"Earth to neutral fault" with TN-C-S supply ?

[JohnW2]

Just found in the instructions for a (FuseBox) AFDD/RCBO (something I doubt I will ever install ) ..

What exactly is an "earth to neutral fault" in a TN-C-S installation ?

Kind Regards, John

 

[Jurassicspark]

What exactly is an "earth to neutral fault" in a TN-C-S installation ?

Quite pointless, but I'm guessing they are referring to an earth to neutral fault within the origin of the installation?

 

[Deleted member 142255]

According to the installation instructions this indicates a neutral/earth fault on the circuit connected to the device.

Disconnecting the outgoing circuit and retesting will prove if it is the device or the circuit, however I would have thought if all dead testing wqss carried out correctly the fault would have already been identified unless this was on an EICR.

 

[JohnW2]

Quite pointless, but I'm guessing they are referring to an earth to neutral fault within the origin of the installation?

That illustrates my very point - that such would surely make absolutely no sense, given that there is an intentional connection between neutral and earth at the origin of a TN-C-S installation ?

They therefore presumably must mean an N-E fault on the circuit downstream of the device. However, whilst that might result in an RCD tripping (if there were other loads on the CU), I can't see how it could stop the device failing to response to pressing the test button, can you?

 

[JohnW2]

According to the installation instructions this indicates a neutral/earth fault on the circuit connected to the device.

Yes, that's presumably what it means - but, as I've just written, how on earth could such a fault cause the device to fail to respond to pressing of the test button?

Disconnecting the outgoing circuit and retesting will prove if it is the device or the circuit ....

Yes, if there is some apparent malfunction of the device, then disconnecting the outgoing circuit would obviously reveal whether the problem was with the device or the connected circuit - but, as above, I still don't understand how a N-E fault on the circuit could result in the test button not working - if it did anything, such a fault would actually cause the device to trip.

 

[Jurassicspark]

I can't see how it could stop the device failing to response to pressing the test button, can you?

The reason why the manufacture has came up with this because I think this can be an issue with RCD test buttons, if the neutral is linked to earth at the origin and a fault links N-E after the RCD, then there is a current path which bypasses the sense coil in the RCD. Very small or no current flows in the coil so no trip.

 

[Jurassicspark]

Someone I know installed an outdoor socket or whatever it was with a N - E fault. RCD test button wasn't working. When disconnecting the faulty circuit the RCD started to function correctly.

 

[Deleted member 142255]

Yes, that's presumably what it means - but, as I've just written, how on earth could such a fault cause the device to fail to respond to pressing of the test button?

Not presumably, specifically stated in the instructions.

Yes, if there is some apparent malfunction of the device, then disconnecting the outgoing circuit would obviously reveal whether the problem was with the device or the connected circuit - but, as above, I still don't understand how a N-E fault on the circuit could result in the test button not working - if it did anything, such a fault would actually cause the device to trip.

I can't see how either. Why not ask the manufacturer to explain, then elighten us with your knowledge?

Edit: It does tend to defeat the object of fitting it if a neutral/earth fault may go undetected.

 

[Murdochcat]

I don’t think this just confined to Fusebox RCBOs …..have come across this on other makes too over the years. Yes odd, but have seen. As above if the test button doesn’t work, disconnect the output and try again

 

[JohnW2]

The reason why the manufacture has came up with this because I think this can be an issue with RCD test buttons, if the neutral is linked to earth at the origin and a fault links N-E after the RCD, then there is a current path which bypasses the sense coil in the RCD. Very small or no current flows in the coil so no trip.

Maybe I need to think more deeply but, at first sight, I don't get that suggestion.

As per the diagram below, as I see it the test current through the L side of the RCD will be the same whether or not there is an N-E fault downstream of the RCD, and the current through the N side of the RCD (zero if there are no loads) will also be the same whether or not there is a downstream N-E fault - so the residual current created by pressing the test button should be the same regardless of whether or not there is a downstream N-E fault/ What, if anything, am I missing?

 

[JohnW2]

I can't see how either. Why not ask the manufacturer to explain, then elighten us with your knowledge?

See what I've just written above.

I doubt that I will be inclined to bother to ask the manufacturer since, as I said, I think it incredibly unlikley that I will ever even dream of installing an AFDD/RCBO - and I've never seen any similar statement from a manufacturer in relation to ordinary RCBOs (which I do use)

Edit: It does tend to defeat the object of fitting it if a neutral/earth fault may go undetected.

That's what seems a bit daft about all this. They don't seem to be suggesting that the device is incapable of responding to N-E faults in the presence of loads (which, as you imply, would be crazy) but, rather, that a N-E fault may stop the device responding to the test button. being pressed However, if there is a significant N-E fault then if there are any loads, it's very likely that the device would trip anyway, regardless of whether or not the test button would work!

 

[JohnW2]

I don’t think this just confined to Fusebox RCBOs ….

I don't imagine it is - I just happened to stumble across that one.

.have come across this on other makes too over the years. Yes odd, but have seen. As above if the test button doesn’t work, disconnect the output and try again

As I've recently written, I really don't understand the mechanism being postulated. As I've said,if there are any significant loads around, one would expect (hope!) that the device would trip even if one didn't press the test button

 

[Deleted member 142255]

That's what seems a bit daft about all this. They don't seem to be suggesting that the device is incapable of responding to N-E faults in the presence of loads (which, as you imply, would be crazy) but, rather, that a N-E fault may stop the device responding to the test button. being pressed However, if there is a significant N-E fault then if there are any loads, it's very likely that the device would trip anyway, regardless of whether or not the test button would work!

Are we not looking close enough?

Your diagram shows the RCD alone.

Reconsider with an AFDD stuck on the front end and a load on the back end, it may become more apparent.

I don't see diagrams for AFDD's that show any more than a black box approach, so that may have some bearing on the situation.

 

[JohnW2]

Are we not looking close enough? Your diagram shows the RCD alone.

It does.

Reconsider with an AFDD stuck on the front end and a load on the back end, it may become more apparent. I don't see diagrams for AFDD's that show any more than a black box approach, so that may have some bearing on the situation.

I suppose you may be right. I know very little about how AFDDs work and (although I clearly may be wrong) had assumed/guessed/whatever that the test button merely tested the residual-current sensing functionality - hence why I just considered an RCD.

However, regardless of thst, I think it's still true that the question is pretty moot, since the device in question is an AFDD/RCBO, and hence obviously should have RCD functionality. That being the case, one would expect that, in the presence of any significant load, an N-E fault would cause the device to trip, regardless of any considerations of the test button functionality, wouldn't one?

 

[Deleted member 142255]

It does.

I suppose you may be right. I know very little about how AFDDs work and (although I clearly may be wrong) had assumed/guessed/whatever that the test button merely tested the residual-current sensing functionality - hence why I just considered an RCD.

However, regardless of thst, I think it's still true that the question is pretty moot, since the device in question is an AFDD/RCBO, and hence obviously should have RCD functionality. That being the case, one would expect that, in the presence of any significant load, an N-E fault would cause the device to trip, regardless of any considerations of the test button functionality, wouldn't one?

You would think so, but as the manufacturers have stated the potential issue in their instructions they must be aware of a set of conditions that would create the issue.

We can speculate from now until domesday, but the answer should come from them in minutes.