Split Consumer unit tripping on lighting when switch 2 way light on

[rfman]

I have just replaced a CU with split load and now the 2 way hall light trips the opposite RCD, existing PVC wiring. Bulbs removed.

I suspect a Line-Line insulation problem.

La-E > 299 M
Lb-E > 299 M
Na-E 17 M
Nb-E 99 M

Hall circuit is on RCDa, other lights on RCDb

Na - Nb > 299 M

La - Lb > 99K Ohm meter measurement, but ~ 30 k on IR test.

Ideas on how to find the weak spot?

Thanks

 

[Hawk81]

I would say its a borrowed neutral

 

[telectrix]

shared neutral on 2 way lighting. put both lighting circuits on the same RCD. if that cures the tripping, you can either leave as is, or find the shared neutral and rectify. usually involves running a 3 core between the switches instead of a T/E

 

[rfman]

Thanks

Would have expected to measure a fixed constant leakage between the 2 Line conductors for a borrowed neutral.

La --------
|
bulb
|
Na ---------
|
bulb 2
|
Lb ----------

Not one that appears to be open circuit continuity test, but fails on IR test. Or am I missing something?


But will take a look. Thanks

 

[Graeme Harrold]

Should have done some basic tests BEFORE changing the CU. IR test on the neutrals of the two circuits should have shown if they joined.....

 

[rfman]

:smile5:

Thanks to everyone for their input. :icon14:

I went back to the circuit & inserted a standard filament bulb in place of the Low energy bulbs and all made sense on the continuity testing. Constant resistance reading between La-Nb. Looks like when the low energy bulbs were in the continuity meter did not pickup the borrowed neutral.

When I had removed the bulb from the Hall circuit there was a only a very high mega reading.

Original testing of wiring was done before I split the lights followed by quick continuity & IR testing.

 

[ezzzekiel]

you would need the loads in and switched on to identify with test meter, or look for a common link at two g switch, or single core commons going to pendants up and down.

you require a neutral upstairs for downstairs circuit.

 

[telectrix]

usually the problem is due to the landing light having it's neutral from the upstairs lighting and the line from the downstairs, often fed from the hall switch.

 

[ezzzekiel]

which is also a very usual setup and one which should be easily identified and rectified for anyone carrying out consumer unit changes

 

[telectrix]

true, but the first time you come across it, it can be confusing. anyone who has come into the trade recently has probably not seen it .

 

[ezzzekiel]

valid point, so expect to see a lot more of it.
placing both lighting circuits onto a single mcb will resolve it although not ideal it is an accepted method, but with people adding spots and chandaliers in everyroom with extractor fans and just about anything else you can think of added to the lighting circuit it is not always an option to employ this method.

 

[telectrix]

good argument for going back to gas lighting.

 

[ezzzekiel]

not with our dwindling gas reserves - candle maybe