Some of the considerations when installing an electrical installation with a TT supply.
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00:00 Introduction
01:00 What is a TT system
05:33 Historical devices for fault protection
07:44 Modern devices for fault protection
10:38 Dual RCD consumer units
12:07 Modern installations
13:00 Submains
14:48 Summary
16:23 Earth Electrodes
19:36 RCD & Electrode Resistance
22:22 RCBOs Switched Neutral
24:06 Maximum leakage current
28:17 Conclusion
CONNECTING NEUTRAL AND EARTH TOGETHER: In the UK, it is not permitted for anyone other than the network operator to make a connection between neutral and earth and use the neutral as a combined neutral and earth. In some situations that may be possible, and the network operator will do that if requested and the appropriate fee paid. The resulting system will be TN-C-S, a combined earth/neutral cable which is split into a separate earth and neutral at the point it enters the property. However the rest of the network needs to be designed appropriately for such an arrangement to be used. The most common option being PME, where the combined N&E conductor is connected to Earth at various points via earth electrodes. Not all networks are suitable for conversion to TN-C-S, and even for those that are, additional work may be required elsewhere before it can be done.
Ah, exactly one of the questions I was going to ask. Here in Australia, the vast majority of installations as far as I know use the Multiple Earthed Neutral (MEN) system, which seems much the same as PME: A property receives a line and neutral conductor and derives an earth from the neutral conductor, though critically there is also a local earth electrode. This also seems a lot less prone to some of the faults you've described before with a broken PEN conductor. Because every property has an earth electrode, the number of faults that could result in much potential on PEN is greatly reduced. I suppose the main obstacle to this in the UK is all the additional work it would require, and things you've mentioned before about difficulties installing earth electrodes.
@@cameronsteel6147 I bought a generator earlier this year - and it's exactly what I've done - connected N to E. As for 'electrodes' It is mentioned connecting generator E to the Earth but as I don't see any benefit at all in doing this, I've left the Earth out of the circuit. Clearly electrodes don't generally make good connections to the Earth so why bother at all ?
Interestingly, we've had N and E connected together in our garage for years. I'm in the process of tracing the connection to remove it ! Currently, it is now removed and the garage is powered up again - as I've disconnected the faulty circuit - yet to still find the connection that shouldn't be there. Sad to say, Dad's 'iffy' wiring. Firstly, I need to find the fault on my milling machine. Then perhaps YTC: 'Machining by Mike' will actually get round to doing some machining !
@@millomweb You should not go disconnecting N & E if you don't know what you're doing. There are many reasons to have them connected, depending on which country you're in. If you don't very much know what you're doing, please call a qualified electrician. You don't want to undo your Dad's "iffy" wiring, just to replace it with your own that is also "iffy". Just because you *think* that an electrode isn't doing anything, doesn't mean that it actually isn't, and removing it could lead to dangerous situations.
@@cameronsteel6147 Did you not read John's comment (the OP) "In the UK, it is not permitted for anyone other than the network operator to make a connection" What electrode are you talking about - welding electrodes ?
Such great knowledge in such a compact time. Can’t wait for our webinars !
Nobody explains as good as you. Clear and concise.
You have explained all the outstanding things I wanted to know about TT systems in this video. Thank you.
Another Triumph John thanks for sharing
Thanks for the pinned note JW - neatly answered my question!
This is an amazing video on TT fault protection. Really just what the doctor ordered
Thanks, John. This has been -- _enlightning_ to say the least. Really appreciate 👍
Why can't You just join incomer n and use that as N andE same asPME
Thank you sir for the great and easily understandable electrical installation system.
John... i really think you should be renting these videos out to the NHS would be so great for relieving stress and anxiety far better than any drugs... so calming relaxing the world becomes a lovely place to be even the price of oil drops and the rain stops.....Best detailed informative and instructive videos out there
JW took off his Gold Rolex Zenith Daytona watch for this video presentation. :)
A really interesting, informative video. Many thanks!
Brilliant channel. Thank you, John.
I love your comment about TT not being Common, I live in Rural Norfolk, and TT is the most common Supply standard as practically all residential properties have Overhead Wires, and most commercial properties too, the only houses with underground TNC-S are the new build developments, my own Bungalow is TT Supply, being built in the 1920's
Really interesting content, thanks for sharing John. 🙏👍
Brilliant explanation john
Thank you Mr Ward.
Great explinations! Thanks!
Really helpful video, thanks.
Great knowledge. Thanks.
Great video, thanks for sharing your knowledge. I am a British electrician (industrial background) living in Thailand. Here most installations are TT, certainly outside of the big cities. The advice from the electrical equipment manufacturers, including Siemens and Square D, is to install an earth electrode and link the earth and neutral bars together in the consumer unit. I can see the benefits of this approach as it reduces the earth loop impedance considerably as you create parallel fault paths. The earth electrode also gives a fault path if for any reason the main neural becomes disconnected or is broken. I would welcome your thoughts on any potential downside of connecting the neutral and earth bars together.
With N&E linked at the consumer unit it's a TN-C-S system that also has a supplementary earth electrode. The risks with that are when the supply neutral is broken, dangerous voltages will appear on all exposed metal items connected to the earth terminal. The earth electrode will reduce those, but would need to have a very low resistance to have any real effect.
I think what your describing is "TN-C-S (Terre Neutral - Combined - Separate)"
That is what you find in every building in Finland, TN-S, TN-C or TN-C-S always with ground electrode.
@@jwflame "The risks with that are when the supply neutral is broken, dangerous voltages will appear on all exposed metal items connected to the earth terminal." but the bird sitting on the wire is not electrocuted - for the simple reason all is at the same potential. By adding an earth electrode, you're adding another current path. If everything is LIVE there is no POTENTIAL to kill. 2. if loss of neutral -> L=N, a no volt release could disconnect the supply the second the neutral is lost. And that is an interesting twist for me - as my NVR is across 2 phases. Should I change it ? I'll have to think about that !
@@millomweb Quite a common shocking problem actually. The water in the bath or shower drain will be at earth and the taps live. You will not need the hair dryer.
I've always been a A&T man myself, hard pressed to choose one over the other. I can understand those who go full TT. AA not so much. God's greatest gift was T. I like my T&A bigger the better. Durable, something you can really grab hold of
awesome video , thanks a lot
the best guy on the web !!
Hi John, Top class lesson, I would really appreciate if you could explain more about EV charging installations and testing, especially with earth electrode. Thanks Sir
kzhead.info/sun/mr6PqJt_mXOtn58/bejne.html
Hi John, another great video. Could you give an example of a situation that would cause a neutral-earth fault at a consumer unit with single pole RCBOs that are all switched off to trip the 100mA time delayed RCD. Thanks.
Thank you John for clearing that up for me , doing my first TT install from scratch next week and was thinking about a upfront RCD no need thou as my tails are short and I have RCBOS 👌🏻 can a tails gland provide the adequate protection for the consumer unit being protected from the tails ?
thanks John
JW... PLEASE SHOW US YOUR TT'S AGAIN.... we miss you Also please wear a pair of TNs when you do... We miss you 🙃
Have not watched the video yet. Just what to say that TT is almost universal in Danmark, and RCD are required on all installations, even old ones. After seeing your previous video about TT, I am happy that this is the case. Broken neural is a serious issue in non-TT systems.
Malta as well. TT only.
sounds the same as Australia
@@closertothetruth9209 We use a Hybrid system in Australia which is a TN-C system from the Power company to the Street and then it changes into a TN-S system with an Earth Rod being installed where the supply enters the property. Looked at as a whole from the Power company to the House it is a TN-C-S system. The only places where you would have a fully TT system in Australia would be in Rural areas.
@@timg6850 i live in the wheatbelt
@@closertothetruth9209 Yeap, That sounds like a Rural area.
Thanks a lot my dear. You just made clear to me a bunch of confusions !! In my appart I see the earthing there given by the company. So is it TN-S design ?. For some the Neutral and the earthing cables are connected on the same board inside the breakers panel of the appart. Is it normal ? I thought they should be separated ? What are the risks. Thanks in advance 😀
In large power transformers, the unequal voltage distribution caused by voltage surges in the winding is prevented by the use of A. static shields. this is correct
Superb
Excellent 👍
Is there anyway to tell if an RCBO has tripped due to earth leakage or overcurrent? I know you can investigate to find out pretty easily, but is there any way to tell just from the initial look at the RCBO?
Really good video thanks John , I have a question, if a TT property had single pole rcbos for all final circuits but no upstream type s RCD, would it need one installing to clear the neutral earth fault (if one occurred) and would it warrant a C2 on an EICR and would an upstream RCD be required to protect fault conditions on main bonding conductors ?
Hi jw.. I have a question, hope you are able to help, possibly a good video topic.. I've just started working on an electrical system at a caravan (static) Park used for residential owners. The main incoming supply appears to be a PME. It appears the some previous contractors have fitted earthing rods at the caravan connections with no Rcd protection on the sub mains boards, only at the caravan connections by way of 30mA rcd and tncs separated with Rod connected to caravan socket via 4mm cpc. Other areas seem to have no visible earthing rods at the caravans or sub main boxes however all swa and earths all connected together.. If I test on mains side of the rcd I get 1.30ohms suggesting its connected to the PME but if I test on rcd side and use triplock function I'm getting 18 to 30 ohms result. Is there a reason for this? And is there an issue here? I know that PME is not permitted but why am I getting such high reading on triplock function if it is a PME system?
Can you explain the Main Earth Neutral (M.E.N. Link) as used in Australia and why we Brits do not use it when we have high earth impedance with rods? Thanks
Hi John, excellent video here. Why do they use 300mA RCDs in agricultural locations please?
John I have a question. If I have an off grid solar inverter (separate garage 20m from house with own ground rod)with an grid AC in for fallback (charge battery or run loads if no sun) is it possible to just run live and neutral as AC in and connect the ground there to the inverter AC out ground which goes to a separate off grid consumer unit in the garage connected to TT? So many videos on this by Will Prowse et al. but wanted to hear your views. Not near extraneous conductors.
20 years ago I was responsible for a Contact Centre with about 1000 PCs. We didn't have problems with the earth leakage currents which were quite large because we didn't have any RCDs, but we did have problems with the inrush current following any power cuts. Most of the MCBs would trip when the power came back on and we would have to run around unplugging most of the equipment before we could turn the MCBs back on! Happy days.
CRT transformers ;) You needed B? type mcb - equiv of the slow blow fuse !
@@millomweb After a couple of power cuts, that’s exactly what we did! Luckily we had a good firm of electrical contractors who did all the electrical work on our site. They sorted this quite quickly.
@@millomweb B is " normal", C is slower, D is often used for motors with high inrush currents. You do need a lower earth fault loop impedance (Zs) when you're using a C or D compared to a B in order to keep the disconnection times down.
@@tomcardale5596 Whatever. I have rewirable fuses here :)
If only electrician courses were as succinct as your video(s), people could no doubt learn more, quicker. Excellent work. I'm curious whether smart sockets/plugs cause any issues due to their standby voltage that might equate some additional earth leakage? All the best to you John.
Most smart sockets and similar are class II, so don't have any earth connection to the internal electronics, so no leakage.
Nice video
Hi jw question not about tt subject but wonder if you can help. Main fuses 60 amp 80 and 100. How much actual current and how long dose it take to blow them. I have seen some right overloaded Fuses but they have not blown
Another oddball case that can cause problems - I've seen RCBOs which have functional earth leads conduct up to 1mA down the FE, contributing to leakage that can cause issues on an upstream RCD
Using RCBOs would cost pennies and thus most new builds have dual rcds boards with type ACs in.
? All brands similar or some with a lot more leakage ?
Great learning lesson here JW but TT is the poor relative of PME and we all know DNO's love their TN-C-S although in the ground they cable them up as TN-S but then present them at the intake as PME
Hi I think in the ground they are making them TNCS.
14:56 key points 1. RCDs for everything 2. JW videos on lunch hour 3. This is how it is
Great video Mr. Ward congratulations and thanks a lot. May I ask a queastion please. Assume you have a 160 kVA 3 phase private transformer (HV/LV) in the countryside and a TT private network. One day you did a loop test and found 0,6 or 0,8 ohms everywhere in the property, what would you do?
It's not impossible to TT systems to have a low impedance like that, but if it's suddenly dropped, then a fault between N&E somewhere is the most likely explanation.
I agree. An unintended shunt between N&E. If that shunt has a low cross section relatively, a fire danger is posible isnt it?
You have mentioned three types of earth electrodes; rods, plates and discs. What about using reinforcing in footings as a electrode in TT systems?
Absolutely fine, and it will make a very good electrode.
We get quite a few TT installations in Gosport due to being boat yards or marinas, where the supply authority can't guarantee not to tn-c-s it in future, so don't provide an earth connection
So the earth electrode goes in the sea ?
@@millomweb oddly pretty close... Shoreline will get you sub 5 ohms. But of course you have to think of long term corrosion
Multiple rods are our answer
@@TheChipmunk2008 Stainless steel.
Interesting the differences between the TT installation and US installations. The TT is in some ways the most similar one I've seen to a US installation, but the lack of a bond from ground to neutral at the entrance is clearly a major issue.
US has normal TNC-S. TT is entirely different.
Only an issue if you are too cheap for RCD (GFCI). Get with the times.
JW wearing his Earth cable style clothing
Could a 100maRCD (delayed) be used instead of a main switch in a CU, if there is a mains isolator supplied by the DNO?? Or do you have to have main switch in CU to RCD (delayed) to MCB >RCBO/RCD30ma ?
Yes, even if there is no DNO isolator. However not actually required if all of the circuits have RCD / RCBOs.
@@jwflame Thanks, have 3 circuits off MCBs to Sub boards so think need RCD(delayed) :)
Are you saying a CU with RCD additional protection is no longer compliant? Even on a new installation I thought that was compliant, even if not desirable?
Depends on what's in the installation, but generally dual RCD units are not compliant for several reasons.
The resistance of soil can change by as must as eight times between wet and dry, that's why the theoretical maximum of 1667 ohms is de-rated to 200 ohms to allow for this, (1667/8=200) therefore the Ra of a 500ma RCD should have a maximum of 12.5 ohms.
Hi John you mentioned not needed rcd up front now if your meter tails are short if you are using reinforced insulation or whatever. What about the fact that a live cable could for any reason touch the consumer unit making it live, it wouldn’t trip so therefore shouldn’t there always be a main switch rcd?
There are always tails with no RCD protection between the meter and whatever switch, RCD or other device is the first one - adding an upfront RCD doesn't change that. Tails into that device or the consumer unit should be securely fixed, so pretty much no possibility of them coming loose.
@@jwflame understand that but say a connection comes out of an MCB for whatever reason… in a TT system very different to a TN . Now a metal consumer unit sitting live that won’t blow the fuse. An RCD at the main switch would stop this right?
@@ja_adam_ It would, but on a TT system MCBs would not be used, it would be all RCBOs with a fixed busbar. The only unprotected items being the incoming tail and the busbar, neither of which are likely to be loose or fall out.
Hello John, It has been 3 month since your last video - I hope you are well??
More videos soon.
@@jwflame you’ve really helped me out with your videos more than my college lecturers just like to say thanks for your time and knowledge.
So why do you not bond the earth spike at the house to the natural? Would this not reduce the resistance back to the transformer.
Because if the neutral breaks you've now made all the metal work in your house live at mains voltage, there are earthing systems that do this, but the wires coming to your house have to be designed such that it's almost impossible to break the neutral without also breaking the line
@@AngDavies Not quite. In such a system there are multiple parallel return paths from appliances: * line->device->neutral->network-neutral (the preferred one) * line->device->neutral=PE->ground-rod->ground->DNO * line->device->neutral=PE->human->ground->DNO * line->device->neutral=PE->ground-rod->ground->neighbours'-rods->neighbours'-PE->(recursion) * line->device->neutral=PE->human->ground->neighbours'-rods->neighbours'-PE->(recursion) The important ones are PE->ground-rod->ground and PE->human->ground. As long as the former has much lower resistance than the latter, the amount of energy going through "human" is low enough. One thing that helps keeping that path good is bonding---if you "insert" PE between "human" and "ground" by e.g. making taps and tubs "PE" and not "ground", you break the "human->ground" connection. (i.e. PE->bonding-wire->bathtub->ground will be a much better path than PE->human->bathtub->ground) You also lower the overall resistance of the PE->ground path in general as more PE-level metal is connected to the physical ground. You can see this in railway operations. There the tracks are the neutral. And they are bonded to ground so well that you can stand on the ground and touch the tracks without any measurable current flowing. (To be fair, if you were to put a locomotive on a 10 metre piece of unconnected track this would not be the case anymore.)
@@HenryLoenwind in such a situation where the earth is acting as the only return path for current, one needs unfeasibly low resistance for it to be safe- for an 80 amp supply, where all is to be sunk through the earth rod, one will need a resistance of well below one ohm to be safe ( such a fault path would bypass any RCD/GFCI protection) which is unobtainable for the most part
@@AngDavies The resistance of that return path isn't that important on its own. Even if it is 100 Ohms, if the parallel return path through a human standing on the ground (near the rod, btw) is 10k Ohms you still get a 100:1 split. That's the idea behind bonding---if everything the human can touch is stone, wood, bonded metal or metal is doesn't have a good ground connection (e.g. bedframe on wooden floor), that human is safe. That's why foundation earthing is the preferred method in countries that ground neutral locally. Not only does it provide a low-resistance ground connection, it sits right there between the human and the real ground. The fault current through the human into the ground now has to go through the foundation to get to ground, passing by/through the foundation grounding, effectively putting the human in parallel with a copper wire. That's quite in contrast to a PEN fault situation. If all that grounds your PE is bonding to gas and water mains and some DNO grounding rod miles away, the floor (or a metal garden fence with a nice deep foundation) may very well provide a much better return path.
@@HenryLoenwind the resistance and current flowing through that return path determine the voltage of that point in the system, in my example 80amps through 1ohm is 80v with respect to true ground, human body resistance is indeed very high, which mean that point will remain at 80v when you touch it and provide an alternative return path. 80v is certainly enough to be dangerous 1/2000th of 80A is still 40ma
Now someone tried to charge an e-car with a power generator. But the loader's safety circuits did not allow that. I think the diesel power generator represents a T-T system. The loader's safety circuits looked for the neutral conductor at one or the other pole (European plugs can be turned), which is connected to earth somewhere in a T-S system or T-C-S system. There was also no grounding of the power generator.
N & E can be connected on my generator. It didn't come with them connected.
My house has an earth wire coming from the utility head, so probably tnc. The cable looks like the old lead covered though, and its a 1940s house. The head looks much newer though, so I'm not sure what's going on in there. All that said, the incoming earth from the utility terminates in a connector block is is not used. I've been put on a TT for some reason, and I'm not really sure why. We have what you might call out buildings with extraneous pipes, but they're connected to the house with a conservatory. I wonder if someone got a bit overzealous about wiring regs and did something silly.
Many old lead covered cable installations have problems, mainly due to the cast iron joint boxes underground corroding and resulting in the earth connection to the lead sheath failing. In theory they should be dug up and repaired/replaced, but that could take months, years or never so TT is the only option left.
4 months without a jw video. I hope he is OK.
Just thought the same thing
I was just thinking I hadn't seen a JW video i n my feed for a while...... :-(
He's fine, you can find him on the e5 group channel.
Free JW
if you could choose your type of installation (TT, TN or whatever), what would it be, and why?
TN-S if available, TT if not.
It's always possible to wire a property as TT, even if it's supplied as TN-C-S, and I have a feeling that John might even do that as he often raises the issues that TN-C-S is vulnerable to supplier-side breaks in the neutral.
Gas.
@@TheEulerID "TN-C-S is vulnerable to supplier-side breaks in the neutral." feed supply through a no volt release contactor. If neutral lost contactor would open disconnecting L
Thank you John for your informative videos. Couple of questions: Why would an electricity provider not provide a TN-C-S earth? And even if it’s not provided, why could you not just replicate a TN-C-S by connecting an earth bar to the neutral wire as close as possible to the electricity meter, rather than installing an earth rod?
Making some unlicensed ad hoc connections in earthing is not a good idea. You could kill someone with it. In 2004 in Finland an electric installer died while fixing storm damage because PEN-fault had put mains voltage on the PEN-wire. This happened in a country where they could expect it. Imagine if they had no reason to expect it? In most places earth rods are used even on TN. Never improvise with earthing. Play according to the rules even if you find them strange.
@@okaro6595 "In 2004 in Finland an electric installer died while fixing storm damage because PEN-fault had put mains voltage on the PEN-wire. " Well that should have tripped the RCD ! "Never improvise with earthing. Play according to the rules even if you find them strange" Well, I've left the Earth out of it regarding my generator. The earthing is done via a connection to neutral. Prior to use, neutral connection is confirmed AND the RCD can be tested PROPERLY to confirm protection circuit is functional.
@@millomweb If you have "left the earth out of it" with your generator where is the circuit to operate the RCD?
If there is a possibility of the neutral supply connection becoming disconnected while the live remains connected then having the installation earth derived from that neutral would, in that eventuality, result in a dangerous voltage on all the cpc wiring in the installation. I have a TT system at my house because the supply arrives overhead on two individual wires, one each for live and neutral. A failure of the neutral wire would not necessarily be accompanied by a failure of the live.
@@davidfaraday7963 Via the green/yellow wire. Incidentally, I've removed the test button from the RCD and fitted my own test button - so the test also confirms the continuity of the protection circuit.
12:50: I agree using a single RCD with MCBs is cheap and certainly not preferable, but where in the regs does it state this type of setup is not compliant?
Division of installation 314.1
@@jwflame Thanks. Presumably this applies to new installations. For existing installations this setup is permitted? FYI, I am not an electrician but an electronic engineer with a keen interest in the regs 😃 I have this type of setup in my house (installed in the 80s). I am considering renting out my house in future and obviously wondering if it would pass the mandatory electrical safety test.
My electrical installation in 1950s was earthed from the underground cold copper water pipe mains and has a earth loop impedance of 5 ohms does this class as a TT earthing system?
Yes, if that was the only connection to Earth. However water pipes haven't been permitted for that purpose since 1966.
Thanks John
Are you sure your main in the ground is copper ? I think they usually switch to iron before going underground.
@@millomweb Hi I am not sure it is copper underground. All I can see is a copper water pipe coming out from the kitchen concrete floor.
@@bobbylee9262 That's not very good ! Copper expands and contracts. Concrete not so much - my aunt had copper pipes fail in concrete :( The copper needs some room for movement.
Would be great if you could remove “MCB” formally bs3871 obsolete devices. 60898’s are circuit breakers ;-)
Very Interesting - As a viewer in New Zealand I have often wondered why you guys do not connect the Neutral and the Earth bars inside the consumer unit together as we do here? In addition we do not switch the neutral conductor at the main switch of the distribution board, just the line (Active) conductor. There is no need to switch the neutral as it is always at earth potential. How are we able to do this but you guys are not?
One good reason for switching both line and neutral, is if there is a broken neutral on the supplier, it can drive the neutral to near full line voltage via loads in other properties. Another is that a neutral-earth fault will still trip an upstream RCD unless both are isolated. Double pole is always the safer option, and in some European countries, double pole MCBs are the norm. As to why the neutral-earth strap is supplied by the electricity supply company, then it is they that know whether it's a TT, TN-S or TN-C-S system, so it's considered safer that the supply company is responsible for that part, and it is not an issue, so why would we want a disruptive change for no reason?
@@TheEulerID Thanks for the explanation, what you say makes sense if your Neutral is not always at Earth Potential. Its just always puzzled me why the UK ended up with so many different earthing arrangements, its very confusing. In NZ we use the Multiple Earthed Neutral system everywhere, so every system is the same, you always know what you are dealing with. The NZ system seems much safer to me!
There are only 3 in the UK: TN-S on older systems (1960s and earlier) supplied via lead sheathed cable usually underground. N&E are separate conductors throughout the supply network. Mostly replaced with TN-C-S now, and even where it appears to be TN-S is probably isn't due to repairs or replacements elsewhere in the distribution network. TN-C-S suppled via CNE cable (combined neutral and earth), used from 1970s onwards and now the most common by far. Usually done as PME, with two or more earth electrodes at various points in the distribution network (similar to the MEN system). At the property the CNE cable is connected to separate N and E on the consumer side, all done before the meter and is the responsibility of the network operator. There should have been earth electrodes at each property but that is never done, which is the main problem with this system in the UK, as a broken NE conductor in the network will result in dangerous voltages on anything connected to the installation earth terminal. TT which was usually only in rural areas with overhead supplies, but can be used elsewhere. A possible solution to TN-C-S problems for items outside such as electric vehicle charging.
@@gordonbuttle272 Why? TN-C-S is not inherently safer and you are at the same danger of a broken neutral, and UK TN-C-S systems also have multiple earths (it's called PME - protective multiple earth). You've provided no evidence that the NZ system is inherently safer. Of all the systems, TN-S is the safest, but as it's a lot more expensive TN-C-S is extremely widely used.
Concern of upstream neutral failure leaving earthed metal items and the neutral at live potential. This can be avoided by the addition of a 'no volt release' contactor that without a live and neutral would disconnect the live from the supply.
is there any reason NOT to have RCDs installed, in ANY type of installation? if i had the money to build my house, i'd want one RCD for the whole house plus several others for the circuits in the house (plus other protective devices, of course). would that be a bad idea?
Generally not, but you can sometimes have nuisance tripping with devices that produce a lot of Heat , such as an Oven , and or are immersed in Water such as a Hot water heater. This is due to the insulation on the elements which can overtime, after several hundred heating cycles, start to leak tiny amounts of current to the CPC (Earth Wire).
Only cases where RCDs are not a good idea are fridges and freezers.
@@okaro6595 why is that?
@@mrxmry3264 It can be expensive to toss the contents to garbage after nuisance tripping.
The main reason to not have them installed is cost. You can install whatever useless device you want, but it will not make an installation more safe. RCD are usually installed for indirect contact protection, that can be achieved in multiple ways. For example if you have a circuit that is made only by double insulated cables/devices in my country technically you don't need an RCD. You also don't need them if you have TN earthing (that is only available in industrial installations with a 15kV forniture with a local transformer). The other reason to not have them is that they can create problem when they trip, that can create problems to the appliances that are connected.
FYI Malta using British standard for electricity. But, they use TT system ONLY.
I think TT is extra hazardous for grounded devices during thunderstorms.
25:46 what happens if the allowable leakage current is exceeded?
Nuisance tripping
@@MrKn1ghts even at currents well below the rated current?
Whats the Advantage from TT over TN-C-S (Terre Neutral - Combined - Separate)?
Doesn't kill people when the combined N&E conductor is broken. Also far better for electromagnetic compatibility as there are no circulating currents through metallic parts bonded to the installation earth terminal.
Please, can one use RCD in the TN-C-S system?
@@KristelJoy14. RCD's can be used on TN-C-S systems, yes. But it cannot protect against harmful/lethal electric chocks in the case of a broken PEN somewhere upstream and a person touching anything connected to 'protective earth'/CPC (that should be routed around (not through) RCD's) which is connected to the PEN downstream of the break.
@@jwflame If the Combined N&E brakes there is still the the Local Grounding. Would be nice to have An extra RCD over the incoming Phases and the combined PEN to monitor for a Differential mode current. Or A RCD like Device to monitor for current on the local Grounding post. TT the the other hand can "kill" (be dangerous in the same way) if the RCD fails and an ground fault occurs. With high impedance grounding the whole house PE system will become live.
@@NikolaiBeier Theoretical you could Have a "master" RCD with PEN an L going trough so if PEN Would fail this can shut down the whole house. But this is not typical an a device like this could falsely triggered by low voltage current's from cable voltage drops. In a TT system one would ideally have a RCD like device to motor for voltage between the local PE and the incoming PEN. In a TN-C-S system it would be nice to have a RCD like device to monitor the current/voltage on the local Grounding lug.
Unrelated to video: I have an external power input socket to my property. I'm guessing it's designed for a generator or inverter. It's on a separate circuit. I thought it might be useful to use it as an external power point for trimmers/lawnmowers etc.. I got the correct plug for it and tried but it doesn't work, testing showed there was no power to it at all. Looking at the consumer unit is where my further confusion lies. The breaker for that circuit is next to the main red switch, but not only that, their switches are bonded together. Turning on/off the main switch also turns on/off the external port circuit. I can't see any logic behind all of that at all.
Designed for connecting a generator. The switch disconnects the grid supply before connecting the generator to the other circuits, to ensure it's only possible to have grid supply OR generator, never both at the same time. Not possible to use it for anything else - you would need to replace the input socket with a suitable output one and rewire at the consumer unit to a suitable circuit breaker.
@@jwflame Ahh, that makes sense, the switch bonded to the main switch is reversed with regard to on/off, I see it now, thanks. Just out of curiosity here, I don't see how this arrangement could operate with the RCDs in the consumer unit. Should the neutral from the generator (what's connected as neutral on the socket) be referenced to a separate earth TT style? That's the only way I can see it in my head. If there's any fault condition in generator mode, fault current would then have a to path to ground via the regular TN-C-S property earth and back to the generator via the ground and not through the RCD causing an imbalance. I was thinking now of using the port with an inverter from my car in emergency situations rather than converting it to an outlet.
@@asmrjunkie6613 The generator would need it's own earth reference. If it was an isolated supply, RCDs would not work.
16:12 is hilarious kzhead.info/sun/lZSnfLihhamYf4U/bejne.html "the Inconvenience of the whole farm loosing power just because somebody dropped a toaster in the bath". very British somehow,.. cracked me up hahaha. thanks for the advice though seriously. I'm building a cabin 250m away from mains and this has helped me wrap my head around electricity set ups a lot sheers
Hi JW, thanks for another great video. I love your delivery and the accuracy of the information provided. I have a question regarding, well, electricians. Recently I watched a video made by an electrician who will remain nameless. In the video they demonstrated that their understanding of electrical theory was sketchy at best and highly worrying at worst. Even their fault finding methodology was hopeless. I don't like normally to criticise people on a personal level but, as you obviously have an in depth understanding of electrical theory I was wondering what your feeling were about the fact that some 'qualified' electricians seem to be slipping through the net. I really wouldn't want the person I mentioned previously working in my own house. This does reflect on the profession in general which seems to me to be grossly unfair to excellent electricians like yourself.
I think it highlights that anyone can upload any old tosh to KZhead. I have no idea who you are referring to btw. The information doesn’t have to safe nor correct. For them it’s possibly just entertainment albeit dangerous entertainment. You can rely on what John says and/or the e5 group 👍
@@richardwilliamson3624 My own background is in electronics and theatre installations, designand installation of both PA and lighting grids, comms etc. I'm always refreshingly reassured by JW's depth of knowledge and more importantly his fundamental comprehension of the electrical theory. I don't want to name and shame but the channel I was referring to features a person who claims to have been working as a professional electrician for 20 years. He didn't understand how to use a some current clamp putting the jaws around both conductors then claiming 'That circuit's okay.' He then went on to demonstrate that he had no idea how to use a PAT tester to check an appliance. He declared that the low resistance (0.4ohms) between the earth lead and the metal housing was wrong as it should be high! Now I suppose anyone could misspeak in a video but his diagnostic sequence started at the consumer unit and worked it's way back to the appliance 🤔 Even after this long winded sequence he couldn't identify the issue which was immediately obvious to most of us commenting... It was a faulty element in the oven, either moisture ingress or a short circuit fault. All very concerning in mho. I'll stick with people I trust lime JW, most of the other videos I watch are electronics oriented.
In the UK, anyone can call themselves an electrician. Membership of competent persons schemes is entirely optional, and their effectiveness is questionable.
@@PurityVendetta 20 years experience of mudling on and guessing! I know the channel you refer and thought similar thoughts.
@@paulf2529 I think the correct recourse would be to report him to his local county council (environmental health) as he could make an environment dangerous.
Please sir why do you use 50volt (50v/30ma =1.6) which I thought it would have been 230 volt
50 volts is the maximum permitted voltage on exposed conductive parts, anything higher than that is considered dangerous.
@@jwflame thanks 👍
I have understood that TT is common in Central Europe. In Finland it is rare because low conductivity of the ground.
I am from Cetral Europe. All our DNO connections are TNC-S . The point where PEN splits to PE and N has to be earthed here with 12ohm of earth resistance.
TN-C-S installations in the UK don't have any local earthing.
@@jwflame What will mine be - 1935 era ? the incoming feed appears to be lead clad with tape wrapped around the outside of that. An earth lead is attached to the lead cladding. At some point, an external circuit was added via an independent switch & orange clad pyro to feed outbuildings. The fuse box in the O/B has an earth wire bonded to iron water pipe rising main - so there is a local earth. The telephone was connected to it's own earth rod !
@@evzenhedvabny6259 Where is 12 Ohms allowed?
@@mernokallat645 12 ohms is the maximum resistance of the earthing electrode.
I do like TTs
Could you clarify why a 23A fault current wouldn't trip a 6A MCB on a lighting circuit? I assume a 6A breaker would trip at well, 6A?
23A will trip a 6A MCB, but only via the thermal trip element, which can take several seconds or minutes depending on the level of overload. For near instant tripping, type B MCBs require 5x the rated current, 30A for a 6A MCB.
No earth provided - so why not use the neutral ?
Hello John, I hope you are well?? Your last video was ~11 months ago? The very best of wishes.
Sizzle away the ground for ever?? Will anybody help me to understand the sentence? Why there was no RCD in lighting circuit?
With no RCD, a fault L-E on a TT system will never be disconnected. Until recently, RCDs were not required for lighting circuits. Hence the 100mA RCD used for the whole installation.
@@jwflame Thanks John. I´ve watched most of the videos on your channel. Love the way you explain, even a person without any technical education can understand. Thanks once again for bringing such informative videos.
Good luck getting below 100 ohms in Poole and Bournemouth urban areas John .
Completely newbie question here, from a non-electrician, who just vastly enjoys watching these electrical videos. Could a qualified spark not simply convert a TT into a (i think_ TNCS? As I believe TNCS is where they just combine the "earth" to the neutral? Or is it more complex that than due to the upstream supply from the DNO not being compliant, as the upstream transformer might not expect the additional current/power?
That's a really interesting question.
I think the DNO infrastructure would need to be updated to provide the earth as part of the incoming supply. That involves digging up roads etc and providing the addition earth back to the substation.
That connection is made in the DNO's equipment, and not allowed to be made in any case. It is for the DNO to decide and connect.
TN-S or TNC-S both require a metallic connection all the way back to the transformer. Only the DNO could do this.
That can only be done by the network operator, and only if the rest of the network is suitable for such a conversion.
Is John ok? a long time with no video (hope he is well)
Have you retired, we are missing your content 🙁
Is it possible to still fit an insulated front-end RCD or would this be categorised as a CU and therefore require a metal enclosure?
Unclear. The wording in BS7671 is 'consumer units and similar switchgear assemblies'. Therefore depends on whether a single RCD in an enclosure is a 'switchgear assembly'. Probably not.
My sisters house has one RCD covering the whole lot, and the RCD is in a box before the consumer unit, and at one point one of the fuses in the consumer unit was replaced with an MCB when a bulb failing resulted in the fuse blowing And the house where i currently live has one RCD covering most circuits (all but 3 circuits are on the RCD, lighting and smoke detectors, the smoke detectors are on their own circuit) At one point someone i know was going to (somehow) convert the consumer unit at the house where i live to dual RCD (and they bought a CHEAP garage consumer unit, which came from china, but thankfully they weren’t happy with it as the RCD was single pole, and it appeared to me to be an MCB with an RCD slapped on the side, so they bought a proper garage consumer unit thing, but to this day it still has not been installed)
I’m an electrician who hasn’t worked for a few years down to health. I wanna get back on it. I’d love w few hours with you to tutor me. Is that something you do?
Watch all his youtube videos you will become a good electrician.
@@ramzg5402 Yeh your right. I watch a lot of KZhead videos about electrical engineering but this guy seems really on it.
In a nutshell.... if i have my gas & water bonded to my csu, can i have pme on a TT system??
How about doing a video about how crap RCD test buttons really are ? They're as crap as using a light bulb to indicate when a circuit is live.
I have some plugs wired with N-E and L-E faults which I use to test my RCDs and newly installed sockets. Just make sure you're alone and not earthed when testing L-E!
@@urveen On the basis that RCDs trip at 30mA - allegedly for safety, then feeding E with 29mA should be ok ! I'm much bemused by the use of a 30mA RCD as a protection device - only to have them bypassed by using multiples in C/Us ! If 60 computers on a circuit trips the RCD then get the computers fixed - there should NOT be ANY leak to earth on any piece of equipment - NO MATTER WHAT ANYONE ELSE SAYS !
@@millomweb yes, N-E fault is an interesting one. Everything stays on until a high current appliance is turned on,for obvious reasons on a TT setup.
@@urveen Having LE is an interesting one too - my father couldn't figure out why the lathe wouldn't work but his neon tester would :)
@@millomweb You are a boomer who cant understand how modern interference filters work.
great break down. but note thats its not 1666.7 ohm. its 1666.66 the round up on the 7 will exide a voltage increase over the 50V. other wise no comment.
I had to laugh - the whole farm could be switched off if someone decided to drop a toaster in the bath! They seem to do some strange things on farms 🤪