great explanation. But if, lets say, a house have two phases and there is no endpoints (no equipment, no lamps, anything, everything is phase+phase) using the neutral, so there is no way to encounter the situation explained in this video, right? Because neutral would ONLY act as earth. In such scenarios, where do we connect the RCD neutral? or do we just leave it disconnected?
It is very rare that there is no electric equipment on. An RCD does not protect against this so forget the RCD in this context. Do not even think of putting an RCD on the PEN wire.
Great explanation. But Im stuck at one point. If the neutral outside the property is broken wouldn't the RCD trip immediately and save the chassis? In your explanation it assumes RCD is not tripping when the neutral is broken. Could you expound it a bit more? Thanks regards.
Oh I just saw that you explain this at 5:55...:)
Although deleted from regulations now, would an Earth Leakage Circuit Breaker trip if one was installed? (an trip back in time to the 70's)
copelandaa It probably would, as it would trip with a voltage difference between the installation earth and an electrode / rod in the ground.
@@jwflame
So best have an ELCB and RCD fitted in series? To be sure?
@@johnburns4017 I am not an electrician, but I think it would be not in series. An RCD would be just like that, but you would put a current or voltage sensing ELCB to sense the current from PEN-N to PE just at the place of split, before the PE is put into main PE block (with any auxilary earthing), and trip the main circuit breaker (disconnecting both L and N). So if there is any reasonable current going via L->N->PE will trip the main supply. The advantage would also be that if the resistance to ground is high, it will still trip. It might also be possible to fit an additional ELCB from the main PE block to the earth, to also trip the main circuit breaker, to measure the current to ground directly. The problem is that in many buildings there might be other earthing devices, and might not be enough. But probably doesn't hurt. It can probably be done even in TN-S system.
Very interesting Roy Meynell. HNC
Hi jw. Been watching lots of your videos. Very informative and clear many thanks. With regards to this particularly scary scenario I had my cutout moved 18 months ago. I noticed the cable was flimsy, thinking it may have been similar to swa that I used to use a lot of back in the day. But as you mentioned in one of the answers the cable is concentric with outer layer being the ne. What are the chances the concentric layer could be severed whilst still leaving the inner live conductor intact? Or does the cable type change up stream?
The outer wires being damaged leaving the inner connected is fairly unlikely. It's more likely that failures occur at joints between cables, there are large numbers of joints, every property has at least one outside.
There can be multiple types of cables in use, particularly on older networks where repairs and partial replacements have been done over the years/decades.
Ah yes that makes sense. Thanks for the prompt response on such an old video!
Really good explanation. Question: why do we not move the RCD just before the connection between E+N? (Or add an additional one) Wouldn't that detect the imbalance and disconnect the live conductor?
Because than if have an earth fault, from live to earth, the current will go to the neutral (via earth and via itself) and you'll get the full current (that flows in live) and the RCD will NOT trip.
For exsample: an applience draws 3 Amps, then you get a shock from it, the current will flow through you (230 mA) and the neutral (2770 mA) and the RCD gets back the full 3 Amps.
John very good video and very useful. I was wondering if you could answer this?
If there was a loose neutral at the cut-out where the link is made would this have the same effect? So effectively the earth is still good but the neutral is loose?
So this would be immediately after the link to earth between the cut-out & domestic meter.
Gordon Watt No, the problem only occurs if the break is before the link, so that both N and E are disconnected from the supply network, but N&E for the installation remain connected together.
@@jwflame If the point at which the PEN became open circuit was further away from the installation, and between this point and the installation there was an earthed electrode, then what would happen in this case? Would the electrode then provide an alternative path back to the source earth? If so, the Line current that that returns from the appliance and back through the installation Neutral, would that then return to earth potential once it reaches the first connection to earth. If this happens, then this would prevent the current returning through the earthed parts of the system?
Great explanation. In this case with a broken PEN would the consumer unit's earth rod connection help form an alternative path and trip the mcb before danger arises?
An earth rod will reduce the voltage on exposed conductive parts, but won't trip anything - current flow through the MCBs / RCDs would just be the normal operating current of the installation even if the earth rod had a zero resistance.
Most installations in the UK don't have local earth rods.
No, earth rods are some 20 ohm or so. That means the current is in the order of 10 amperes.
Hi John, great video. What would happen if there was an earth rod attached to the TN-C-S system? Would that bring any improvement at all? It seems in Ireland all systems have TN-C-S connected also to an earth rod.
An earth rod would divert some of the current in the event of a fault, and reduce the voltage on exposed metal parts. However unless the impedance was extremely low, the effect would be insignificant.
Yes it would act as another path to 0v potential so it would keep the whole thing grounded. It would also decrease your Ze reading and help with earth loop impedance as it's a parallel path.
What impedance value would be needed for an Earth electrode so that the installation is ok in any Neutral fault? In Australia we wack the rod in and never test it, sortajust hope for the best.....I know in the UK its tested until suitible results, whats a good result?
If also independent earthing rod present with an impedance below 20 ohm sth, then it greatly improves the safety and save life probably.
@Mark H Google AS/NZS 3000 Earth fault loop impedance , NZ and Aus is using TNCS system with Earth rod installed and yes you will need very low Imp for Earth Loop to be working
excellent info John . would it be prudent to install an earth electrode to attempt to reduce the PD between exposed conductive parts and earth in the case a broken supply neutral/earth?
You can install one, but it won't do very much unless it has a very low impedance of only a few ohms, which in reality is very difficult to achieve.
So i install an Earth rod in Australia on a TNCS supply for no real safety reason???
@@jwflame Hi John, first let me thank you for your very good programmes, I have found them very informative. I now live in rural Thailand and severed neutral overhead cables are very common, I know of at least 3 in our local area. The last was about 2 weeks ago, late afternoon our appliances stopped working and the lights started flickering, the voltage fluctuated between about 120/180. I walked into the main part of the village and every thing was normal street lights and our in-laws supply all good in fact the only street light not working was directly outside our house, my wife did not want to phone the electric utility because she thought the problem was in our house.
Our house is at the end of a branch line, so at first light I walked the line and found the break at our next door neighbours extremely overgrown tree/man eating bush. My wife phoned the utility and they responded very quickly. This will happen again because of the many overgrown trees between our house and the nearest transformer.
@@jwflame Part 2 of an extremely long winded post. Our property has NO protective earth or ground rod and despite the Thai electrical regs closely following that of Australia with regards to the MEN link most other properties in the village are similar with just two prong plugs and sockets. The village is wired for MEN with neutral to earth links about every 4th pole. I am going to install a solar system, so wanting to do it correctly and not trusting local Thai electricians, I have been researching the subject.
My conclusion was to first rewire the house to Thai regs with regards to the MEN link/earth rod and earth wiring. The regs call for a 2.4M ground rod to be installed no testing required (the cost of my own meggar test equipment rules out me testing the ground rod myself). I now have a quandary do I go the MEN link way and potentially turn a single earth fault into a whole house earth fault or ? Any suggestions would be greatly appreciated.
Perhaps people with TN-C-S supplies should fit an ELCB or a 1A fuse to their main Earthing wire in the consumer unit.
Seems like TN-C-S trades cheaper cabling cost (for the utility) with consumer safety. My country primarily uses TT earthing only with TN-S for larger buildings like office towers and the like
Seems to me that if you put a high current NVR between the consumer unit and the meter any loss of the PEN conductor would cause it to cut out and disconnect the line to the installation almost immediately.
I think the most practical system is the tn-c-s system we have multiple Earth neutral system used effectively in Australia and New Zealand and I think it's the only practical earthing system.
We also use 11kv/230v single wire earth return and that is remarkably effective for remote sites. So does Australia.
The SWER system was invented by an New Zealand electrical engineer New Zealander back in the 20s or 30s by a Mr Mandino.
My understanding is that in a summer house/shed with no extraneous conductive parts and a wooden floor then its ok to bring the TNCS earthing arrangement outside of the house via SWA etc . I have to say though the potential for a nasty belt from a Metal CU in the situation you describe is concerning.
It's often safe because appliances expected to be used outside are class 2 (double insulated) so no exposed metal parts connected to the earth wire.
@@anahatamelodeon but items used in a summer house/ shed are mostly just normal household items . Not garden items