Scolmore Electrical Products
This official sponsor may provide discounts for members

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

happyhippydad

-
Esteemed
Arms
Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:  American Electrical Advice Forum

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

-
Arms
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

-
Mentor
Esteemed
Arms
@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.
[automerge]1596117059[/automerge]
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.
 
Last edited:

pc1966

Esteemed
Arms
Supporter
@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.
 
Last edited:

happyhippydad

-
Esteemed
Arms
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.
[automerge]1596117059[/automerge]


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.
[automerge]1596117059[/automerge]


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?
 

Pretty Mouth

-
Esteemed
Arms
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

-
Esteemed
Arms
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

-
Mentor
Esteemed
Arms
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

-
Esteemed
Arms
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:
[automerge]1596132739[/automerge]
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:
[automerge]1596133220[/automerge]
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)?
 
Last edited:

pc1966

Esteemed
Arms
Supporter
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

-
Mentor
Esteemed
Arms
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

Esteemed
Arms
Supporter
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.
 

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

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
Top