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.

 

[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.

 

[GBDamo]

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?

Thanks for the response,

I have ramp tested the RCD before and after results were mid 20's mA (Can't remember exactly) and also clamp tested the main earth which didn't show any leakage however at that exact time the RCD was not tripping.

Did the IR first at 250 and as it was low did it again with everything unplugged at 500 where I got the 2-3 MOhms readings.

 

[Octopus]

2 - 3 Meg ohms is perfectly acceptable

 

[GBDamo]

2 - 3 Meg ohms is perfectly acceptable

I Know but it concerns me when one circuit is 200+MOhms and another is 2-3, I like to know why.

 

[GBDamo]

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.

No, Never have I lopped a flex off and plunged the house into darkness, sending an irate customer into a tirade of expletives as they had lost three hours of work on their PC.

My polite suggestion they use autosave did not improve matters.

Every days an education.

 

[marconi]

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

and Murdoch's #9.

I know that redundant cables are properly sealed and earthed using kits such as these when they are in the industrial, power distribution and ship setting - see:

3M Cold Shrink Cable Abandonment Kits - https://www.cablejoints.co.uk/sub-product-details/3m-hazardous-area-cable-joint-kits/cable-abandonment-3m-cold-shrink-cscak

I don't know what the rules and regulations say about the redundant cables in the domestic/office setting. I think it odd though to not completely disconnect, isolate and label up such cables (including cpc and armouring) in the domestic/office setting (eg the shower cable in this post) so that such cables are safely made no longer part of the electrical wiring and thereby cannot play any further part in the flow of any currents in that installation.

 

[Octopus]

^^ label up unused circuits ..... It's bad enough that new fuseboards don't get labeled buy the idle!

 

[sparksfly]

Customer called nuisance tripping every night approx 10:30 ish this week in a very large house and outbuildings.
Ramp tests fine. Had a low IR 0.23 meg (250v) reading with everything plugged in, tracing through 4x DB's to get to the main N-E cause which served 2x outside lights in a remote barn. Disconnected them and investigated another DB which the RCD had also tripped once for the underfloor heating, IR fine but MCBs were very loose on busbar. At 1.22M ohms (@250V) on the main DB (NOT unplugging anything) I left happy things were much better but played the cautious card. Customer called to say the last 2 nights were fine. Sometimes you simply cannot reasonably isolate all electrical items connected that give you a very low reading to find the faults as quick as possible. 3 hours later and god knows how many steps including a bit of jogging keeps one fit along with the mental exercise !!