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.