Functional earth on a RCBO, can it be connected to the DNO earth with a TN-C-S supply.

[ericmark]

This started on the screwfix forum, I internet hunt found

What is the purpose of Functional Earth (FE) on Acti9 iC60 RCBO?
If there is a break in the neutral, the product is supplied by redundancy via the FE (Line to FE supply instead of Line to Neutral), allowing it to continue to perform its protection function.
This configuration allows the behavior of Voltage Dependent (VD) RCDs with FE to be compared with that of Voltage Independent (VI) RCDs.

We are told there are two types of RCD, Voltage Dependent (VD) and Voltage Independent (VI), and with a Voltage Dependent (VD) RCD we have a functional earth so it will still work with a loss of the neutral, so the big question is if the neutral and the earth are combined, as with a TN-C-S supply, can the functional earth be the earth supplied by the DNO, as loss of PEN would mean both neutral and earth are lost together?

Until that thread I had never considered the problem of using the same connection for a functional earth as for neutral.

"Voltage Dependent Technology" RCDs supplied via the FE
(Functional Earth)
In the event of loss of supply (required to address the fault and trip), there are two
categories of behavior:
b Category 1: the RCD does not open and continuity of service is optimum,
because the load is powered on when the voltage is restored, but the RCD is unable
to operate if the loss of power supply is the result of a neutral break.
The presence of line voltage is a hazard because the RCD is no longer able to
perform its protection function in the event of contact with the line potential
b Category 2: the RCD opens if there is loss of power supply (including in the event
of neutral break). After the voltage is restored, the RCD must be reclosed to power on
the load again.
To improve the level of protection provided by category 1 RCD, in the event of neutral
breaking, the product is supplied by redundancy via the FE (line to FE supply instead
of line to neutral), allowing it to continue to perform its protection function.
This configuration allows the behavior of VDT RCDs to be compared with that of VIT
RCDs.

This is page 9 of document on earth fault protection the more I read the more I think the functional earth should go to an earth rod which is not connected to the DNO earth.

However my copy of BS7671:2008 seems to say both earths should be connected together. What does the team think?

 

[EFLImpudence]

I don't really know much about Voltage dependent or not but as no one else seems interested...

the more I read the more I think the functional earth should go to an earth rod which is not connected to the DNO earth.

Yes presumably it should not be only connected to the DNO earth.

I can find no literature which mentions anything about the FE being connected to a broken/lost neutral in a TN-C-S situation.

However my copy of BS7671:2008 seems to say both earths should be connected together. What does the team think?

Does that matter? In fact would it not be beneficial for everything concerned?

If they were connected then the FE would not have to be specifically connected to the rod - just the earth bar as it is now.


Also - was it not recently proposed that an electrode should be included in a TN-C-S installation?

 

[AndyPRK]

Interesting question.

Seams silly to fit an earth rod just for the RCBOs. If you are going to fit a rod, you may as well use it for the whole instalation. (which seams to be the trend the industry wants to go)

 

[trojanhawrs]

It would make a difference if the neutral was lost after the fusehead, otherwise seems redundant for TNCS.

 

[ericmark]

I think the chance of loosing the PEN is remote, but also loosing the neutral without also loosing earth is also remote, so if protecting against the remote chance that the neutral is lost, then also should be protecting against remote chance the PEN is lost.

 

[flameport]

With a TN-C-S supply, not only can the FE lead be connected to the network earth terminal, it must be connected to it.
You can only have a single earthing type for an installation.

TN-C-S as used in the UK won't have any earth electrodes in the vast majority of installation and relies entirely in the network operator earth terminal, which is the same conductor as the neutral.

Most other countries have local earth electrode(s), but those are all connected to the same earth terminal, and it's still TN-C-S. The advantage there is if the incoming CNE conductor is broken, the local earth electrodes will provide an alternative earth path which will reduce the touch voltage on exposed parts. How effective that is depends on the resistance of the electrodes. For any meaningful effect, electrode resistances of under 10 ohms are required.
However this is not a magic solution to anything as if those electrodes have a very low resistance <1 ohm, then broken CNE conductors will go unnoticed with all of the installation current returning via the electrodes and possibly other electrodes of nearby properties depending on where the break in the CNE conductor is.

If you want an earth electrode not connected to the network provided terminal then it's a TT installation, and nothing in it must be connected to the CNE terminal in any way - and that includes where metal service pipes or structures are shared with adjacent properties. With such shared services they must all have the same earthing arrangement - you can't make one of them TT with the rest TN-C-S as the shared conductive parts will make the whole lot TN-C-S, and the 'TT' efforts will have no proper earth connection and will be relying on the shared conductive parts which could be changed or removed at any time.

 

[EFLImpudence]

With a TN-C-S supply, not only can the FE lead be connected to the network earth terminal, it must be connected to it.
You can only have a single earthing type for an installation.

TN-C-S as used in the UK won't have any earth electrodes in the vast majority of installation and relies entirely in the network operator earth terminal, which is the same conductor as the neutral.

Most other countries have local earth electrode(s), but those are all connected to the same earth terminal, and it's still TN-C-S. The advantage there is if the incoming CNE conductor is broken, the local earth electrodes will provide an alternative earth path which will reduce the touch voltage on exposed parts. How effective that is depends on the resistance of the electrodes. For any meaningful effect, electrode resistances of under 10 ohms are required.
However this is not a magic solution to anything as if those electrodes have a very low resistance <1 ohm, then broken CNE conductors will go unnoticed with all of the installation current returning via the electrodes and possibly other electrodes of nearby properties depending on where the break in the CNE conductor is.

If you want an earth electrode not connected to the network provided terminal then it's a TT installation, and nothing in it must be connected to the CNE terminal in any way - and that includes where metal service pipes or structures are shared with adjacent properties. With such shared services they must all have the same earthing arrangement - you can't make one of them TT with the rest TN-C-S as the shared conductive parts will make the whole lot TN-C-S, and the 'TT' efforts will have no proper earth connection and will be relying on the shared conductive parts which could be changed or removed at any time.

Are you sure?

What is the difference between an additional earth electrode and an extraneous-conductive-part?

 

[trojanhawrs]

Are you sure?

What is the difference between an additional earth electrode and an extraneous-conductive-part?

Extraneous conductive parts should be bonded to the MET. As long as the earth electrode is, nothing.

 

[flameport]

What is the difference between an additional earth electrode and an extraneous-conductive-part?

Very little, other than one is intended as an earth electrode and the other one is not.

ECPs can't be used as earth electrodes as their presence is not guaranteed, as they could be removed or replaced at any time.
However when present, they will act as earth electrodes.

If you add earth electrodes to a TN-C-S system, it's still a TN-C-S system, and those electrodes must be connected to the MET.

 

[ericmark]

For any meaningful effect, electrode resistances of under 10 ohms are required.

So 230/10 = 23 amps, the question then is how low can the earth electrode go before the current is too much for earth wires to handle? I do have an earth rod with a TN-C-S system when supplied by the DNO, but it is really there so if the DNO supply fails, then my system becomes TN-S. And the inverter bonds to earth and neutral, without that relay in the inverter it would be an IT system which is normally only used with bathroom shaver sockets.

It is only a single rod, so would not think the resistance is very low, however only there because I have battery back up to run central heating and freezers, most homes with a TN-C-S supply will not have an earth rod.

But it still raises the question if the PEN is lost, will the RCBO still work? Not even sure how you would test it? In real terms the RCBO's are in the consumer unit, which will be dead with loss of DNO supply, only the RCD sockets and one FCU to boiler will still work, I have never tried turning off the main isolator and testing the RCD sockets, I know they work with DNO power, not tested without.

Looked at the installation certificate to see what they recorded.

Not looked before, I actually watched him measure the earth loop impedance, so why it is not entered on the paperwork not a clue. They fitted the RCD sockets, so should have been some where so record, I know they work as when I got my new loop impedance and RCD tester I selected them to test it with, as easy to reach and reset, so do know they work.

However I would clearly fit the functional earth wire with a RCBO, but not so sure it would work, if I lost the PEN, either with or without my earth rod? Likely it would as unlikely less than 50 volts line to PEN, Flameport has shown the tests rig with EV supplies, and how it can fail to go above or below the critical voltage to cause the EV system to trip. So seems likely it would still work.

However still can't see how we could loose the neutral without loosing the earth, so seems rather pointless.

 

[EFLImpudence]

Very little, other than one is intended as an earth electrode and the other one is not.

but you said electrodes were not allowed with TN-C-S.

ECPs can't be used as earth electrodes as their presence is not guaranteed, as they could be removed or replaced at any time.
However when present, they will act as earth electrodes.

We are talking about an electrode being there for when The PEN is lost so the FE RCBO will still work.

If you add earth electrodes to a TN-C-S system, it's still a TN-C-S system, and those electrodes must be connected to the MET.

I don't think anyone is disputing that.

 

[flameport]

but you said electrodes were not allowed with TN-C-S.

I did not say that.

 

[EFLImpudence]

I did not say that.

Sorry, I must have misunderstood.

With a TN-C-S supply, not only can the FE lead be connected to the network earth terminal, it must be connected to it.
You can only have a single earthing type for an installation.

 

[trojanhawrs]

We are talking about an electrode being there for when The PEN is lost so the FE RCBO will still work.

The point is, if the pen is lost and you have an earth electrode not connected to the MET then you're introducing a dangerous potential into the installation. If it is connected that's fine, but still makes the FE connection near redundant as it'll be at the same potential as the neutral.

 

[flameport]

With a TN-C-S installation the FE lead connects to the MET, which will be connected to the supplier network earth.
If there are earth electrode(s) then they also connect to the MET, and via that to the network earth terminal.
It's all connected together.

This is what's not possible:

the more I think the functional earth should go to an earth rod which is not connected to the DNO earth.

because if such an arrangement was created, you now have a path via the tiny FE lead between the network earth (via the neutral) and an earth electrode. In the event of any fault, a substantial current will melt that lead and probably destroy the RCBO.