testing for a borrow neutral

[DirtyDog368]

hello all,

i just installed a new cu, which some of you may have read in another one fo my threads. everything was fine until i tried to connect one of the ligthing circuits to the board but it made the RCD trip every time.

i did some reading and suspected the ole borrowed neutral dilema.

Now i just want to run something by you so bare with me.

Imagine the new cu is installed
all circuits connected (except light circuit) and working fine
connect earth to earth bar of light circuit and all ok
connect nuetral to neutral bar and pop goes one of the RCDs
connect nuetral to the other neuitral bar and pop goes the other RCD
connect L1 to mcb and pop goes RCD
connect L1 to another mcb on other RCD and that pops too.

does this sound like borrowed neutral.

I tried to connect linked circuits into one mcb and but its RCD still tripped.

So i am thinking perhaps this is not a borrowed earth but perhpas insulation reistance problem. However the mcb never trips so if there is leakage of current it is too small to trip mcb but big enough to trip rcd.

can any one suggest a test method to either prove or dicprove this.

thanks very much


DD368

 

[andyb]

Am I missing something or is testing for a shared neutral simple?

take a resistance reading from the relevant live and work your way down the neutrals and see which ones are connected. If there is more than one connection you have a shared neutral.

I think you may be sort of missing my point Peter.

What I'm saying is this. Yes i know that there are ways to test for borrowed neutrals, but it is not one of the tests i carry out.

If a customer wants a new consumer unit, then the test I carry out before removing their existing board is an insulation test to make sure that the new board can be fitted without rcd problems.

If ok then I fit the new board and carry all relevant tests. If a borrowed neutral shows up, then I deal with it.

Do you really undertake the test you describe before you change a board? If so, what do you do when you find a borrowed neutral?

 

[telectrix]

it's always best to find any borrowed neutrals before changing a board. that way you can allow for the rectification in your price and no shocks to the customer when you tell him " sorry, mate, just found a problem. it's going to cost an extra £150 to sort it"

 

[Big n Daft]

"Do you really undertake the test you describe before you change a board? If so, what do you do when you find a borrowed neutral?"

Er yes, then inform the customer of the extra work required, if they wont pay just be naughty and shove both lights on the same rcd.

 

[GLENNSPARK]

I think you may be sort of missing my point Peter.

What I'm saying is this. Yes i know that there are ways to test for borrowed neutrals, but it is not one of the tests i carry out.

If a customer wants a new consumer unit, then the test I carry out before removing their existing board is an insulation test to make sure that the new board can be fitted without rcd problems.

If ok then I fit the new board and carry all relevant tests. If a borrowed neutral shows up, then I deal with it.

Do you really undertake the test you describe before you change a board? If so, what do you do when you find a borrowed neutral?

Ok Andy. yep it is easier to test for a borrowed neutral on a split load board........using the test i refered to earlier.....breakers on and RCDs off , Incomer off (of course) start on 1 side and just drop each breaker off in turn until it clears. Make a note of it and move on. Do the same on the other side until it clears again and you have the 2 offending circuits (nearly always landing lights)........

 

[Inteificio]

What he said ;-)

 

[DirtyDog368]

hello guys, guy who started thread here.

this has all been very educational and this thread has helped me to appreciate testing and how to translate results into a practical sense of how circuits could be interconnected. thanks to all your suggestions and helpful tips. seems the megger 1553 are destined to be very good chums from now on.

thanks

dd368

 

[pushrod]

Is it me or is this all simle stuff that can be found using a test meter? The guy who started this thread needs to think if he's in the right game, trial and error is not good for fault finding.
Stop think and do it to a logical system.

yes, then inform the customer of the extra work required, if they wont pay just be naughty and shove both lights on the same rcd.

-A touch of irony there