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)

 

[Strima]

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.

 

[Midwest]

What are the chances of the neutral leg being broken between the service head outside your property and your meter without causing some other fault? Pretty much zero.

pme = protective multiple earth. It’s earthed at pretty much every joint; every time the dno do any work on a cable they pme it if not already and put an earth rod in.

you could always bang an earth rod in on a tncs system if it makes you feel better. That’s what other countries do and I think was considered for the 18th?

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).

 

[jakew009]

Talking to an ENWL jointer a few weeks ago they don't necessarily PME their cable joints

I guess different networks will have different policies. WPD guys when replacing a transformer the other day said the only way you could guarantee a tns connection is if you had your own transformer on site.

 

[Dartlec]

I guess different networks will have different policies. WPD guys when replacing a transformer the other day said the only way you could guarantee a tns connection is if you had your own transformer on site.

The last couple of 'TN-S' supplies I've tested have had Ze of 0.3 or so, suggesting that they are likely TNC-S in reality. I've also heard others say that large parts of the country are effectively that, regardless of the service head.

 

[LastManOnline]

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.

"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?
We create equipotential bonds in the home by eliminating the resistance between two conductive services. No resistance equals no volt drop.

 

[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.