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

[EFLImpudence]

I missed a crucial "not".

Obviously no reason to have the electrode "not" connected to the MET.

 

[AndyPRK]

I thought FE leads were to allow the electronics in the RCBO to work, rather than to carry any current.

I didn't think there was a connection between the FE and N in the rcbo ?

 

[AndyPRK]

I don't think there is a connection between the FE and N as I have some wylex rcbos with FE, and a time delayed RCD at the origin.

The RCD would trip if there was a connection?
They wouldn't risk using those tiny wires if there was any chance of current flowing.

 

[flameport]

I thought FE leads were to allow the electronics in the RCBO to work, rather than to carry any current.

They are - that's literally the definition of functional earth.

I didn't think there was a connection between the FE and N in the rcbo ?

There won't be directly connected, but there must be some component(s) between them, and applying 230V AC across the neutral and FE leads will inevitably damage something.

They wouldn't risk using those tiny wires if there was any chance of current flowing.

In normal and proper use there isn't any current - until you start connecting the FE lead to an earth electrode that's separate from the rest of the installation as was suggested in the original post, and then the incoming CNE conductor goes open, resulting in 230V or more between the neutral and FE lead, a voltage which will remain there until something goes on fire as even tripping the RCBO won't disconnect that voltage.

 

[JohnW2]

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

Absolutely no difference, electrically - only the words differ, and electrons don't understand words.

 

[SUNRAY]

...There won't be directly connected, but there must be some component(s) between them, and applying 230V AC across the neutral and FE leads will inevitably damage something...

Let me get this straight, you are saying safety devices are designed to blow up/cause damage if the incoming neutral is broken?

 

[flameport]

Let me get this straight, you are saying safety devices are designed to blow up/cause damage if the incoming neutral is broken?

Please read the whole thread for context before springing to incorrect assumptions.

This:

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.

suggested connecting the RCBO FE lead to a separate earth rod and not to the earth terminal.
If that method of incorrect installation was done, and the CNE incoming cable failed, it could put 230V or more between the RCBO neutral and the FE lead.

In reality this will never happen, as the RCBO FE lead would be connected to the main earth terminal in accordance with the manufacturers instructions. Even with a failed CNE incomer, the FE lead and neutral would still be at the same voltage as they both connect to the same shared N&E terminal.

 

[SUNRAY]

Please read the whole thread for context before springing to incorrect assumptions.

This:

suggested connecting the RCBO FE lead to a separate earth rod and not to the earth terminal.
If that method of incorrect installation was done, and the CNE incoming cable failed, it could put 230V or more between the RCBO neutral and the FE lead.

In reality this will never happen, as the RCBO FE lead would be connected to the main earth terminal in accordance with the manufacturers instructions. Even with a failed CNE incomer, the FE lead and neutral would still be at the same voltage as they both connect to the same shared N&E terminal.

I have been following the whole thread but not read the document.

My comment is based solely on your statement and for clarity I include what your statement replies to:

...

I don't think there is a connection between the FE and N as I have some wylex rcbos with FE, and a time delayed RCD at the origin.

There won't be directly connected, but there must be some component(s) between them, and applying 230V AC across the neutral and FE leads will inevitably damage something...

Which I assumed (correctly or incorrectly) you meant the damage will be internal to the RCBO.

On that basis, loss of the supply neutral where the earth (connected to the neutral centre point) and line are still present "...will inevitably damage something..." and my assumption being damage to the component(s) within the RCBO "between the FE and N".

 

[flameport]

Yes, the RCBO would be damaged if it had 230V between the neutral and FE lead.
When connected correctly that is impossible, as the neutral and earth terminal will be at the same potential, even if the incoming supply cable has a break in the combined N&E conductor.

The damage would only occur in the specific circumstances suggested in the opening post of this thread.

 

[JohnW2]

Yes, the RCBO would be damaged if it had 230V between the neutral and FE lead.

Do any of us actually know what goes on within these things and hence what, when present, these FEs are connected to - or is everyone just guessing?

Tangentially ... I thought that, for a long time, all "RCDs" (RCCBs), as well as RCBOs, had 'electronics' necessary for their properoperation. If so, why do we (at least I) not see some RCCBs with FEs?

 

[AndyPRK]

Plenty of videos on YouTube on the inside of rcbo.

I assumed RCDs were a toroid with 3 windings. I guess the type A must be more advanced.

 

[SUNRAY]

Yes, the RCBO would be damaged if it had 230V between the neutral and FE lead.

Are you sure of this?

It seems a bit unlikely that any manufacturer would contemplate such a ridiculousy stupid and hazardous product on the market.

When connected correctly that is impossible, as the neutral and earth terminal will be at the same potential, even if the incoming supply cable has a break in the combined N&E conductor.

I'm not even considering a combined N & E, only separate from the star point; TN-S (as I believe I have at home) or TT.

The damage would only occur in the specific circumstances suggested in the opening post of this thread.

 

[JohnW2]

Plenty of videos on YouTube on the inside of rcbo.

I'm sure there are, and if/when I have some time I may look - but I was wondering whether anyone already knew the answer.

I assumed RCDs were a toroid with 3 windings. I guess the type A must be more advanced.

If I understand correctly, only the earliest RCDs used just current from the sense coil directly to operate the trip mechanism. Subsequent ones have seemingly invariably added electronics to the equation (hence might be perceived as needing an FE to ensure there was always voltage to power the electronics) - I imagine, conceptually, to 'amplify' the current from the sense coil.

 

[ericmark]

If I understand correctly, only the earliest RCDs used just current from the sense coil directly to operate the trip mechanism. Subsequent ones have seemingly invariably added electronics to the equation (hence might be perceived as needing an FE to ensure there was always voltage to power the electronics) - I imagine, conceptually, to 'amplify' the current from the sense coil.

It seems two methods with electronics, active RCD's fail safe, loose the supply and it auto disconnects, or the FE and passive type.

But question remains, with loss of PEN will they still disconnect if there is an earth fault?

 

[JohnW2]

It seems two methods with electronics, active RCD's fail safe, loose the supply and it auto disconnects, or the FE and passive type.

Indeed, but (a) that 'NVR' functionality does not necessarily require electronics and (b) that introduces an additional complication to the discussion - so it would probably be best to just thing/talk about passive RCDs, as found in CUs.

In any event, I don't know exactly what "loose the supply" means in terms of active RCDs. 'Loss of PEN/CNE' will reduce the L-N pd as seen by the RCD, but I don't know how much reduction is required before it interpret that as "lost supply" - do you?

But question remains, with loss of PEN will they still disconnect if there is an earth fault?

Indeed, but that question is pretty moot - both 'loss of PEn' and 'earth fault' are very rare events, so the probability of the two occurring simultaneously must be pretty close to zero.