Still unsure about testing for borrowed neutrals.

[electek]

Hi , I know this has been covered before but i'm still unsure and concerned about nuisance RCD tripping after replacing a consumer unit if there are borrowed neutrals , especially on the lighting circuits.

Last time i posted regarding this I was left confused as there were many different ideas and suggestions about how to test for this but no definite 'this is what you do'.

Can anyone explain to me exactly what to test for and how to do it to prevent problems after fitting a new CU.

Any help with this is much appreciated.

Chris.

 

[Spudnik]

Continuity and/or IR test between neutral conductors.

If no reading on continuity or a infinate reading on IR then they is no connection between the two.

 

[oldtimer]

OK first things first if you give the old CU a Ze-PFC-IR tests and everything is good to go count the lives and the neutrals at the board and it should be the same ie 12 reds 11 blacks pay attention to the cable sizes ie 10,6,4,2.5mm should be paired up but you may find 2 x 1,1.5mm reds and 1 x 1,1.5mm blacks

 

[baldsparkies]

On domestic, its about checking for continuity between neutrals on radial fed circuits. Continuity would point towards borrowed neutrals. But remember to have the loads switched on ie lighting circuits bulbs in switches on, if you get my drift. Remember ring mains will have continuity anyway, at least they should. Most problems are with the landing light on the 2 way. The quick answer is to put both circuits on the same rcd but seperate breakers many do this.

Officially rewire the landing light with the downstairs circuit.

 

[electek]

But what if the lighting circuit is wired in the three plate method from what i can workout you would have to make sure all the switches were on and all the lamps were good otherwise a blown lamp or open switch could indicate that everything was ok but still you could have a borrowed neutral - does that make sense ?

Thanks

Chris

 

[electek]

Baldsparkie , you got in there just before me as i was typing !

Thanks for your help , i understand what you say about switches on , bulbs in , that in fact was my question so you've already answered it.

Two other questions - Is it ok to insulation resistance test with the switches closed and lamps in ? Also what if you have lighting transformers e.t.c. surely they'll give false results.

Also you say put both circuits on same RCD but different breaker but I thought that was not allowed because if RCD trips you lose all lights ?

Thanks again for your help

Chris

 

[slocm3105]

On domestic, its about checking for continuity between neutrals on radial fed circuits. Continuity would point towards borrowed neutrals. But remember to have the loads switched on ie lighting circuits bulbs in switches on, if you get my drift. Remember ring mains will have continuity anyway, at least they should. Most problems are with the landing light on the 2 way. The quick answer is to put both circuits on the same rcd but seperate breakers many do this.

Officially rewire the landing light with the downstairs circuit.

I'm sure its not just me who would mention this, but shouldn't you put both lighting circuits into the SAME MCB if they have a borrowed neutral?!?!

 

[baldsparkies]

I'm sure its not just me who would mention this, but shouldn't you put both lighting circuits into the SAME MCB if they have a borrowed neutral?!?!

Ask yourself this A wylex rewirable board has 2 5amp circuits. They have been in service for 25 years and despite having a borrowed neutral for the landing light, the occupier has never had a problem. By all means you can go for one breaker. But if your going down the good electrical practice route, then its a landing light re wire.

 

[baldsparkies]

Baldsparkie , you got in there just before me as i was typing !

Thanks for your help , i understand what you say about switches on , bulbs in , that in fact was my question so you've already answered it.

Two other questions - Is it ok to insulation resistance test with the switches closed and lamps in ? Also what if you have lighting transformers e.t.c. surely they'll give false results.

Also you say put both circuits on same RCD but different breaker but I thought that was not allowed because if RCD trips you lose all lights ?

Thanks again for your help

Chris

Is it ok to insulation resistance test with the switches closed and lamps in ? Not really you answered your own question with the bit about trannies. Any resistive load between lives and neutrals will mess your insulation readings.

I thought that was not allowed because if RCD trips you lose all lights ? True, but if your downstairs and the upstairs lights are still on they wont help much anyway.
In the real world its more usefull to keep power circuits on different rcds.

Hope the above helps Chris

 

[Risteard]

Ask yourself this A wylex rewirable board has 2 5amp circuits. They have been in service for 25 years and despite having a borrowed neutral for the landing light, the occupier has never had a problem.

If there is a borrowed neutral then supplying this through two separate MCBs renders those circuits DANGEROUS. They must be one circuit.

 

[baldsparkies]

If there is a borrowed neutral then supplying this through two separate MCBs renders those circuits DANGEROUS. They must be one circuit.

So give a situation that would render this dangerous to the house owner.
Are we saying the original circuits have been dangerous since completion of the install 25 to 30 years ago and whilst connected to the original c/u. Thats a long time and a lot of use, there are a lot of very lucky home owners, if as you say its dangerous. (I know where your coming from but think about it)

 

[davelerave]

there was a quesion recently

i think testing at mcbs should be easier

 

[m25maintenance]

ir test with line neutral joined, and earth

 

[davelerave]

the light/bulb that's borrowing a neutral will need to be in- circuit when testing afaik

 

[mark80]

@baldsparkies. The situation is dangerous when say you change an upstairs light for the customer in the bedroom for example. You turn off the upstairs MCB and then test for voltage. As youd expect thered be no voltage and you then begin to carry out the work. Once you have disconnected the core and seperated them, someone accidently turns on the 2 way switch for the landing light. One of the neutrals in your separated cables is now live and ready for you to touch =) that is why it is dangerous.
Live supply is linked from downstairs light and then travels to upstairs light, obviously if the switch is in the on position this will send a live supply through the lamp, down the neutral which you may have disconnected making it live.