TN-CS Combined With TT Question

[OnlQQker]

So I've just watched this and the chap explains all the different Earth arrangements used in the UK. He mentions about how in a TN-CS earth system if the neutral was broken just before it came into your house then this would be very dangerous. So I was just wondering if a TT system combined with a CN-CS would be safer.
Or am I missing something he is saying and barking completely up the wrong tree? (which is always quite possible)

 

[LastManOnline]

Thank you! Having completed the last of a very nice Italian wine from near Naples, I am now enjoying a decent and reasonably price French wine.

Well you seem to be working your way across the continent very fluidly!

 

[pc1966]

Well you seem to be working your way across the continent very fluidly!

Yes, doing my bit!

I have some decent Spanish wine as well, and there is even good English wine from Nyetimber if you like the Champaign style. Going further north is a struggle for good grape growing weather, but locally the is Cairn O'Mohr who make good wine from fruits.

 

[LastManOnline]

T

https://myenergi.com/wp-content/uploads/2019/09/Protection-against-damaged-PEN-Conductor-r1_0.pdf

Thanks again for posting that. A truly informative read and the kind of info I, ve been on the lookout for for sometime.

 

[Midwest]

https://myenergi.com/wp-content/uploads/2019/09/Protection-against-damaged-PEN-Conductor-r1_0.pdf

Still scratching my head on this.

So the myenergi device, has a voltage monitoring device and this ‘alternative device’ (an RCD that monitors the current flowing to earth)?

So looking at the later, a person may endure around a 30mA shock before the device disconnects, and the device still needs a reference point with Earth. Doesn’t this reference point still need to be 2-10m from other rods or buried metallic objects?

Bearing in mind ventricular fibrillation can occur at 30mA. Doesn’t appear sorted to me.

 

[pc1966]

So looking at the later, a person may endure around a 30mA shock before the device disconnects, and the device still needs a reference point with Earth.

It is basically an RCD that looks at CPC current as well. So yes they can get up to 30mA before it trips but that is the same as any other RCD for "additional protection". Not fun, but in most cases not fatal.

I don't think it needs any Earth reference which is the main selling point.

Basically if the volts are out of range it opens L, N and E (probably a normally-open contactor, which would deal with an close-by open N+E fault that would rob a protection circuit of its operating power supply), and if it detects a 30mA leakage current on CPC (and presumably L&N for normal RCD protection?) it also disconnects.

 

[LastManOnline]

Still scratching my head on this.

Primarily it ties the local ground to the same potential as the metal furniture, if this was not the case then the ground - and thus the feet of the drunk leaning on the lamp post could raise to some random potential in the event of a fault elsewhere - such as a line-ground fault at the next lamp post - or that one itself so with say 100....230V actually connecting to the ground/drunk's feet, whilst the lamp post itself would be tied to earth at the substation.

Tying the local ground to the same potential as the metalwork in the area prevents this, its one of the principles of equipotential bonding used throughout the 18th.

Rather than of little value, it's of great value

The resistance value to earth isn't unspecified at all; it depends on the load or the unbalance of a 3 phase load - so 500W would be 100ohm, through to 5kW which would be 9ohm - with corresponding values between.

There, s more than one head been scratched at the moment. I am genuinely perplexed at a number of the comments in Julie's post and would be very happy to be corrected
First, the "equippotential zone" created by the rod which "ties the local ground to the same potential as the metelwork".

Equipotential means "same potential". That would require zero "touch voltage" at the rod. Zero "touch voltage" at the rod would require a rod with zero resistance.

Second. Regarding the drunk with his feet on the earth who is simultaneously touching the earthed metelwork. Is, nt the earth he is standing on also tied to earth at the substation,? similar to the the earthed metelwork he is touching?

Third :If there was a line to earth fault at the pole further up. In a tncs system the voltage at the rod will never be more than the voltage across the neutral that's, its connected in parallel with. So let's say 50 volt approx. However that rod still has a contact resistance with the ground of dozens of ohms. And as with any rod there will be step voltage that gradually reduces. Where then does the voltage come from that poses such a danger at the feet of the drunkard.?
Hope there is at least a little sense in what's written above.

 

[Rockingit]

There, s more than one head been scratched at the moment. I am genuinely perplexed at a number of the comments in Julie's post and would be very happy to be corrected
First, the "equippotential zone" created by the rod which "ties the local ground to the same potential as the metelwork".

Equipotential means "same potential". That would require zero "touch voltage" at the rod. Zero "touch voltage" at the rod would require a rod with zero resistance.

Second. Regarding the drunk with his feet on the earth who is simultaneously touching the earthed metelwork. Is, nt the earth he is standing on also tied to earth at the substation,? similar to the the earthed metelwork he is touching?

Third :If there was a line to earth fault at the pole further up. In a tncs system the voltage at the rod will never be more than the voltage across the neutral that's, its connected in parallel with. So let's say 50 volt approx. However that rod still has a contact resistance with the ground of dozens of ohms. And as with any rod there will be step voltage that gradually reduces. Where then does the voltage come from that poses such a danger at the feet of the drunkard.?
Hope there is at least a little sense in what's written above.

My interpretation of what Julie was suggesting is that in a perfect stand-alone circuit it's exactly that - same as in any circuit load - the pd varies on the circuit dependant on where you measure it versus the load. So if there was only one energy source on the literal world and one drunk and one lamp post it becomes some maths to work out that A to B to C where B is a factor of distance/resistance/voltage, diminishing from A = 230v (say) to C = 0v. But by having another conductor route from A > C in the form of a cable we create a parallel path so now B is B/2 (or whatever resistances are involved). Now imagine that we have multiple sources of energy and multiple parallel paths and rapidly we end up with B/∞ which clearly is 'not a lot'. This is what happens when we 'ground' multiple times. If we all of us walked around in our daily lives with stupidly long fly leads attaching our bodies to the neutral points on the nearest sub station we wouldn't need cpc's or bonding to keep us safe - but as that's as ridiculous as it is impractical we instead have as many points around us at anyone time which ARE linked back, and we use technology such as RCD's to account for the stage in-between us and that nearest point.

Pedants..... yes, that's a very simplistic view!

 

[LastManOnline]

My interpretation of what Julie was suggesting is that in a perfect stand-alone circuit it's exactly that - same as in any circuit load - the pd varies on the circuit dependant on where you measure it versus the load. So if there was only one energy source on the literal world and one drunk and one lamp post it becomes some maths to work out that A to B to C where B is a factor of distance/resistance/voltage, diminishing from A = 230v (say) to C = 0v. But by having another conductor route from A > C in the form of a cable we create a parallel path so now B is B/2 (or whatever resistances are involved). Now imagine that we have multiple sources of energy and multiple parallel paths and rapidly we end up with B/∞ which clearly is 'not a lot'. This is what happens when we 'ground' multiple times. If we all of us walked around in our daily lives with stupidly long fly leads attaching our bodies to the neutral points on the nearest sub station we wouldn't need cpc's or bonding to keep us safe - but as that's as ridiculous as it is impractical we instead have as many points around us at anyone time which ARE linked back, and we use technology such as RCD's to account for the stage in-between us and that nearest point.

Pedants..... yes, that's a very simplistic view!

No argue with Julie's reasoning about regarding about role improved earthing and bonding in general play.
But it's the point about creating an equiopotential zone between the metelwork and the local ground through the sinking of an earth rod at the base of the lamp.
Let's take a practical example. In the link from "pretty mouth" on this thread "my energy" state that to keep the touch voltage at 70 volts at an earth electrode during a fault at their EV socket would require a rod with a resistance of 2.5 ohms, which they regard as been impractical to achieve. That is thus 70 volts potential difference from the top of the rod measured measured against the ground 2 to 3 feet outwards from the rod.Does that qualify as an "equiopotential zone"?
What then qualifies Julie, s example as an equiopotential zone?

 

[Rockingit]

My brain hurts now. An EQPZ is simply an 'area' (as defined by the duration of a low enough resistance) where no pd exists between two points - so going back to basics we bond the neutral of our energy to actual earth, our water pipe also to it, gas etc etc, such that if I happen to grab onto the kitchen sink at the same time as brushing against the toaster (whatever...) then because they are both part of the same eqpz then no volts can exist between the two conductors. Theoretically, if you had long arms and grabbed the neighbours toaster then you could take a belt if it was from a different energy source.

 

[LastManOnline]

My brain hurts now. An EQPZ is simply an 'area' (as defined by the duration of a low enough resistance) where no pd exists between two points - so going back to basics we bond the neutral of our energy to actual earth, our water pipe also to it, gas etc etc, such that if I happen to grab onto the kitchen sink at the same time as brushing against the toaster (whatever...) then because they are both part of the same eqpz then no volts can exist between the two conductors. Theoretically, if you had long arms and grabbed the neighbours toaster then you could take a belt if it was from a different energy source.

Can totally relate to the "hurting brain" bit. But your point is the same conventional electrical principal we have all been taught. An EQPZ means no PD between 2 points. And that is not achievable with a standard rod (or even several)

 

[Julie.]

Sorry about responding a bit late to the party, I haven’t been on-line.

Sorry if it wasn’t too clear – I will attach a few pictures to try to explain.



Firstly, if we didn’t use PME or any bonding to the ground, then each case would look like this:

In this case assuming we are say ½ way along, and the earth impedance is say 400 ohm each side of the lamppost then the potential of the ground at the point of the lamppost would be around ½ of the fault voltage – so say 115 – 120V – if we touch the lamppost – which is held at zero then we have a 120V shock hazard!

If we now bond this lamppost to the ground – say by a 100 ohm connection then the fault current would go through the 400 ohm (A) and then both the 400 ohm (B) and 100 ohm in parallel.

This time then the voltage would only be 38V or so – so a much better situation.

Obviously, there are cases where the earth impedance is lower, but then the earth rod resistance would be lower as well, but for close up situations, it’s possible to get cases which aren’t much better than no bond at all

In practice however we would usually have many similar bonds to ground similar to the picture below

In this case, each point, such as the blob in the centre has a resistance to all the earth bonds around it, the closer ones may be 70 ohm, further ones 120 ohm etc – but if we take an average of 100 ohm and there are 10 of them, then the overall is around 10 ohm; however the real advantage is the effective isolation of each point from more distant ones:

In this case each the two example points end up fairly disconnected from each other – a mini near equipotential zone – in this case the equivalent circuit would look like this:

This gives a touch voltage around 5V or so for the same fault and ground resistance as the simple examples above.

Now, as has been pointed out, these aren’t true equipotential zones as any resistance at all would give rise to a voltage drop, but this is the case throughout all of the real electrical systems – even if you use 10mm^2 to bond everything throughout a house, then there will still be some resistance, and therefore some voltage drop in the event of a fault, but the regs generally use “substantially the same potential” as an EP zone/bonding criteria.

The next issue is the loss of a PEN, so with 500W this would represent a load resistance of 100 ohm or so, our situation would be this:

Effectively there would be a potential of 230V on the body of the lamp, however with the 100 ohm earth rod, it looks like this and we would have around 115V touch potential.

Although this is no good – it is better than having no earth rod, not worse.

Obviously all the above is just an oversimplification, but it illustrates that multiple bonds to ground gives both low overall resistance to ground, plus areas that form equipotential type zones - fairly disconnected from the influences of other areas forming similar zones.

 

[LastManOnline]

Sorry about responding a bit late to the party, I haven’t been on-line.

Sorry if it wasn’t too clear – I will attach a few pictures to try to explain.



Firstly, if we didn’t use PME or any bonding to the ground, then each case would look like this:View attachment 61748
In this case assuming we are say ½ way along, and the earth impedance is say 400 ohm each side of the lamppost then the potential of the ground at the point of the lamppost would be around ½ of the fault voltage – so say 115 – 120V – if we touch the lamppost – which is held at zero then we have a 120V shock hazard!

If we now bond this lamppost to the ground – say by a 100 ohm connection then the fault current would go through the 400 ohm (A) and then both the 400 ohm (B) and 100 ohm in parallel.

View attachment 61749
This time then the voltage would only be 38V or so – so a much better situation.

Obviously, there are cases where the earth impedance is lower, but then the earth rod resistance would be lower as well, but for close up situations, it’s possible to get cases which aren’t much better than no bond at all

In practice however we would usually have many similar bonds to ground similar to the picture below

View attachment 61750
In this case, each point, such as the blob in the centre has a resistance to all the earth bonds around it, the closer ones may be 70 ohm, further ones 120 ohm etc – but if we take an average of 100 ohm and there are 10 of them, then the overall is around 10 ohm; however the real advantage is the effective isolation of each point from more distant ones:

View attachment 61751
In this case each the two example points end up fairly disconnected from each other – a mini near equipotential zone – in this case the equivalent circuit would look like this:

View attachment 61752
This gives a touch voltage around 5V or so for the same fault and ground resistance as the simple examples above.

Now, as has been pointed out, these aren’t true equipotential zones as any resistance at all would give rise to a voltage drop, but this is the case throughout all of the real electrical systems – even if you use 10mm^2 to bond everything throughout a house, then there will still be some resistance, and therefore some voltage drop in the event of a fault, but the regs generally use “substantially the same potential” as an EP zone/bonding criteria.

The next issue is the loss of a PEN, so with 500W this would represent a load resistance of 100 ohm or so, our situation would be this:
View attachment 61753
Effectively there would be a potential of 230V on the body of the lamp, however with the 100 ohm earth rod, it looks like this and we would have around 115V touch potential.

View attachment 61754


Although this is no good – it is better than having no earth rod, not worse.

Obviously all the above is just an oversimplification, but it illustrates that multiple bonds to ground gives both low overall resistance to ground, plus areas that form equipotential type zones - fairly disconnected from the influences of other areas forming similar zones.

Am also late back to the party! You put a lot of work into that reply. Appreciated. Have gotten through most of your calculations and they speak for themselves. You have demonstrated your point that it's better to have an earth rod at the base of the streetlamp than not have one. In your illustrations numbers 5 and 6 (dealing with open PEN) you demonstrated that the touch voltage at the lantern would be 115 volts instead of 230 volts.
I struggle a little however with some of your assumptions.
Firstly 115 volts is "better", but is that acceptable ? , safe? If the lamp wattage is reduced, will the touch voltage not rise higher again?
Secondly to describe the above situation as an "equipotential zone" is employing the term in a way I have not seen before. "equi" meaning equal or same. Its sometimes referred to as a "safe zone". The term conveys the idea of safety in people's minds, sometimes defined as an area where electric shock won't be experienced. I appreciate that you are perhaps implying a "more equal zone" but its important to make the distinction because of the mistaken belief (come across it here in ROI frequently) that the neutral can be "tied down to earth" via a 100 ohm rod. Your own calculations demonstrate that there will actually be a significant PD at the rod.
One final point that has me a little confused is the example you gave of the drunk touching the lantern and receiving a shock coming from the ground which flows back to the substation via the street lamp. Is the drunk not standing on "earth" while also touching "earth" (the street lamp is earthed)? In theory there should be no PD there?

 

[Midwest]

I’ve had to put a couple of lids back on lamp posts. Never seen any earth rods. But isn’t the lamp post an earth rod itself?

 

[LastManOnline]

I’ve had to put a couple of lids back on lamp posts. Never seen any earth rods. But isn’t the lamp post an earth rod itself?

It could be depending on how its secured to earth

 

[pc1966]

Is the drunk not standing on "earth" while also touching "earth" (the street lamp is earthed)? In theory there should be no PD there?

The region very close to the rod (how close depends on the depth of the rod, etc) will be pulled closer to the lamppost, so they should get a much smaller shock than the volts to true Earth.

Again it is not a magic solution, and there will be dangerous voltages possible, but it is definitely less dangerous with the rods!