Discuss Bizarre dead short between Neutral and Earth in the Electrical Forum area at ElectriciansForums.net

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1shortcircuit

Firstly, please let me explain the fault on the circuit HAS been rectified. This was achieved by splitting the offending circuit (kitchen ring) into two radials and putting each leg on their own 16A mcb on the same RCD.

Attended a property to do some remedial works on the kitchen ring, New dual RCD protected board already installed and just there to put rubber grommets into back boxes. Such a joy:tongue3:

Anyway as I always test circuits before working on them I found a dead short between Neutral and Earth but RCD holds?

RCD tests came back well within specified tripping times on x1 and x5 tests but RCD failed to recognise this dead short?

Anyone have any suggestions as to why this RCD will test fine but not do it's job as intended?

All testing was done with circuit completely removed from Consumer Unit. Faulty leg found between two sockets so removed and as said made ring into two radials, each on their own mcb (same RCD). Comment sticker left on consumer unit explaining circuits 2 & 3 both energise kicthen.

Kind Regards

1SC
 

DPG

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Definately zero ohms, ie. on a low ohms range?? Daz
 
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1shortcircuit

So the resistance could be as high as 9999 ohm's?
Erm, quite possibly?

So whilst the IR reading was low enough to require further investigation and rectification there may have been enough resistance between Neutral and Earth to not trip the RCD.
 

DPG

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I'd measure on a low ohms range and get a figure in ohms rather than megohms. Daz
 

oldtimer

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Also take a link of cable and short neutral and earth at the CU just to confirm then say the cooker circuit is on the same RCD then get your r1 r2 test plug at at the cooker outlet short out N-E
 
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1shortcircuit

and then look for the nail.
Well it seems the kitchen cabinets MAY have been screwed to the wall and from what I can tell the cabling has not followed safe zones so it could have been a screw I guess??? Could be here all day guessing which is why I just cut out the faulty link and created two radials

Also take a link of cable and short neutral and earth at the CU just to confirm then say the cooker circuit is on the same RCD then get your r1 r2 test plug at at the cooker outlet short out N-E
I was wondering this myself, Just to prove that in the event there is a fault the RCD does actually do what it's supposed to do.

I did contact Elecsa, who I must add have been excellent on the two occasions that I have needed to contact them and even they said it doesn't make sense.

Thanks for everyone's input:thumbsup
 

nickblake

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Ah ha i bet its a nail like tel has said had one simular all readings were ok RCD tripping time hunky dory , with the circuits operating all fine then RCD would trip for no reason drove me mad trying to find it new RCD's etc as you do tested circuit all clear then the RCD would trip , located the fault to a capping nail that was straight through the cable it was intermittantly shorting and tripping the RCD pig to find so it could well be an intermittant short
 
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1shortcircuit

This circuit has not created any nuisance tripping since the board has been replaced and the property has been in full use :thumbsup
 

oldtimer

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The thing is what lessons should be taken from this well yep you have a low reading but at the same time it costs nothing to do some other tests as well even if you just jott them down a bit like a British Gas guy who tests a socket and says big problem here you have no earth but if he tested some other sockets ie random he may have said this socket has no earth but the others are ok
 

vernam616

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try it on another RCD and see if it holds

RCD may be faulty, maybe you got decent trip times but doesnt mean it cannot be faulty
 
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1shortcircuit

RCD may be faulty, maybe you got decent trip times but doesnt mean it cannot be faulty
Kind of defeats the object of carrying out the correct tests in sequence though wouldn't you agree?
 

vernam616

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Yes i suppose it does im not disagreeing with you, however that would be my first try, lets face it what could cause the RCD NOT to trip? when we all know it should be
 

Brooks-Electric

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I would suggest neutral on the faulty leg is interconnected with a neutral of another circuit (likely to be cooker). I am assuming you only removed the circuit in question to carryout the testing. Therefore leakage back through other neutral to earth at transformer. I would suggest removing faulty neutral and carrying out test between that and the neutrals of the other circuits on that RCD.
 
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1shortcircuit

I would suggest neutral on the faulty leg is interconnected with a neutral of another circuit (likely to be cooker). I am assuming you only removed the circuit in question to carryout the testing. Therefore leakage back through other neutral to earth at transformer. I would suggest removing faulty neutral and carrying out test between that and the neutrals of the other circuits on that RCD.
I've tested for borrowed Neutrals between other circuits as this is what I first suspected. Also, whilst the circuit was removed I tested for continuity of both Neutral legs to the main earth bar of consumer unit and nothing :thumbsup

Does it the circuit trip under load?
Nope, not a single trip and circuit has been under normal daily usage

Thanks for all your help everyone, much appreciated as always.
 

Des 56

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Erm, quite possibly?

So whilst the IR reading was low enough to require further investigation and rectification there may have been enough resistance between Neutral and Earth to not trip the RCD.
The thread title is "bizarre dead short"we will have to call you 2shortcircuit from now on :smile:
Your reading has only confirmed that you dont have a million ohms,add a bit of resistance of the cpc back to the met and its not that bizarre I would say for the rcd to hang on in there
 

Jimmy Boy

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That will be fine just write down for 'Design ' Electrical Trainee and Signed of by NICEIC supervisor ..Of course I am being flippant..? ' Tony Cable'..my autobiography I woke after 5 weeks from a coma...To find myself as an Electrician lol
 
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1shortcircuit

The thread title is "bizarre dead short"we will have to call you 2shortcircuit from now on :smile:
Your reading has only confirmed that you dont have a million ohms,add a bit of resistance of the cpc back to the met and its not that bizarre I would say for the rcd to hang on in there
It would seem you have hit the nail on the head.

The earth leakage is obviously working within the tolerance of the 30mA RCD therefore not tripping it however at a later date this could become worse and lead to nuisance tripping or the amount of leakage combined with that of other circuits could eventually step out of the tolerance level again causing nuisance tripping.

At least my actions of finding and rectifying the fault were correct and should eliminate any tripping of the device.

**** note to myself, Learn more, test more and understand more ****


Thanks again to everyone that has added their input, great forum.... I continue to learn more everytime I log on:thumbsup
 
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