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Topic Title: Tripping RCD after new CU
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Created On: 31 October 2013 07:02 PM
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 31 October 2013 07:02 PM
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Grayston

Posts: 19
Joined: 12 December 2012

after fitting a new CU the rcd trips on the sockets, 70's house so all sockets on one circuit.
I have tested all sockets with readings all similar, one had loose earth which gave a higher reading until re-tightened.
spent along time trying to work it out, but then noticed that the litchen and dining room lights did not work when the socket MCB was off! these are in an extension so thinking that someone has wired them from a socket.
I guessing this would trip the RCD, especially if they have taken a live from the socket, but neutral to the lighting?
i've taken a couple of sockets off to see if theres any tell tail signs of spurring, but nothing so far. Really dont want to start taking floor boards up if theres another way.
anyone got an ideas or have come across this before?
Cheers
G
 31 October 2013 07:06 PM
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24Hour

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Have you tested the circuit prior to restoreing supply , or has the bang test been put into use.
Neutral earth fault, most likely or wrong neutrals in dd neutral blocks

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Yes i do do 24/7 everyday of the FLAMIN year.
 31 October 2013 07:20 PM
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mikejumper

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Originally posted by: Grayston

.........these are in an extension so thinking that someone has wired them from a socket........

........i've taken a couple of sockets off to see if theres any tell tail signs of spurring, but nothing so far.......
G

Could be JB'd under the floor.
Any sign of a FCU anywhere.
 31 October 2013 07:31 PM
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Grayston

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neutrals on the board are fine, sockets tested fine, rcd tested, but strange that it didnt trip at 1/2 times, didnt trip at 1 times either, did trip at 5 times at 33ms.
swapped the rcd to double check and still faults, moved both lighting circuits and sockets to same side of board to see if that would help, but no. thinking the neutrals were all together and may work.
the rcd will hold when the socket circuit is switched on, you can put electric air fresheners on too, but anything bigger, TV, Fridge will trip the RCD

only thing i can think of is to find out where the lighting connects to the sockets and disconnect, unless there any other options?
 31 October 2013 07:41 PM
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AdrianWint

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Originally posted by: Grayston

the rcd will hold when the socket circuit is switched on, you can put electric air fresheners on too, but anything bigger, TV, Fridge will trip the RCD


This is a classic symptom of a neutral-earth fault ..... the RCD holds until anything other than a very small load is switched on. You really need to do an insulation test N-E but first you must disconnect the ring circuit completely ( L, N & E) from the board otherwise you will get misleading results!
 31 October 2013 08:09 PM
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Grayston

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i do think your right, i'll do that test again tomorrow, pretty sure i did disconnect the ring, but after lots of hours working it gets abit fuzzy.
i guess then it back to hunting down the fault,
 31 October 2013 09:07 PM
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aligarjon

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What insulation resistance readings have you got written down on your certificate, that might give you somewhere to start looking ?

try testing your rcd's with no loads connected to make sure they are operating correctly

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
 01 November 2013 08:22 AM
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Grayston

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will check today, but good idea to check rcd's with no load, i am puzzled to why one trips when switching say the tv on, but doesnt trip when testing??
 01 November 2013 05:21 PM
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budha

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had something similar recently after a cu change, turned out to be faulty power shower pump which had been spurred off a spur on the upstairs sockets.

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budha,
nice talking with you.
 01 November 2013 05:21 PM
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budha

Posts: 158
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had something similar recently after a cu change, turned out to be faulty power shower pump which had been spurred off a spur on the upstairs sockets.

-------------------------
budha,
nice talking with you.
 01 November 2013 05:21 PM
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budha

Posts: 158
Joined: 09 June 2008

had something similar recently after a cu change, turned out to be faulty power shower pump which had been spurred off a spur on the upstairs sockets.

-------------------------
budha,
nice talking with you.
 01 November 2013 05:21 PM
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budha

Posts: 158
Joined: 09 June 2008

had something similar recently after a cu change, turned out to be faulty power shower pump which had been spurred off a spur on the upstairs sockets.

-------------------------
budha,
nice talking with you.
 01 November 2013 06:21 PM
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OMS

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Originally posted by: Grayston

will check today, but good idea to check rcd's with no load, i am puzzled to why one trips when switching say the tv on, but doesnt trip when testing??


Think about it - as you increase load then you increase voltage drop on the line conductors - which then presents as a consequential voltage rise on the neutral conductors.

With a N-E fault, some of the current driven by that voltage rise is flowing to earth via the fault, some via the RCD - as you increase the load, whilst the proportions stay the same, more current flows in each path as the votage between N + E increases until you get to the RCD tripping threshold and out it goes.

The clues are all there - particularly the very sluggish operation of that RCD at 5 x I delta n - which is a fail incidentally.

Keep in mind, a fault in "your" house and high loads next door will exhibit the same effect !!

Regards

OMS

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Failure is always an option
 07 November 2013 04:56 PM
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Grayston

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got to the fault in the end, the kitchen extension had a lighting circuit spurred from it and then a socket spurred which was below the work surface, behind the cupboard backing. Could be seen through 2 holes that had been cut to plug in believe it or not an extension cable to power the fridge and freezer!
tested and found a neutral to earth fault. thought it would have been a screw through the cable, probably when the work surface was fitted, but when i eventually got to the socket i found the neutral wired into the earth and earth wired into the neutral!!
 07 November 2013 07:22 PM
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rutts

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Out of idle curiosity didn't you get a low insulation reading in your tests before you energised the circuit?
 07 November 2013 09:07 PM
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leckie

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Pretty obvious that no IR tests were carried out. This is why I keep banging on about testing before you swap the CU.
 07 November 2013 09:16 PM
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rutts

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Yeah. It was a rhetorical question.
 08 November 2013 11:33 AM
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Grayston

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yes, but also found that my fluke was no reading correctly, tried another meter and it appeared.
but no didn't do IR before renewing the CU, will do next time though
 09 November 2013 08:30 AM
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leckie

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Have a read through BPG6 on this link.
http://www.esc.org.uk/industry.../best-practice-guides/
 09 November 2013 06:35 PM
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frspikeyhead

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Just out of interest the crossed connections surely wouldn't show up on an IR test if the socket in question was a plastic back box and the appliance un plugged.
IET » Wiring and the regulations » Tripping RCD after new CU

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