TN-CS Combined With TT Question

[LastManOnline]

On street furniture I was under the impression that the electrode on a TNC-S was there in case of a vehicle hitting the equipment and potential damage to the supply cable, this is also in additional to tying the equipment down to local earth potential.

How in practice would you "tie the equipment down to local earth potential"?

 

[pc1966]

"tying the local ground to the same potential as the metelwork.." I, may be missing what you are getting at here, but to create an equiopotential bond between the metelwork and the ground would require a rod with virtually zero resistance?

No, you just need enough of a ground rod or mat that it pulls the local Earth up to a similar potential.

It is very hard to pull the PME down to close to true 0V, that needs a very low rod Ra, but much easier to pull up the local surroundings so the touch/step potentials are tolerable. While that may not stop equipment being damaged by 300V+ AC supply, but it might stop someone nearby from getting a life-threatening shock from any extraneous parts.

 

[LastManOnline]

No, you just need enough of a ground rod or mat that it pulls the local Earth up to a similar potential.

It is very hard to pull the PME down to close to true 0V, that needs a very low rod Ra, but much easier to pull up the local surroundings so the touch/step potentials are tolerable. While that may not stop equipment being damaged by 300V+ AC supply, but it might stop someone nearby from getting a life-threatening shock from any extraneous parts.

But if we take a rod in a TT installation. Under fault conditions the top of the rod could well be at near mains potential. And for argument sake, 2 to 3 feet further out may be at zero potential. I am trying to grasp how that could constitute an "equiopotential zone"?

 

[pc1966]

But if we take a rod in a TT installation. Under fault conditions the top of the rod could well be at near mains potential. And for argument sake, 2 to 3 feet further out may be at zero potential. I am trying to grasp how that could constitute an "equiopotential zone"?

That is a very short & poor rod!

Yes it is not a simple thing to do, but having something down in the tens of ohms range probably implies a reasonable region of equipotential from it.

 

[LastManOnline]

That is a very short & poor rod!

Yes it is not a simple thing to do, but having something down in the tens of ohms range probably implies a reasonable region of equipotential from it.

Agreed. But its more the principle I am looking at. To create an equipotential zone in a home we use bonding conductors that would need to be below 0.5 ohms. What sort of resistance then would the rods have that are sunk at the base of a street lamp require in order to create a similar effect?. Or is my reasoning off the wall here?

 

[pc1966]

0.5 ohm is very low - but the sort of thing for keeping N down to a few volts.

Realistically if you have local rods in the tens of ohms I would expect to see local touch potentials kept down below 50V or so. However, this is Sunday and I have had more wine than is reasonable to attempt any computation of this!

 

[LastManOnline]

0.5 ohm is very low - but the sort of thing for keeping N down to a few volts.

Realistically if you have local rods in the tens of ohms I would expect to see local touch potentials kept down below 50V or so. However, this is Sunday and I have had more wine than is reasonable to attempt any computation of this!

First and foremost you enjoy your Sunday (and your wine!). The main problem I have with Sunday is how quickly it becomes Monday. Will get back to this another time.

 

[pc1966]

First and foremost you enjoy your Sunday (and your wine!). The main problem I have with Sunday is how quickly it becomes Monday. Will get back to this another time.

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.

 

[Pretty Mouth]

I think this point has been raised before, and I think I'm right in saying they are some 300 injuries per annum, from a broken PEN conductor (source HSE, though I haven't a link for that.....at the moment).

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

 

[pc1966]

Well that confirmed the Zappi is basically a 3-pole RCD. Which is a sensible approach, so long as you have zero-volt (or unusual volt) releasee as well.

 

[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)