Discuss Understanding the relationship between TNCS, TNS and TT in the Electrical Forum area at ElectriciansForums.net

happyhippydad

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I have never actually had to make an electrical system into a TT. I have added rods in order to get an effective Ra but never had to (for example) install a TT on a new garage or shed.

It's best I give an example with regards my question..

A new garage has a supply taken to it from the house. The house is PME and this has been extended to the garage, lets say 4mm 3 core SWA. The 3rd core is perfectly adequate as an earth. Also, the installer puts a copper rod in the ground and attaches this to the earth bar in the new garage CU. Is this OK? Is this actually better? Is it now dangerous?

I've got a few ideas but I'd rather not influence the line of thought and just leave it as above.

Cheers all. :)
 

Megawatt

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I have never actually had to make an electrical system into a TT. I have added rods in order to get an effective Ra but never had to (for example) install a TT on a new garage or shed.

It's best I give an example with regards my question..

A new garage has a supply taken to it from the house. The house is PME and this has been extended to the garage, lets say 4mm 3 core SWA. The 3rd core is perfectly adequate as an earth. Also, the installer puts a copper rod in the ground and attaches this to the earth bar in the new garage CU. Is this OK? Is this actually better? Is it now dangerous?

I've got a few ideas but I'd rather not influence the line of thought and just leave it as above.

Cheers all. :)
@Happpyhippydad yes it is safe and needed. What he did was bond the neutral bar to the ground rod. Without it your voltage would fluctuate.
 

davesparks

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@Happpyhippydad yes it is safe and needed. What he did was bond the neutral bar to the ground rod. Without it your voltage would fluctuate.
I hope that they haven't done this as it is dangerous and illegal to combine the neutral and earth in a consumers installation in the UK.
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A new garage has a supply taken to it from the house. The house is PME and this has been extended to the garage, lets say 4mm 3 core SWA. The 3rd core is perfectly adequate as an earth. Also, the installer puts a copper rod in the ground and attaches this to the earth bar in the new garage CU. Is this OK? Is this actually better? Is it now dangerous?
Good question, this has the potential to be a very interesting discussion.

The third core is perfectly adequate for earthing (cpc) but not for main bonding if any extraneous conductive parts are present in the outbuilding (based on the assumption that the required size of bond is 10mm)

I assume in this scenario that main bonding is not required at the shed?

Based on these assumptions that earth rod is not necessary.

What is the earth rod actually doing electrically? That depends a little on its Ra, generally its hard to get a low enough Ra that it will make a practical difference to the earthing of a PME supplied installation. You would only start to notice it affecting the Zs of the circuit, and at the origin, if the Ra was a couple of ohms or less.
If the Ra is that low then the fault current that could flow through the rod could be higher than the 4mm CPC could safely handle and it could be non-compliant.

That earth rod, in my opinion, will in itself be an extraneous part because it is introducing an earth potential into the installation. As such it would need to be connected back to the MET by 10mm copper or equivalent.
Based on that logic it is in fact making the installation worse rather than better.

As you probably know I am an advocate of earth electrodes being connected to the MET of all installations with a PME earth, I think 10mm would be the minimum acceptable conductor size for this.
 
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pc1966

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@Megawatt as already said, in the UK only the supply authority (DNO) is allowed to bond N & E together for various safety reasons, largely to do with the consequences of an open 'neutral' making metalwork live, and to limit spurious current circulating around the earthing structures of multiple buildings, etc.

It is common in the UK for the bond point to be at the supply cut-out and that is called TN-C-S here (Common N & E to the supply point, Separate after) but within any normal installation N & E will not be bonded together. In this case the DNO is expected to have multiple low-impedance earth rods at points along the conductor (hence the PME name given for protective multiple earth). That is not always successful, and there are PME faults putting folk at risk disturbingly often across the UK.

The main advantage is to save conductor costs over the traditional TN-S system by not running separate N & E to everywhere (secondary advantage is usually a lower supply fault impedance Ze so easier to achieve fast disconnection by fuse or MCB).

The main disadvantage is the PME open-circuit fault consequences that are comparable to the TN-C risk of metalwork going live and high currents flowing in to anything bonded to true Earth (e.g. metal water pipes, etc).

There was talk of having UK properties being fitted with earth rods for TN-C-S as they are built, etc (which I think is similar to the USA arrangement) but they would go to the 'E' after the DNO point in any case (which is not isolated by incoming switch, etc) and definitely not to N. But that has not made it in to our regulations yet.

Of course with a TT setup you only have the rod(s) for earth and so in practice you need RCD protection for all circuits to have any real hope of disconnecting on a fault as it is really hard to get below ten-ish ohms for a couple of rods, and you really need one ohm or below in many cases.
 
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happyhippydad

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I hope that they haven't done this as it is dangerous and illegal to combine the neutral and earth in a consumers installation in the UK.
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Good question, this has the potential to be a very interesting discussion.

The third core is perfectly adequate for earthing (cpc) but not for main bonding if any extraneous conductive parts are present in the outbuilding (based on the assumption that the required size of bond is 10mm)

I assume in this scenario that main bonding is not required at the shed?

Based on these assumptions that earth rod is not necessary.

What is the earth rod actually doing electrically? That depends a little on its Ra, generally its hard to get a low enough Ra that it will make a practical difference to the earthing of a PME supplied installation. You would only start to notice it affecting the Zs of the circuit, and at the origin, if the Ra was a couple of ohms or less.
If the Ra is that low then the fault current that could flow through the rod could be higher than the 4mm CPC could safely handle and it could be non-compliant.

That earth rod, in my opinion, will in itself be an extraneous part because it is introducing an earth potential into the installation. As such it would need to be connected back to the MET by 10mm copper or equivalent.
Based on that logic it is in fact making the installation worse rather than better.

As you probably know I am an advocate of earth electrodes being connected to the MET of all installations with a PME earth, I think 10mm would be the minimum acceptable conductor size for this.
I hope that they haven't done this as it is dangerous and illegal to combine the neutral and earth in a consumers installation in the UK.
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Good question, this has the potential to be a very interesting discussion.

The third core is perfectly adequate for earthing (cpc) but not for main bonding if any extraneous conductive parts are present in the outbuilding (based on the assumption that the required size of bond is 10mm)

I assume in this scenario that main bonding is not required at the shed?

Based on these assumptions that earth rod is not necessary.

What is the earth rod actually doing electrically? That depends a little on its Ra, generally its hard to get a low enough Ra that it will make a practical difference to the earthing of a PME supplied installation. You would only start to notice it affecting the Zs of the circuit, and at the origin, if the Ra was a couple of ohms or less.
If the Ra is that low then the fault current that could flow through the rod could be higher than the 4mm CPC could safely handle and it could be non-compliant.

That earth rod, in my opinion, will in itself be an extraneous part because it is introducing an earth potential into the installation. As such it would need to be connected back to the MET by 10mm copper or equivalent.
Based on that logic it is in fact making the installation worse rather than better.

As you probably know I am an advocate of earth electrodes being connected to the MET of all installations with a PME earth, I think 10mm would be the minimum acceptable conductor size for this.
Yes, no extraneous in the new shed.
If there was a broken PEN, wouldn't the RCD trip immediately (if there was a rod installed <1667ohms)
as any current would be flowing to earth?
 
Yes, no extraneous in the new shed.
If there was a broken PEN, wouldn't the RCD trip immediately (if there was a rod installed <1667ohms)
as any current would be flowing to earth?
The point at which the E and N are combined is at the cutout, ie. upstream of any RCDs. In the event of such a fault, the current flowing through the RCD would still be equal in both L and N, as it wouldn't be diverted to earth until it reached the cutout.
 

suffolkspark

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Theres a 15 year old housing estate near where I live, ive worked in about 5 of them (theres about 100) every detached garage is wired in 3 core 2.5mm swa off the non rcd side of the boards to a 2 way board in the garage with a rod stuck in aswell and linked back to it, so there all wrong 😅 taylor wimpey I believe, I shake my head every time
 

Lucien Nunes

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If there was a broken PEN, wouldn't the RCD trip immediately
As PM succinctly puts it, there's no reason for the RCD to know anything about the problem, because the circuit downstream of it isn't faulty. The current flowing to earth is not leakage, just normal load trying to returning to the DNO's supply, finding the resistance of the incoming PEN higher than it should be, therefore creating a higher potential to real earth than there should be, all upstream of the RCD.

With the PEN broken there might not even be any current flowing; if nothing extraneous is connected or bonded to the MET (i.e. the only way the installation can 'see' earth is via the PME supply) then the neutral side of the load is completely open-circuit, all the lights go out but the entire earthing system jumps up to 230V.

yes it is safe and needed. What he did was bond the neutral bar to the ground rod. Without it your voltage would fluctuate.
At the risk of banging on about it, and only because the constrast between the two systems has not been pointed out in this thread specifically:
In the US it is normal for neutral and ground to be linked at the main panel of the customer's installation. In the UK, only the network operator is allowed to make the connection within their own sealed equipiment. The customer is given a neutral wire (from the meter) and a ground wire (direct from the service terminal) and is not permitted to connect them together at the panel or anywhere else.
 

happyhippydad

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The point at which the E and N are combined is at the cutout, ie. upstream of any RCDs. In the event of such a fault, the current flowing through the RCD would still be equal in both L and N, as it wouldn't be diverted to earth until it reached the cutout.
Of course! Obvious :blush:
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So in terms of a broken PEN and whether the RCD trips or not it makes no difference if the new garage supply is PME, TT or both. Can these assumptions be made:

1. Everything connected to the MET in the house CU will be at 230V?
2. If the garage has used the house PME AND a rod has been installed then this simply means the rod will also be at 230V? Actually worse than without a rod.
3. If the garage is on it's on TT then everything connected to the MET in the garage CU will still be at 0V to earth?

If scenario 2 above happens, will the ground itself not become 230V? What is the potential difference actually between, the rod and the ?????. If the ground itself is at 230V how would you get a shock touching the rod if the rod is in the ground?

I need to stop the questions there :dizzy:
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One more question...

In the case of TN-S. Would the rod then be useful on the garage if somehow the earth from the supply cable to the transformer got broken? It would then become a TT system and an earth fault would still trip the RCD?

I'm guessing a broken earth on a TN-S is pretty unlikely? Also, there is likely to be Main bonding cables which may well be sufficient to trip the RCD so even with TN-S there is not really a benefit to having a rod installed (there just isn't an increased danger)?
 
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pc1966

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1. Everything connected to the MET in the house CU will be at 230V?
It depends on the location of the PME fault.

If it is feeding just that one property then yes, it will be the best part of line voltage. If it is the PME of several properties on different phases it could be anywhere 0-230V depending on the balance of loads.

2. If the garage has used the house PME AND a rod has been installed then this simply means the rod will also be at 230V? Actually worse than without a rod.
Yes, that is a danger (see below).

3. If the garage is on it's on TT then everything connected to the MET in the garage CU will still be at 0V to earth?
Yes, that is the main advantage of a TT arrangement here. Disadvantage is the need for a reliable rod locally (though under your control).

If scenario 2 above happens, will the ground itself not become 230V? What is the potential difference actually between, the rod and the ?????. If the ground itself is at 230V how would you get a shock touching the rod if the rod is in the ground?
If you look up step/touch potential you will find more about this.

Depending on the rod depth / layout you might see 230V over a meter or so (short rod), or it might be very much lower (deep rod or earth mat/wire, etc). Also the risk is very dependent on the probability of anyone touching the rod (which ought to have a cover anyway) or nearby soil and being reasonably conductive to the (further way or true) Earth as well. So in some case that is not very risky, but other (e.g. around a pool/hot-tub) you have a genuinely risky situation for a PME fault.

Exactly the same worry has come to the fore with EV chargers as cars might be plugged in to charge and being washed at the same time. So some chargers go TT, others seek to disconnect L/N and E in a fault is detected. That last part has until recently been a big no-no in the regulations, with usually only the withdrawal of a plug/socket providing isolation including CPC/Earth.
 

davesparks

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2. If the garage has used the house PME AND a rod has been installed then this simply means the rod will also be at 230V? Actually worse than without a rod.

If scenario 2 above happens, will the ground itself not become 230V? What is the potential difference actually between, the rod and the ?????. If the ground itself is at 230V how would you get a shock touching the rod if the rod is in the ground?

I need to stop the questions there :dizzy:
Actually I'd say you are asking the right questions, you don't need to stop asking them, just pause until you have good answers before asking more.

I'll try to answer the bit I've quoted, and try to break it down in to small logical points.

For the sake of argument I'll assume the broken PEN affects only the service cable to this one installation, I'll also a ssume that a number of purely resistive electrical loads are connected and switched on at the moment of the PEN fault occurring.

If we first consider the installation without any connections to the mass of earth then at that moment in time that the PEN breaks the MET rises up to line voltage, this is because it is connected to the line via the N-E link, neutral, load and then line conductor.
The earth bar at the garage CU will also rise up to line voltage.


Next consider what happens if the earth rod was a perfect (zero ohms) connection to earth. In this case the MET will not rise to line voltage, the full neutral current will flow via the earth rod back to the substation N point, the installation will continue to work as normal and the broken PEN may go unnoticed. However that 4mm CPC connecting the earth rod to the MET may not be able to carry the full neutral current and could result in further problems.

These are the two extreme ends of the scale and are unlikely to occur in my opinion.

In reality something in between will happen.

The earth rod has a resistance to Earth, with the broken PEN fault this resistance is put in series with the resistance of the loads in the installation. You could simplify this to a diagram of two resistors in series with the MET connected to the middle point between the two resistors.
This will behave in accordance with the laws of resistors in series, current will flow and voltage will drop across each resistor, it will form a potential divider.

If the resistance to Earth of the rod is exactly equal to the resistance of the connected load then the MET will sit at a voltage to earth exactly half the line voltage, and a current will flow that is half of the normal load current.

If the resistance to Earth of the earth electrode is 9 times the resistance of the electrical loads then the MET will be at 9/10ths of line vokatge above earth, and a current will flow which is 1/10th of the normal load current.

If the rod is 100 times the resistance of the loads (this is becoming a more realistic value) , the MET will be at 99% of line voltage and 1% of the normal current will flow.

Hopefully this goes some way to making sense (and hopefully my maths is right for the last part)

The earth rod doesn't raise the earth in general up to line voltage, the earth rod will rise up to the same voltage as the MET and this will 'taper off' in the earth around it back down to 0 volts, if you Google voltage gradients around earth rods you'll likely get pictures with lots of concentric circles around the earth rod showing this.
This is why the top of an earth rod should be below ground level, to avoid the vokatge gradients appearing on the surface. This is what I was taught at college and the text books had pictures of cows standing with their legs in two different circles of the voltage gradient from an earth rod sticking up above the ground surface and getting electrocuted.
 

pc1966

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In the case of TN-S. Would the rod then be useful on the garage if somehow the earth from the supply cable to the transformer got broken? It would then become a TT system and an earth fault would still trip the RCD?
In most case having additional earthing paths via dedicated rods or by means of metal water pipes, etc, is a good thing. But of course one must never rely on service pipes as an earthing means (unless specifically with the agreement of the pipe owner, but that is not a good path to try and pursue anyway).

If you have RCD additional protection, as pretty much most new circuits will have anyway, then you would become TT in this case and be OK even though faulty and less safe than before. But obviously not if any circuits depend on the OCPD for clearing!

Why less safe? Basically as the RCD is more complex than the MCB so more likely to fail, and you might have a fault that is not RCD protected (e.g. supply line to CU case) that would cause a big BANG! in the TN-S / TN-C-S case but when TT-ing would just roast the earth rod and leave metalwork at a dangerous voltage.
 

UNG

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I'm guessing a broken earth on a TN-S is pretty unlikely? Also, there is likely to be Main bonding cables which may well be sufficient to trip the RCD so even with TN-S there is not really a benefit to having a rod installed (there just isn't an increased danger)?
Not as unlikely as you think,
Mate of mine had got a new MFT and not being used to it thought it was giving him the wrong readings, I just happened to be in the area when he rang me so I called in with my MFT and did a neutral - earth loop test and got nothing we called the DNO after around 10 hours of digging and jointing the DNO repaired the fault they reckoned at least half the street had no earth for many months

On another occasion my mate's daughter in law's friend was having electrical issues in her rented property with appliances failing, my mate offered to go around at no cost as he was concerned, the property was on a looped supply from next door and had been fitted with an earth rod and connected to the main earth terminal as the DNO's earth was poor after some head scratching my mate rang me talked me through what he had found, my first suggestion was to knock next door and ask if he could test their earth and found their earth was from the earth rod next door, DNO was called and yet again the digging and jointing crew were on overtime again. The worrying part of the second one was the landlord was also an NICEIC approved electrical contractor and one of his lads had put the earth rod in because somebody told him it would get messy ringing the DNO during an overheard phone conversation. The DNO guy was less than impressed when he was told this as the earth rod was earthing about 6 properties

And another one I was called out to customer had spilt some wine and a socket was in the way RCD in the board tripped it was reset and she still had no power, found another RCD in the external meter cupboard as well, at some point somebody had installed an earth rod probably because the DNO's earth terminal returned a reading of 157Ω called the DNO and asked them if they could check it as it looked like quite a new service head didn't put it though as an urgent so a week or two later the DNO fixed it

The DNO guy's we have spoken to over the last few years reckon that there are probably quite a few earthing issues they don't know about and rely on the electricians on the ground to call them with any faults found during testing

Not a massive fan of earth rods due to the number of times they are not properly installed and the potential bigger problems they can cause
 

magnoliafan89

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If you had a shed that had any source of water in it say a tap or whatever would it be necessary to rod it with a 10mm going back to the MET?
 

pc1966

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If you had a shed that had any source of water in it say a tap or whatever would it be necessary to rod it with a 10mm going back to the MET?
If the supply is TN-C-S and the water pipe is conductive enough to the Earth to be "extraneous" then yes.

But if a plastic supply pipe then no.
 

pc1966

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This is another cost-based reason to look at a TT supply for an out-building - if you are not installing a 10mm copper-equivalent CPC for other reasons (relating to total current capacity, etc) then going TT allows you to bond with phase-sized conductors instead.
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What if it was a TT or TNS??
If TT supply then no.

If TN-S then no. But there is a potential in the future for the DNO to change to TN-C-S and then the installation ought to be reconsidered for issues like this!
 

davesparks

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If you had a shed that had any source of water in it say a tap or whatever would it be necessary to rod it with a 10mm going back to the MET?
No, having a source of water does not change anything from an earthing and bonding point of view.

Bonding is concerned with extraneous conductive parts, such as metal pipes, not what is carried by a pipe.

If a metal pipe brings the water into the shed, and that metal pipe introduces a potential into the shed then it would require bonding, but this does not "rod it with a 10mm going back to the MET" whatever that means?

The options are either to install a main bond of the appropriate size from the MET, or to set up a separate TT system if the first option is not viable.
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This is another cost-based reason to look at a TT supply for an out-building - if you are not installing a 10mm copper-equivalent CPC for other reasons (relating to total current capacity, etc) then going TT allows you to bond with phase-sized conductors instead.

If TT supply then no.

If TN-S then no. But there is a potential in the future for the DNO to change to TN-C-S and then the installation ought to be reconsidered for issues like this!
Going TT doesn't allow you to 'bond with phase sized conductors instead" it changes the required size of bond yes, but the main advantage is the location at which the bond connects, the MET of the TT system instead of the MET of the original installation.

"if TNS then no" is also incorrect, TNS systems are subject to exactly the same rules regarding main bonding in outbuildings, extraneous partsust be bonded with the correct size of main bond back to the MET. The difference with a TNS, especially in larger installations, is that the required size for a main bond is smaller than for PME and so is easier to achieve.

You could even still encounter the same situation with a TT system, though it is less likely.
 
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pc1966

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Going TT doesn't allow you to 'bond with phase sized conductors instead" it changes the required size of bond yes, but the main advantage is the location at which the bond connects, the MET of the TT system instead of the MET of the original installation.

"if TNS then no" is also incorrect, TNS systems are subject to exactly the same rules regarding main bonding in outbuildings, extraneous partsust be bonded with the correct size of main bond back to the MET. The difference with a TNS, especially in larger installations, is that the required size for a main bond is smaller than for PME and so is
True, I should have been precise than answering 'no' to the assumed question about the 10mm minimum.

As you say, in all cases you need to look at the various minimum CPC requirements based on the electrical and physical situation.
 

Simon47

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Something to bear in mind when thinking about ground voltage gradients is the effect on animals. Cows & horses have legs a fair distance apart - and their feet are in direct contact with the ground. So they are at much higher risk and places like farms & stables need much more care over earthing and bonding.
Some may recall an incident a few years ago at a racecourse. A horse got jumpy and dropped dead. Turned out other horses had been jumpy in the same place - a cable fault was found which was causing ground voktage gradients, enough to kill the unfortunate horse.
 
@Happpyhippydad yes it is safe and needed. What he did was bond the neutral bar to the ground rod. Without it your voltage would fluctuate.
That an earth rod prevents "the voltage from fluctuating" is an explanation I have heard before but have yet to see the the science behind it. The voltage at the rod in a TNC-S system is always dependent on the voltage dropped over the main neutral as the rod is connected in parallel with it.
 

davesparks

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That an earth rod prevents "the voltage from fluctuating" is an explanation I have heard before but have yet to see the the science behind it. The voltage at the rod in a TNC-S system is always dependent on the voltage dropped over the main neutral as the rod is connected in parallel with it.
Megawatt is referring to the USA system where they make the N-E link at the main DB by installing earth rods there.
I think this is sometimes the only N-E link on the system, making it technically TNS, but I may be misunderstanding this.
 
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Lucien Nunes

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That an earth rod prevents "the voltage from fluctuating" is an explanation I have heard before but have yet to see the the science behind it.
I have heard that too, and asked the same question without adequate answer. The implication is that the PEN is of too high a resistance and the voltage drop in it too severe, but the Ra of the earth rod is somehow low enough to provide an effective parallel path thereby mitigating the voltage drop. Since a typical customer's earth electrode Ra is 100 - 1000 times higher than a typical Ze, the reduction of voltage drop it can provide will be in the order of 0.1 - 1% (of the variation, not the supply voltage). Noting re. voltage drop that in a US domestic installation the PEN is normally a centre conductor between split-phase lines.

If an electrode of vanishingly low Ra were provided, the overall Zpn would be reduced and the voltage drop mitigated, but only because a significant fraction of the load current (and possibly other peoples' load currents) would return via the electrode. We accept that these currents do flow, hence the bonding requirements for TN-C-S, but surely no regs would sanction relying on the electrode as part of the load circuit absent a solid PEN connection, (other than for a SWER supply.)
 
Megawatt is referring to the USA system where they make the N-E link at the main DB by installing earth rods there.
I think this is sometimes the only N-E link on the system, making it technically TNS, but I may be misunderstanding this.
But TNS by definition requires a separate earth from DSO to consumer. Earth rods (while they may be added) are not strictly speaking included in the arrangement.
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I have heard that too, and asked the same question without adequate answer. The implication is that the PEN is of too high a resistance and the voltage drop in it too severe, but the Ra of the earth rod is somehow low enough to provide an effective parallel path thereby mitigating the voltage drop. Since a typical customer's earth electrode Ra is 100 - 1000 times higher than a typical Ze, the reduction of voltage drop it can provide will be in the order of 0.1 - 1% (of the variation, not the supply voltage). Noting re. voltage drop that in a US domestic installation the PEN is normally a centre conductor between split-phase lines.

If an electrode of vanishingly low Ra were provided, the overall Zpn would be reduced and the voltage drop mitigated, but only because a significant fraction of the load current (and possibly other peoples' load currents) would return via the electrode. We accept that these currents do flow, hence the bonding requirements for TN-C-S, but surely no regs would sanction relying on the electrode as part of the load circuit absent a solid PEN connection, (other than for a SWER supply.)
👍Agreed
 
Yes, no extraneous in the new shed.
If there was a broken PEN, wouldn't the RCD trip immediately (if there was a rod installed <1667ohms)
as any current would be flowing to earth?
This is a widely held but erroneous belief in the electrical Industry. When the PEN breaks and someone touches bonded metalwork that becomes live, rcd, s will NOT trip. Reason is the fault current flows first through L of rcd and then back out N of rcd and subsequently through the unfortunate individual in contact with bonded metalwork. Quite simply this individual has become the "new neutral" from the rcd, s point of view (though a higher resistance one).
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In most case having additional earthing paths via dedicated rods or by means of metal water pipes, etc, is a good thing. But of course one must never rely on service pipes as an earthing means (unless specifically with the agreement of the pipe owner, but that is not a good path to try and pursue anyway).

If you have RCD additional protection, as pretty much most new circuits will have anyway, then you would become TT in this case and be OK even though faulty and less safe than before. But obviously not if any circuits depend on the OCPD for clearing!

Why less safe? Basically as the RCD is more complex than the MCB so more likely to fail, and you might have a fault that is not RCD protected (e.g. supply line to CU case) that would cause a big BANG! in the TN-S / TN-C-S case but when TT-ing would just roast the earth rod and leave metalwork at a dangerous voltage.
Mayby I misunderstood but how would the earth rod be roasted?. The standard resistance of most rods would not allow that to happen
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I hope that they haven't done this as it is dangerous and illegal to combine the neutral and earth in a consumers installation in the UK.
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Good question, this has the potential to be a very interesting discussion.

The third core is perfectly adequate for earthing (cpc) but not for main bonding if any extraneous conductive parts are present in the outbuilding (based on the assumption that the required size of bond is 10mm)

I assume in this scenario that main bonding is not required at the shed?

Based on these assumptions that earth rod is not necessary.

What is the earth rod actually doing electrically? That depends a little on its Ra, generally its hard to get a low enough Ra that it will make a practical difference to the earthing of a PME supplied installation. You would only start to notice it affecting the Zs of the circuit, and at the origin, if the Ra was a couple of ohms or less.
If the Ra is that low then the fault current that could flow through the rod could be higher than the 4mm CPC could safely handle and it could be non-compliant.

That earth rod, in my opinion, will in itself be an extraneous part because it is introducing an earth potential into the installation. As such it would need to be connected back to the MET by 10mm copper or equivalent.
Based on that logic it is in fact making the installation worse rather than better.

As you probably know I am an advocate of earth electrodes being connected to the MET of all installations with a PME earth, I think 10mm would be the minimum acceptable conductor size for this.
That situation of more current flowing through the rod than the CPC could handle could only occur if the OCPD was incorrectly rated. After all what flows through the CPC must first flow through the L. But in any case 1,2 or even 3 rods will ne er get you you under 1 ohm resistance and its not a situation I could ever envisage occurring. You are correct about the 10mm to the rod. This is the generally accepted size. Most of that 10mm is specified however fir mechanical protection rather that currant carrying capacity. For current carrying capacity a 4 ft rod (the standard in most TNC-S) countries will never require more than a 1.5mm
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True, I should have been precise than answering 'no' to the assumed question about the 10mm minimum.

As you say, in all cases you need to look at the various minimum CPC requirements based on the electrical and physical situation.
Interestingly, your initial comment about "Bonding with phased sized conductors" seemed reasonably to me (still does!). I do not work with TT so I am speculating here. If the extraneous metal part (copper tap) in the shed was turned out to have a reasonably high resistance, would it then be possible to Bond with phase size conductors?
Secondly would it be acceptable practice in UK to insert some plastic pipe in place of the copper and remove this Extraneous part?
 
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pc1966

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Mayby I misunderstood but how would the earth rod be roasted?. The standard resistance of most rods would not allow that to happen
Roasted is a bit of an exaggeration, but if you have 230V on the rod with say 30 ohm resistance you have around 1.8kW being dissipated all of the time (mostly in the soil immediately around the rod.

It was more the point that the OCPD won't disconnect even though a significant amount of power is going astray.
 

davesparks

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That situation of more current flowing through the rod than the CPC could handle could only occur if the OCPD was incorrectly rated. After all what flows through the CPC must first flow through the L.
You have missed the point of my post, the point of it was to describe the theory of tjso hypothetical situation. I described a number of scenarios to try and give a full explanation of the theory.

The OCPD will not have any bearing on this, you have again misunderstood my post, plus an OCPD does not monitor the current in the CPC.
In the hypothetical example I gave of the earth rod, which is connected to the CPC of the garage circuit at the garage,being in perfect contact with the mass of earth and a broken PEN occurring. The CPC of the garage circuit will be connecting neutral at the cutout, via the MET, to the earth rod. All of the installation's neutral current will flow through that cpc which may be undersized.

Not also that one of the conditions of this hypothetical situation was that a number of purely resistive loads were connected.
 
Roasted is a bit of an exaggeration, but if you have 230V on the rod with say 30 ohm resistance you have around 1.8kW being dissipated all of the time (mostly in the soil immediately around the rod.

It was more the point that the OCPD won't disconnect even though a significant amount of power is going astray.
👍
Roasted is a bit of an exaggeration, but if you have 230V on the rod with say 30 ohm resistance you have around 1.8kW being dissipated all of the time (mostly in the soil immediately around the rod.

It was more the point that the OCPD won't disconnect even though a significant amount of power is going astray.
Agreed👍. Third time trying to post this. Has to be 3 words minimum apparently.
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You have missed the point of my post, the point of it was to describe the theory of tjso hypothetical situation. I described a number of scenarios to try and give a full explanation of the theory.

The OCPD will not have any bearing on this, you have again misunderstood my post, plus an OCPD does not monitor the current in the CPC.
In the hypothetical example I gave of the earth rod, which is connected to the CPC of the garage circuit at the garage,being in perfect contact with the mass of earth and a broken PEN occurring. The CPC of the garage circuit will be connecting neutral at the cutout, via the MET, to the earth rod. All of the installation's neutral current will flow through that cpc which may be undersized.

Not also that one of the conditions of this hypothetical situation was that a number of purely resistive loads were connected.
The "Theory of tjso." Not a misprint?
 
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It should say this, 'the theory of this hypothetical situation' , its a typo.
You are correct. I misunderstood. I assumed we were talking about TT installation with one 3 x 4mm circuit connected to rod with very low res.

Regarding the bonding of the a, metal water pipe in a shed connected on PME, would 10mm still be required if it was established that the res of the waterpipe was for arguments sake 500ohms...and if so why?
 

pc1966

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Regarding the bonding of the a, metal water pipe in a shed connected on PME, would 10mm still be required if it was established that the res of the waterpipe was for arguments sake 500ohms...and if so why?
Yes.

The simply "why" is the regulations say so.

The actual reason is (I suspect) that it might be 500 ohm when you checked it, and later someone on another supply bonds to a branch of it and you now have sub-1 ohm to path true Earth and the opportunity of a large PME fault current passing that way.
 
Yes.

The simply "why" is the regulations say so.

The actual reason is (I suspect) that it might be 500 ohm when you checked it, and later someone on another supply bonds to a branch of it and you now have sub-1 ohm to path true Earth and the opportunity of a large PME fault current passing that way.
Of course and rightly so. However what I am specifically thinking of is a situation like a shed on private property where for arguments sake the waterpipe is going directly to the house not to the neighbour or street. If there is 4 feet of copper in the ground and the rest is plastic to the house. Basically you now, have an earth rod (500 ohms) and cannot import any significant fault current under a PME fault. This would still require a 10mm bond?
 

davesparks

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Of course and rightly so. However what I am specifically thinking of is a situation like a shed on private property where for arguments sake the waterpipe is going directly to the house not to the neighbour or street. If there is 4 feet of copper in the ground and the rest is plastic to the house. Basically you now, have an earth rod (500 ohms) and cannot import any significant fault current under a PME fault. This would still require a 10mm bond?
Yes it still requires a minimum 10mm bond if that is the required size of bond for the supply, the regulations don't allow for any leeway in this.

To vary the size of main bonding according to individual circumstances would result in quite a few pages of new regulations setting out the calculations, assessments and rules surrounding this. I would imagine it to be a nightmare to regulate for, and a nightmare to actually apply in real life, and almost impossible to confirm at initial verification and periodic inspection.

If you were to allow parts 500ohms or over to have a smaller bond then what about a part at 495ohms? Also what if it is 500ohms in the summer when you test it and this drops dramatically when the ground becomes waterlogged in the winter?

As an electrician you are unlikely to be able to ascertain the length of buried metal, and certainly can't know what changes might occur in the future.

The main bonding sizes are based on a worst case scenario, this covers all other scenarios.
 
Yes it still requires a minimum 10mm bond if that is the required size of bond for the supply, the regulations don't allow for any leeway in this.

To vary the size of main bonding according to individual circumstances would result in quite a few pages of new regulations setting out the calculations, assessments and rules surrounding this. I would imagine it to be a nightmare to regulate for, and a nightmare to actually apply in real life, and almost impossible to confirm at initial verification and periodic inspection.

If you were to allow parts 500ohms or over to have a smaller bond then what about a part at 495ohms? Also what if it is 500ohms in the summer when you test it and this drops dramatically when the ground becomes waterlogged in the winter?

As an electrician you are unlikely to be able to ascertain the length of buried metal, and certainly can't know what changes might occur in the future.

The main bonding sizes are based on a worst case scenario, this covers all other scenarios.
My figure of 500 ohms was arbitrary. My purpose was to use a figure of resistance which would render it impossible to import damaging PME currents (damaging from a heat point of view). And the example was designed more as an exercise in common sense where the contractor on the ground would be allowed to make a decision that would save unnecessary wiring. After all to conclude that a 4ft length of copper pipe, at the end of a garden might someday be dug up and replaced with a metal one is a possibility, but is it likely?

Will the mains water services someday go back to been metallic? possible but is it likely?
These are possibilities rather than practicalities
As you mentioned in another post, we cannot cover every situation.

Regarding the 500 ohms been significantly reduced in resistance due to been "waterlogged". Well as you correctly pointed out the resistance of rods goes up and down all year round. But a resistance of 500 ohms will never be reduced to a few ohms even lying directly in water.
Length of buried pipe is, as far as I am aware not something sparks need to be concerned about. It's just the resistance.
I reread your post regarding the hypothetical situation and it was spot on.
 
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