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)

 

[OnlQQker]

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!

So electric cars for example could this theory still work?

I'm on about the other end of the supply lead, but I'm not to sure how cars are grounded, though I have seen anti static flaps touching the road from them in the past.

 

[LastManOnline]

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!

Yes less dangerous. But 115 volts is still VERY dangerous. We seem to have adopted a "river with 2 crocodiles is safer than a river with 4 in it" type of reasoning to some degree here.
Remember this discussion started with the viewpoint that "an earth rod ties the lamppost to SAME potential as the local grounds". Julie, s own calculations disproved that.

 

[LastManOnline]

So electric cars for example could this theory still work?

I'm on about the other end of the supply lead, but I'm not to sure how cars are grounded, though I have seen anti static flaps touching the road from them in the past.

You will find the link "pretty mouth" posted very helpful regarding the use of earth rods with EV chargers.

 

[Julie.]

The point i was trying to illustrate, was that if there is no reliable connection to ground, then there would be a shock risk.

if there is a bond, whether the connection is made using a rod, or incidental via the foundations then that's the situation described where I include a ground connection.

There are some situations where the street furniture is isolated, in some cases the foundations comprise aggregate of stone and sand, which does not make a good connection to ground, and so on.

 

[LastManOnline]

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!

Have you looked again at the scenario as originally described.? The drunk is standing on the ground when there is a rise in potential at his feet. Now in reality the earths potential is constantly rising and falling with respect to the live. But the lantern is also connected to true earth and hence rises with the ground. So I, m trying to establish how he is at risk of shock. Ironically we have been discussing quite a bit about equiopotential zones. Well true earth is connected directly to the neutral at the substation and the neutral is connected directly to the lanterns earth (TNC-S) so it should not be possible for the drunk to have a PD developing across his body (theoretically)

 

[Julie.]

Yes less dangerous. But 115 volts is still VERY dangerous. We seem to have adopted a "river with 2 crocodiles is safer than a river with 4 in it" type of reasoning to some degree here.
Remember this discussion started with the viewpoint that "an earth rod ties the lamppost to SAME potential as the local grounds". Julie, s own calculations disproved that.

No, they do show that multiple earthing rods do bring the general situation to one of minimal potential difference.

The one particular time it doesn't is when there is an open PEN

The only options to solve this are either to not use a combined conductor at all only TNS, or to ensure that every single piece of metalwork likely to come in contact with line, neutral, or earth(cpc) must be bonded to earth with overall resistances that are approaching zero.

If we take your view that an equipotential zone must have absolutely no voltage differences, then it is purely a theoretical concept as no matter how solid we interconnect anything, in practice there will be some resistance.

 

[Julie.]

Have you looked again at the scenario as originally described.? The drunk is standing on the ground when there is a rise in potential at his feet. Now in reality the earths potential is constantly rising and falling with respect to the live. But the lantern is also connected to true earth and hence rises with the ground. So I, m trying to establish how he is at risk of shock. Ironically we have been discussing quite a bit about equiopotential zones. Well true earth is connected directly to the neutral at the substation and the neutral is connected directly to the lanterns earth (TNC-S) so it should not be possible for the drunk to have a PD developing across his body (theoretically)

No the lamp is not connected to "true earth" as you put it.

That's the whole point of the two scenarios, one has a connection to ground, and one doesn't, by definition the one which isn't connected by a reliable connection will not be connected.

 

[LastManOnline]

No, they do show that multiple earthing rods do bring the general situation to one of minimal potential difference.

The one particular time it doesn't is when there is an open PEN

The only options to solve this are either to not use a combined conductor at all only TNS, or to ensure that every single piece of metalwork likely to come in contact with line, neutral, or earth(cpc) must be bonded to earth with overall resistances that are approaching zero.

If we take your view that an equipotential zone must have absolutely no voltage differences, then it is purely a theoretical concept as no matter how solid we interconnect anything, in practice there will be some resistance.

Agreed. I am comepletely on the same page as you here. Either multiple earthing rods or better still TNS. I was, nt implying that the equipotential zone would be anything different that what we currently enjoy in a properly bonded house.

 

[LastManOnline]

No the lamp is not connected to "true earth" as you put it.

That's the whole point of the two scenarios, one has a connection to ground, and one doesn't, by definition the one which isn't connected by a reliable connection will not be connected.

But is, nt the lantern connected to true earth via the substation?

 

[Julie.]

But is, nt the lantern connected to true earth via the substation?

Which scenario?

With an open PEN, then no! - and that's the situation when there is circ 120V where I said it was better than 240V but no good.

In the other scenarios, yes it's at zero potential, but the ground surrounding it may not be if there is no bonding and there is a fault elsewhere

 

[LastManOnline]

Which scenario?

With an open PEN, then no! - and that's the situation when there is circ 120V where I said it was better than 240V but no good.

In the other scenarios, yes it's at zero potential, but the ground surrounding it may not be if there is no bonding and there is a fault elsewhere

Sorry. I should have described which scenario. So the TNC-S supply system is healthy (no open PEN). The lantern is connected to true earth at the substation and hence connected to the drunks feet. So basically the drunk feets and the lantern are at the same potential. Now let's let take the scenario of the ground at the drunks feet rising in potential with respect to the lantern. You propose that could happen by a line to earth fault at the next lamp. Normally speaking in a healthy TNCS system virtually all the fault current flows back via, the neutral. There will naturally be a small flow down through the DNO, s rod, s depending on the volt drop across the DNO, s neutral and the resistance of the rod. There will also be a "step voltage" extending outwards as there would be with any other rod but that zone would be distancewise a long way from the drunk. So what kind of fault had you in mind that would result in a dangerous PD at the drunks feet?

 

[Julie.]

Sorry. I should have described which scenario. So the TNC-S supply system is healthy (no open PEN). The lantern is connected to true earth at the substation and hence connected to the drunks feet. So basically the drunk feets and the lantern are at the same potential. Now let's let take the scenario of the ground at the drunks feet rising in potential with respect to the lantern. You propose that could happen by a line to earth fault at the next lamp. Normally speaking in a healthy TNCS system virtually all the fault current flows back via, the neutral. There will naturally be a small flow down through the DNO, s rod, s depending on the volt drop across the DNO, s neutral and the resistance of the rod. There will also be a "step voltage" extending outwards as there would be with any other rod but that zone would be distancewise a long way from the drunk. So what kind of fault had you in mind that would result in a dangerous PD at the drunks feet?

Please show which picture!

You state that i have the high voltage in my example - which is the situation where there is no rods fitted - no local earthing arrangements.

Then describe how it can't have a voltage because of the local earthing arrangement.

As examples where there could be a fault to the ground

Ohl cable drops
Next door neighbour cuts lawnmower cable exposing line conductor, which sits in puddle
Individual mis-wires a fitting, connecting the line conductor to the enclosure
....
....

I doubt i said a line to earth fault, as that would not describe the situation I raised

 

[LastManOnline]

Please show which picture!

You state that i have the high voltage in my example - which is the situation where there is no rods fitted - no local earthing arrangements.

Then describe how it can't have a voltage because of the local earthing arrangement.

As examples where there could be a fault to the ground

Ohl cable drops
Next door neighbour cuts lawnmower cable exposing line conductor, which sits in puddle
Individual mis-wires a fitting, connecting the line conductor to the enclosure
....
....

I doubt i said a line to earth fault, as that would not describe the situation I raised

No. You specifically gave the example of " a line to ground fault at the next lamp post". See your post no 12.
You speculate that a voltage of 100 to 230 volts could arise at the drunks feet from such a fault, which in reality could not happen for the reasons I outlined in my last post.

 

[Julie.]

No. You specifically gave the example of " a line to ground fault at the next lamp post". See your post no 12.
You speculate that a voltage of 100 to 230 volts could arise at the drunks feet from such a fault, which in reality could not happen for the reasons I outlined in my last post.

Exactly!


If there is a line to ground fault, the current will pass through the ground, as there is a finite resistance, there would be a voltage drop along the route - perhaps 228V at the point of the fault circ 115/120V 1/2 way along etc.

So if you are stood on the ground at the half way point, you have circ 120V at your feet.

Now if the lamppost is correctly earthed with a solid PEN/TNS type connection this will be held to the Zero potential of the substation. therefore you will be at 120V or so whilst the lamppost is at 0V (or near to).


If you have a fault to earth, then the fault would pass through the PEN/TNS type connection.

 

[LastManOnline]

Exactly!


If there is a line to ground fault, the current will pass through the ground, as there is a finite resistance, there would be a voltage drop along the route - perhaps 228V at the point of the fault circ 115/120V 1/2 way along etc.

So if you are stood on the ground at the half way point, you have circ 120V at your feet.

Now if the lamppost is correctly earthed with a solid PEN/TNS type connection this will be held to the Zero potential of the substation. therefore you will be at 120V or so whilst the lamppost is at 0V (or near to).


If you have a fault to earth, then the fault would pass through the PEN/TNS type connection.

No. That is not correct. Remember we are dealing with a TNC-S system here, not TT. The fault current thus flows back through the DNO, s neutral (largely) and not down through earth. Assuming there is a PME at the lamp post (100 ohms) where the fault occurs than that is simply an extra circuit through which current will flow. But the entire voltage present at the DNO, s rod will be dropped across it just like it would with any other earth electrode here. So not only will there be no dangerous voltage at the "halfway point",(an unspecified distance) there will in fact be no dangerous voltage 10 feet from the electrode as it will have been dropped in its entirety across the (presumed) 100 ohm resistance of the rod