Why are earth bonding conductor sizes different for TT and TN-C-S (PME) earthing arrangements?
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- Опубликовано: 9 июн 2024
- The earth bonding conductor sizes depend upon the type of earthing system used within an electrical installation. eFIXX viewer Lee Turner asked us why does a TT system require 6mm2 and a TNCS need 10mm2.
Question answered by Gary Parker - Technical Manager - ECA.
eFIXX electricians technical questions are selected from comments on our videos and social media channels - please add questions you'd like answered in the comments.
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🎦 Presented by
Joe Robinson - Technical editor eFIXX
Gary Parker - Technical manager - Electrical Contractors Association
#efixx #morepower #earthing 
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To be picky, table 54.8 is based on the size of supply PEN conductor, not neutral conductor (although really just a change of terminology).
Also, do the regs stipulate 6mm2 for TT? A MPB conductor can be 6mm2 min. - 25mm2 max., not just 6mm2. The question is though, on TT, why is it 6mm2 min, when the earthing conductor only need be 2.5mmm2 if mechanically protected and 4mm2 if not mech protected? The earthing conductor only needs to be big enough to carry the current that will fire the RCD, and there's no diverted neutral currents for the MPB conductor to carry, so why does the MPB conductor need to be bigger than the EC on TT? Sorry, but the presenter merely repeated what's in the regs, without explaining why it is so. Regarding earthing, it seems the regs are really designed for TN systems, with TT just thrown in the mix as a side issue, and applying TN to TT is good enough.
The earth on an rcd system is basically just an Ariel
I'm not sure this does answer the question. Is it really to do with a broken combined neutral-earth on TN-C-S? Surely the point about bonding on a TN-C-S system is that, in the event of a live/line to metal infrastructure fault (gas pipe, plumbing etc.) then the impedance of the bonding must be low enough that it will cause the MCB to trip.
If there's a supplier-side break in the combined neutral/earth, then the current flowing through all the active electrical devices will try and flow through the bonding connection to anything connected to earth. However, I'd wager that it not going to be able to carry large amounts of current. Even a metal water or gas pipe (increasingly replaced by plastic in both cases) isn't going to have a low enough impedance to earth to carry tens of amps, and certainly nowhere near the difference between a 10 mm^2 or 6 mm^2 cable. It's likely in the case of a break in the neutral on supply side that the potential of anything bonded to the buildings PME is going to rise to high levels as the earth impedance is going to be in the tens of ohms, even with metal gas and water pipes. Compare that to 10 metres of 6mm^2 cable (about 0.05 ohms).
TT is different. It has to be designed with protection to cope that the earth impedance is tens of ohms. It might trip a 6A MCB on a line to earth fault, but that's about it. Consequently, having ridiculously large profile bonding conductors serves little purpose.
So, I think that the larger profile bonding conductor on TN-C-S is all about the ability to trip something like a 40A MCB on a line to metal plumbing/gas pipe fault and for that, you need very low bonding resistance. With a TT system, if there's that same fault, there's no way that the resistance to earth of a conducting rod is going to trip that MCB. Even in the best case, it will be lucky if it can carry 10 amps and the difference between 6mm^2 and 10mm^2 binding cable is entirely irrelevant.
Man, cant you simplify this statement into laymans terms without sounding pompous and long winded.. What are you basically stating here without trying to sound like a scientist!?
@@Surferant666 I trained as a physicist, and that's what my degree is in, so I plead guilty to the outlook of a scientist.
As for explaining it in layman's terms, then it does require some circuit analysis and theory. If I had the ability to comment with charts and drawings, it would make it much easier. I'm sure John Ward would be able to do a better job, and he has some relevant videos on earthing systems, one of which explicitly states that the impedance to ground of a TT system is high (normally in the tens of ohms) and a big cross-sectional bonding conductor would serve no purpose as the earthing rod can't carry more than a few amps.
In any event, I don't think the explanation on the video actually explains anything.
@@TheEulerID i retract my previous statement. Youre just over technical as we all are lol. I try to use analogies to explain complex situations but sometimes it gets lost in translation being a nuclear engineer.
Engineers know a little about a lot.
Scientists know a lot about a little.
Facts
@TheEulerID I find this very interesting and never thought of it till you mentioned it: ''Surely the point about bonding on a TN-C-S system is that, in the event of a live/line to metal infrastructure fault (gas pipe, plumbing etc.) then the impedance of the bonding must be low enough that it will cause the MCB to trip''.
The explanation for bonding of extraneous conductive parts (what you called metal infrastructure) is always that it's supposed to reduce touch voltages in the event of a live/line fault to the exposed conductive parts of equipment. E.g having one hand on a toaster with a live frame with the other on an unbonded water pipe you would have a substantial voltage across you. With the water pipe bonded to the main earth terminal which the toaster frame is also bonded to through the earth wire ( ''equipment grounding conductor'' in the US) they will both become live at the same voltage and not current will flow through you and so no shock. This always left me wondering what about fault currents generated at the instant the fault occurs which will trip the over-current protection and/or the RCD and clearing this danger immediately? Is it to reduce this touch voltage in case those fail and the fault just sits there?
Your explanation that I quoted basically treats the extraneous conductive parts like exposed conductive parts that are normally bonded, whereas the need to bond extraneous conductive parts is always explained in terms of eliminating the possibility of touching these earthy metal things inside the building in the event of faults which make live the exposed conductive parts of the electrical installation/equipment. Because according to your explanation a metal desk should be bonded too. I'm genuinely curious about all of this and wanting to learn as much as I can, not trying to be a smart ass!
@@Orgakoyd The bonding is also there, of course, to make sure that different conductive parts that somebody might come into contact with are all at the same potential, but you don't need ones with a large cross-sectional area to achieve that, which is where I think the difference is.
Incidentally,the 18th edition regulation 542.1.2 requires new foundations to have a foundation earthing system (something which is becoming more common across the world) which, ought to lead to a much more reliable earth contact for TT systems with considerably lower earth loop impedance. That might then change some of these regulations. What I also expect to happen is that this will change the game for TN-C-S systems too so that it provides some protection against PME faults by requiring supplementary earthing. In the past metal gas and water pipe delivery systems often masked these by providing a relatively low impedance path to neighbours properties (the diverted neutral issue). With services provided in plastic, this can't happen of course.
So, the whole area of supplementary earthing systems is, I think, going to change the rules in the future.
Would be good if you could do a video showing in plain English how to test if an exposed conductive part or extraneous conductive part requires bonding. There are figures ranging from 50v/0.03mA = 1666.67 Ohms. 23000 & 26000 Ohms etc.
We’ll see what we can do
Don't those figures relate to different things though? 50v/30mA gives 1. the max. value for earth electrode resistance (411.5.3), 2. max. value between simultaneously accessible exp. and extr.c.parts re suppl bonding, and 3. checking that an extr.c.part is effectively connected to the MET (so it's already known it's an extr.c.part) (415.2.2), whereas 23kOhms (22kOhms at the 10mA level) refers to measurement between a part and the MET to determine whether it actually is an extr.c.part (GN8, Sect. 6.1).
That's an easy one, get your megger put one clip on the earth bar or DB earth and one clip on the lump of metal in question. Set it at 250 v, push the button and if you get 3.5 M/OHM or greater it does not need bonding as it is just a lump of metal. anything less and it will need bonding.
I leave in Malawi central Africa.
Our supply system here ,PEN conductor is grounded along the route but entry to the house ,say single phase ony neutral and phase enter the premises and customer creates earthing by providing an earth electrode.
Is this TT or TNCS?
Please do a full video covering earth neutral bonding on a sun sync inverter giving special attention on the wiring using either contactor or relay .
sorry but that was explained like a polititian, answered without really giving an answer. 😕😉
Was clear enough for me
The question was how was the sizes decided not how to choose the size from a chart. So yes like a politician...
So basically if the Earth is being used as a phase return, you need to match that size of the incoming Earth/Phase return?
Interesting. Flat clips on a round cable
Ha ha dog rough job
Not only does the bonding conductors have to take the building's load current but also that of other installations connected to the DNOs mains cable downstream of a broken CNE/PEN conductor on the mains cable.
I've always wondered, with so many extraneous conductive parts and large bonding wires, is it possible that a broken PEN conductor to a building can go completely unnoticed?
got a 125 amp commando socket on a pme, they have a fly up to a marquee the total run is 100m or so my commando socket is about half way. a guy got a whack off a metal case of some stage equipment. I'm thinking its difference in potential i'm thinking of ditching the PME at the commando and TT with 100ma time delayed rcd at the commado socket does that sound right scratching my head ha
I notice your protective Earth label was missing 👀
Well Spotted Eagle man 😝
More importantly who paints a wall such a bright green 0:57 🤔
Hi , in ring circuit when 2.5 mm2 used and the two ends of each wires of cable connect to 32 amp circuit breaker , I think if this breaker which is single 32 amp replace to (double poles) circuit breaker (MCB) that's rated to 16 amp, so when we have issues in point in cable of the ring circuit or cable have cut in conductors so for that will not make cable get over heated when having load excited 16 amp and the breaker will isolated the current so we will have Warning of fault , this is an opinion because it will reduce a lot of fuses or protection points.
I think you're trying to say "why not break a ring final circuit into two radial final circuits on two MCBs? If so, there's nothing wrong with doing this so long as you split the ring entirely and don't leave it connected together anywhere. I'm not a fan of ring final circuits and can probably count on one hand the number we've installed in the last 3 or 4 years. Ring finals are the 'old' way of doing things back when you covered an entire floor (or even house) with one socket circuit to save both cable and fuseboard space. You could get away with a 4-way board for a whole house in some instances. Lights, Cooker, Sockets & Immersion Heater. There's many houses still wired like this no doubt
This comment appears to be entirely irrelevant to the subject of the video. Are you sure you are commenting on the correct one? This is about earth bonding, not ring circuits.
Steve Jones I’m fairly certain that joe said in the video to leave any questions you might have in the comments for future video’s
@@paullyons4624 he did, at 1:33.
@@iggifer
Yes that's right but not two separate single mcb it's will be one double rated for 16 amp and nutral combine in nutral bar so for that any over current on each end of wire will trip the breaker and safe the circuit specialty when there's damage in some point of cable or have been cut by mistake so for that when we have load excited 16 amp in one end of cable and the breaker trip that's will give you Warning that's the ring circuit have break in it or cable been cutting ...
You could have been bit more pragmatic, mr jobsworth
Covering arses and BS🤣🤦♂️