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

 

[Knobhead]

Standard IR tests, have you done them?

 

[DirtyDog368]

yes but how would the readings differentiate between IR problem or borrowed neutral scenario?

 

[telectrix]

IR test each lighting circuit L/N, L/E, N/E, with all loads removed. bet you've got either a N/E reversal or a N/E short.

 

[andyb]

hello all,

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.

Think about what you are asking.
The mcb is rated 6 amps & the rcd is rated 30ma

yes but how would the readings differentiate between IR problem or borrowed neutral scenario?

An IR test will not highlight a borrowed neutral.
If the rcd is tripping because of earth leakage due to low IR, that would of shown up with your testing before energizing the circuit.

 

[Rockingit]

From experience, the first place I'd go looking is on the landing. 2way switch downstairs, probably a 2G along with another light thats on the downstairs ring, and the light on the landing is 2way switched but wired from the upstairs (or vice-versa). Easy way to test is to put both circuits on the same RCD and then go around the place a switch at a time.

 

[Jimmyb]

turn all lights on then IR test between the live for one light circuit and the neutral from the other, if poor IR result go turn the lights off and IR test again if the results are now good then it a borrowed neutral, switch each light on one by one and test inbetween untill the problem one is found. it will prob be the landing light having the live fed from downstairs switch

 

[Deleted member 9648]

Borrowed neutrals (or lives) can be identified by IR testing between circuits..(disconnected).....standard test proceedures ( continuity-polarity-IR) will identify any other problems before energising circuits.....swapping around energised circuits in the hope that trips wont trip is not the way to do it.

 

[ackbarthestar]

I can remember being shown that IR testing should be for both individual circuit and for separation of circuits, not only be between the P/N, PE/P and PE/N of each circuit but a basic IR/cont test between the neutrals of each lighting circuit and between the lives and the neutrals of each socket circuits. If there is a crossed neutral then that should pick up the problem.

 

[andyb]

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.

No it doesn't (assuming you moved the neutrals over also)

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

This proves it.

Have you carried out an insulation test on these circuits? and if so what was the result?

It seems that you are using trial & error to find the problem,

 

[GLENNSPARK]

Think about what you are asking.
The mcb is rated 6 amps & the rcd is rated 30ma



An IR test will not highlight a borrowed neutral.
If the rcd is tripping because of earth leakage due to low IR, that would of shown up with your testing before energizing the circuit.

Rubbish.....you can IR between the neutral bars at 250V with lamps in and switches on.....if it comes back flat then you have a borrowed neutral ...thats assuming all other IR tests are good of course.....

 

[Big n Daft]

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.

 

[andyb]

Rubbish.....you can IR between the neutral bars at 250V with lamps in and switches on.....if it comes back flat then you have a borrowed neutral ...thats assuming all other IR tests are good of course.....

It's not rubbish Glenn
When would you ever do what you say in a normal days work?

The test you are talking about would show the same result in a continuity test between neutrals as well. Which is my rubbish point. A borrowed neutral would not show up during normal insulation resistance tests.

 

[ezzzekiel]

to be honest apart from looking for link at downstairs switch dont usually bother checking for them, if it turns out to be there then a new cable is ran regardless so cant see the point.

 

[Inteificio]

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.