Domestic favour, tripping RCD and redundant circuits

[GBDamo]

I have been asked to look into a random tripping RCD.

The installation is quite simple, now only only four circuits,

None RCD protected side has only the lights and a redundant 16A MCB 2.5mm radial for which no accessory could be found, possibly disconnected outside lights but the circuit was still live, this has been disconnected and put in connectors in the CU.

RCD side has a 16A MCB 2.5 radial with only one double socket outlet, a 32A MCB ring doing the majority of the sockets (possibly with a ring off the ring but that's another story), a 20A 2.5mm radial doing living room sockets and finally a 40A MCB 6mm redundant circuit serving a removed shower.

All IRs pass although the 20A radial and 32A ring both had low readings at 2-3MOhms. All other tests pass comfortably.

However the 40A ex-shower circuit still had the Neutral and CPC terminated in the CU, the other end could not be located, probably cut off and plastered in the bathroom wall.

I have disconnected the the Neutral and CPC for the ex-shower and made them safe in terminal blocks in the CU.

After 4 days there have been no more random tripping whilst I've tidied up some loose ends I'm concerned that I haven't really found the fault.

Could a Neutral and CPC buried in a damp wall like this cause an RCD to trip?

Any thoughts welcomed.

 

[Andy-1960]

Yes

 

[SparkyChick]

What he said ^^^^

 

[Wilko]

Yep, a good chance the old shower circuit could cause this. Perhaps continuity / ir test that cable and see?

 

[Octopus]

But he said he's disconnected the shower ...

I would recommend the customer keep a record of when it trips.

 

[SparkyChick]

But he said he's disconnected the shower ...

I would recommend the customer keep a record of when it trips.

I read it as the shower was connected when it was tripping and in the process of investigating the tripping, he disconnected the old shower cable.

 

[Octopus]

I read it as the shower was connected when it was tripping and in the process of investigating the tripping, he disconnected the old shower cable.

I think you are correct ...

 

[GBDamo]

I think you are correct ...

To clarify the electric shower was replaced by a bar mixer before my friend bought the house but the circuit for the shower is still in place. That is it runs from the CU to a location unknown, assumed to be cut off and plastered into the bathroom wall.

The live had been disconnected in the CU but the Neutral and CPC had not.

This tripping has been on and off for the three years they have had the house.

 

[Octopus]

Good response.

Yes leaving the neutral and earth in a wall is never a good idea.

I would leave the cpc connected to the cu

 

[GBDamo]

But he said he's disconnected the shower ...

I would recommend the customer keep a record of when it trips.

Been done Mordoch, it's totally random no correlation with anything or any appliance. Has even gone overnight with nothing on, literally nothing.

 

[Octopus]

^^ sometimes these things happen.

I had been investigating random tripping in a very big 3 story house a couple of years ago. Dual RCD board

In the end I recommended the customer changed to a rcbo board ....
Which was implemented.

A few months later the 3rd story socket circuit started to trip.... Eventually I found a cable disappearing down a wall behind the shower room below. I disconnected the spur and it's never tripped again.

Very trying to deal with.

 

[Risteard]

You could connect all cores of the former shower cable to the Earth bar in the DB.

 

[GBDamo]

Its nice to find a screw through a cable or some such that you can point to and say "that's it" but it's not always the case.

Just don't want to be getting a free breakfast under false pretenses.

 

[GBDamo]

You could connect all cores of the former shower cable to the Earth bar in the DB.

Just to ensure it it doesn't become and remain live due to coming into contact with another live conductor?

bed time now, thanks for the assistance all.

 

[Risteard]

Just to ensure it it doesn't become and remain live due to coming into contact with another live conductor?

bed time now, thanks for the assistance all.

Well, perhaps, although the more likely scenario is someone reconnecting it in the DB. Hopefully if all cores are earthed they may stop and think. (No guarantee there though, I'm afraid. Stupid never fails to surprise...)

 

[Pete999]

What he said ^^^^

What the both said^^^^^^^^^^^^^^^^^^^^^^^

 

[marconi]

I think the state the shower cable had been left in - particularly at its far end - could allow N-cpc or N to 'earth/terra-firma' currents to flow depending on the potential of the N with respect to cpc or earth/t-f.

There might also be some interaction between the redundant shower cable and the lower IR reading socket circuits through the fabric of the building especially if dampness is present.

As Murdoch might have said - 'did you clamp and measure earth leakage currents before and after the changes you made'? Or ramp test the RCD before and after?

I'd also take gander and do some IR measurements on those appliances which are plugged in which could suffer earth leakage even when switched off by the functional controls eg: kettles, ovens, immersion heaters, central heating, ...or in combination accumulate a high earth leakage current.

 

[marconi]

I think the state the shower cable had been left in - particularly at its far end - could allow N-cpc or N to 'earth/terra-firma' currents to flow depending on the potential of the N with respect to cpc or earth/t-f.

There might also be some interaction between the redundant shower cable and the lower IR reading socket circuits through the fabric of the building especially if dampness is present.

As Murdoch might have said - 'did you clamp and measure earth leakage currents before and after the changes you made'? Or ramp test the RCD before and after?

I'd also take gander and do some IR measurements on those appliances which are plugged in which could suffer earth leakage even when switched off by the functional controls eg: kettles, ovens, immersion heaters, central heating, ...or in combination accumulate a high earth leakage current.

I'd want a bit more investigation done on the low IR socket circuits. What voltage did you do the IR tests?

 

[Pete999]

What he said ^^^^

What the both said^^^^^^^^^^^^^^^^^^^^^^^

 

[The Ghost]

As @marconi stated vis-a-vis the earth neutral thing, how many times have we switched off a circuit and shorted N-E and the trip trips? As the N + E were connected still to the respective bus bars...Trip! with a bit of moisture involved.