Understanding the relationship between TNCS, TNS and TT

[HappyHippyDad]

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.

 

[LastManOnline]

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]

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.

 

[LastManOnline]

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]

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.

 

[LastManOnline]

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.

 

[LastManOnline]

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.)

Hi Lucien. Like you I have heard no adequate answer to what to me is currently "a claim" that is yet to be proven and this particular thread has not shed any extra light on the matter. So, a suggestion, let's "pool resources" as it were and see if we can shed a little more light on it.
Firstly I would state
1) that the voltage at the rod is entirely dependent on the voltage across the DSO, s neutral (in a TNC-S system) as it forms a, parallel circuit across it
2) Based on the above I would further state that the rod has no effect on the neutral and thus plays no role in "tying it down to earth potential".

Anything you would add to that?

 

[Simon47]

You can't say that you are putting a resistance in parallel with something, and then say that it cannot have any effect on it.
If you fit a local earth electrode and strap it to the DNO's PEN via the MET - then it will have an effect on neutral-earth voltage. Under normal circumstances it'll be a negligible effect as there should be very little N-E voltage present. But should there be a broken PEN core, then the combination of the loads on the 3 phases downstream of the break and the resistances of any other parallel earth paths will determine the N-E voltage in the system.
If the loads are relatively well balanced and/or the earth resistances low then the N-E voltage will be low. If the loads are very imbalanced and/or the earth resistances high, then the N-E voltage will be correspondingly high.
Obviously, from a user safety PoV it would be best if every property had a low impedance load earth electrode, and thus in combination having a fairly hard N-E link regardless of where any PEN break is located. Retrofitting these would, as has been pointed out when it was mooted as a new reg, be difficult and/or costly for many properties as a retrofit - but I would have thought a fairly easy and inexpensive thing to do for new builds (drop some rebar, or copper tape, in the foundations and connect to that).

 

[Lucien Nunes]

The particular point that Edmond and I had picked up on was a specific statement that has cropped up a few times on the forum regarding USA installations. In cases where a homeowner has reported voltage fluctuations, especially where overvoltage between one hot and neutral of a split-phase installation is occurring and indicative of a high-resistance PEN, a recommendation has been made to check / repair the ground rod connection.

As per my post #24, that seems unlikely to do anything at all, and if it does, is surely concealing the actual fault.

 

[Simon47]

As per my post #24, that seems unlikely to do anything at all, and if it does, is surely concealing the actual fault.

Yes and yes.
My understanding of US electrics is that (compared to us) they tend to have many more transformers (each serving fewer properties), each located closer to the properties served. So less opportunities for lost PENs.

 

[LastManOnline]

You can't say that you are putting a resistance in parallel with something, and then say that it cannot have any effect on it.
If you fit a local earth electrode and strap it to the DNO's PEN via the MET - then it will have an effect on neutral-earth voltage. Under normal circumstances it'll be a negligible effect as there should be very little N-E voltage present. But should there be a broken PEN core, then the combination of the loads on the 3 phases downstream of the break and the resistances of any other parallel earth paths will determine the N-E voltage in the system.
If the loads are relatively well balanced and/or the earth resistances low then the N-E voltage will be low. If the loads are very imbalanced and/or the earth resistances high, then the N-E voltage will be correspondingly high.
Obviously, from a user safety PoV it would be best if every property had a low impedance load earth electrode, and thus in combination having a fairly hard N-E link regardless of where any PEN break is located. Retrofitting these would, as has been pointed out when it was mooted as a new reg, be difficult and/or costly for many properties as a retrofit - but I would have thought a fairly easy and inexpensive thing to do for new builds (drop some rebar, or copper tape, in the foundations and connect to that).

You can't say that you are putting a resistance in parallel with something, and then say that it cannot have any effect on it.
If you fit a local earth electrode and strap it to the DNO's PEN via the MET - then it will have an effect on neutral-earth voltage. Under normal circumstances it'll be a negligible effect as there should be very little N-E voltage present. But should there be a broken PEN core, then the combination of the loads on the 3 phases downstream of the break and the resistances of any other parallel earth paths will determine the N-E voltage in the system.
If the loads are relatively well balanced and/or the earth resistances low then the N-E voltage will be low. If the loads are very imbalanced and/or the earth resistances high, then the N-E voltage will be correspondingly high.
Obviously, from a user safety PoV it would be best if every property had a low impedance load earth electrode, and thus in combination having a fairly hard N-E link regardless of where any PEN break is located. Retrofitting these would, as has been pointed out when it was mooted as a new reg, be difficult and/or costly for many properties as a retrofit - but I would have thought a fairly easy and inexpensive thing to do for new builds (drop some rebar, or copper tape, in the foundations and connect to that).

The post we were responding to made the claim that the

You can't say that you are putting a resistance in parallel with something, and then say that it cannot have any effect on it.
If you fit a local earth electrode and strap it to the DNO's PEN via the MET - then it will have an effect on neutral-earth voltage. Under normal circumstances it'll be a negligible effect as there should be very little N-E voltage present. But should there be a broken PEN core, then the combination of the loads on the 3 phases downstream of the break and the resistances of any other parallel earth paths will determine the N-E voltage in the system.
If the loads are relatively well balanced and/or the earth resistances low then the N-E voltage will be low. If the loads are very imbalanced and/or the earth resistances high, then the N-E voltage will be correspondingly high.
Obviously, from a user safety PoV it would be best if every property had a low impedance load earth electrode, and thus in combination having a fairly hard N-E link regardless of where any PEN break is located. Retrofitting these would, as has been pointed out when it was mooted as a new reg, be difficult and/or costly for many properties as a retrofit - but I would have thought a fairly easy and inexpensive thing to do for new builds (drop some rebar, or copper tape, in the foundations and connect to that).

The post I was responding to had to do with how an earth rod at the consumers Installation could prevent the voltage on the Neutral from "fluctuating". While Lucien mentioned that this point has been raised on the forum in connection with U. S. installations, I have also come across it here (R.O.I.).Again I have yet to see an adequate explanation for this assertion.
Think about it. As I mentioned previously the volt drop over the neutral determines the volt drop across the rod, as they are connected in parallel. So logically then, its the neutral that infuences the earth rod and not the other way around.
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The particular point that Edmond and I had picked up on was a specific statement that has cropped up a few times on the forum regarding USA installations. In cases where a homeowner has reported voltage fluctuations, especially where overvoltage between one hot and neutral of a split-phase installation is occurring and indicative of a high-resistance PEN, a recommendation has been made to check / repair the ground rod connection.

As per my post #24, that seems unlikely to do anything at all, and if it does, is surely concealing the actual fault.

Agreed

 

[HappyHippyDad]

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.

Are you saying you are an advocate of earth rods on PME but only with a low enough Ra?

What is the actually use of this earth rod though? An Ra even of 2 ohm is only going to give a PFC of 115A which will not blow a 100A fuse or even an 80A fuse in the cut out.
A sub 1 Ra would though.
I suppose a low enough Ra of around 2 would be sufficient to trip the MCB's in the consumer unit?

 

[LastManOnline]

Are you saying you are an advocate of earth rods on PME but only with a low enough Ra?

What is the actually use of this earth rod though? An Ra even of 2 ohm is only going to give a PFC of 115A which will not blow a 100A fuse or even an 80A fuse in the cut out.
A sub 1 Ra would though.
I suppose a low enough Ra of around 2 would be sufficient to trip the MCB's in the consumer unit?

You have answered your own question really. An earth rod is only of use if it allows sufficient current to flow in order to operate the safety devices in the system it forms part of.

 

[static zap]

An earth rod is only of use if it allows sufficient current to flow in order to operate the safety devices in the system it forms part of.

Would it not lower touch potentials as well ?
( Not ideal but better than now't )

 

[LastManOnline]

Would it not lower touch potentials as well ?
( Not ideal but better than now't )

Yes if it had a resistance value similar to the neutral its connected in parallel with. However its actual resistance value (speaking about the 4 ft galvanized rod used in most TNC-S countries) will in reality be a hundred times or more. The result? During a line to earth fault in a "healthy" TNC-S system, the rod plays a negligible role. During a broken neutral fault it actually becomes a live extraneous part that becomes a potential hazard.

 

[pc1966]

It is only if most properties have rods to the total Ra for the section post-fault is low enough does it help to keep potentials down. Even then it would need a lot of rods.

But then again it also needs something to be monitoring this and disconnecting the segment / getting someone to fixing it as well!