Neutral to earth fault affecting both RCDs

[Roadhousezamora]

Hello everyone, I'm newly registered having read the boards from the shadows for a while. I have a question about neutral to earth faults and RCDs. I have read around this as much as I can but being relatively newly qualified my experience isn't as much as the collective members here so I thought I'd ask.

I had been asked to install a new extractor fan in a kitchen, I have done some pre work testing on the cabling present and it all looked good so I proceed to fit the extractor. When filling out the minor works cert I have struggled to get the Rcd to trip. I know I should have tested this as well before I started the work but as I said I'm new and clearly a bit naive. Initially the Rcd auto test didn't trip at 1/2 on both sides of the wave, tripped the Rcd both times at 1x then gave me a 50v error at 5x, the test button worked.

I did a bit of reading and found that the fault could be a neutral to earth fault. I pulled out each neutral at the board and IR tested between individual neutrals and the earth bar of all the circuits on that side of the board and found no problems >200M on each circuit. When carrying out a global IR test (neutral bars to earth bar) I see there is a fault but on the other side of the board. I pulled all the neutrals out on the other side and identified a neutral to earth fault on the sockets ring upstairs. I have chased this to a cable between two sockets and will obviously now have to replace this but my question was, can a neutral to earth fault on the other side of the board affect both RCDs? Was the 50v warning on the rcd I was initially testing due to the neutral to earth fault I have identified on the other side of the board?

From my reading I have seen that this type of fault can cause the RCDs to not trip at all and wondered if the fault could be affecting my ability to test this RCD?

Any help would be appreciated and any additional info will be provided swiftly.

If it helps anyone the RCDs are actually Lewden type A RCCBs (63A 30mA)

Thanks

 

[Aaron b]

Are you testing the RCD at the board or on a circuit? No experience with meggers (and we don't install RCD's often for utilities) but I would have assumed that the test had failed because the voltage on the earth conductor was at 50V. I assume the cable resistance was too high to achieve 250mA at <50V. Was there a '>' or a '<' in front of the warning?

I don't understand how a neutral earth fault would cause an RCD not to trip. The only reason I can think of for the fault on the socket circuit not to trip would be if it was sticky from lack of use (or a DC leakage issue).

If the cable isn't accessible and you cant install a new one, could you disconnect the faulty cable and derate it to 20A?

 

[Aaron b]

Sorry, I see the guy's above beat me to it, I don't know why I assumed a merger either... ?

 

[Roadhousezamora]

I was reading someones explanation and what I took from it was that the amount of current lost from the neutral was less than 30mA and therefore not triggering the RCD... Didn't fully understand how that happens though.

Yeah I may well have split the ring into two radials, there is a spare way in the board as well.

I have been testing on the circuit and not at the Rcd itself with no load so will try this when I'm next there.

If the Rcd responds at the device itself I will re add the loads but remove the neutral earth fault and see if they continue to operate.

I really appreciate this, being new my confidence when it comes to this sort of fault finding isn't great but I'm keen to make sure I get to the bottom of it, for myself as much as the client.

 

[pc1966]

What was the Zs at the test points used where the RCD failed? (Using the no-trip option, obviously!)

Seeing >50V warning to me sounds like the earth/CPC is exceptionally poor the test current is pushing it up above 50V.

 

[pc1966]

It's a Dilog 9110 (can't afford anything premium right now lol) it's ok but prone to being temperamental.

Have you tried a new set of probes?

I have the 9118 and it is fine (though I question the no-trip accuracy in some low Zs cases) and so far not seen the 50V warning. Probes are "consumables" as eventually the flexing breaks the conductors or the plugs get weak and don't establish as good connection.

 

[Roadhousezamora]

Zs was 0.4, on the circuit I was installing the extractor on.

With regards to leads, no I haven't replaced them yet, I will look at picking up a set if I can't seem to find a solution on the system itself, I haven't had them that long but it's a good suggestion for me to look into, thank you

 

[Aaron b]

I was reading someones explanation and what I took from it was that the amount of current lost from the neutral was less than 30mA and therefore not triggering the RCD... Didn't fully understand how that happens though.

Ah that makes sense for why that fault didn't cause the RCD to trip, I was thinking more of a direct short from a screw or the like but you're right it could be a lot lower than 1M and not trip I suppose. Assuming in most cases a 30mA RCD's only operate above 20mA..... 230V/0.02A=11.5kohm. Any chance something could be wet?

Hopefully checking your leads and using a long lead to check the earths should identify the problem with the RCD test.

 

[Roadhousezamora]

Ah that makes sense for why that fault didn't cause the RCD to trip, I was thinking more of a direct short from a screw or the like but you're right it could be a lot lower than 1M and not trip I suppose. Assuming in most cases a 30mA RCD's only operate above 20mA..... 230V/0.02A=11.5kohm. Any chance something could be wet?

Hopefully checking your leads and using a long lead to check the earths should identify the problem with the RCD test.

Yes possibly, there is an external wall that the faulty cable is buried on if there's water from the outside I suppose it could be wet but I would probably expect to see that on the plaster on the I side. Will get the damp meter on it next time, thanks

 

[Lucien Nunes]

To clarify with certainty:

An N-E fault downstream of an RCD can cause it to fail a test. because some of the leakage current generated by the tester can 'get back into' the neutral via the fault and go back through the RCD, hence reducing the imbalance. Equally, the N-E fault can defeat the RCD's own test button, because that might simply send some current from the L-in terminal to the N-out terminal, only for it then to disappear down into the CPC via the fault without returning through the RCD to create the imbalance.

But this only happens on circuits protected by the RCD in question. An N-E fault on a circuit protected by a different RCD won't affect the first one. That is logical: N & E are always connected together upstream of the RCD anyhow, whether at the origin (TN-C-S) or the substation (TN-S) so it doesn't matter if they are also connected at an upstream fault.

What can happen that 'crosses over' from circuits on one RCD to another, is the effect of a heavy load current showing up an N-E fault and causing a trip. In that case, it doesn't matter where the load is, whether on the RCD that trips or another. Anything that creates voltage drop in the supply neutral can raise the N-E voltage and drive current through the fault. The fault will however always be on the one that trips.

 

[Roadhousezamora]

As a quick update, the RCD on the non fault side was indeed faulty itself, the RCD in its place is functioning. It would seem as stated above there was no link between the neutral to earth Fault on the other side with this faulty RCD.