shared neutral

[beanzntoast]

Hi all,Maybe a simple 1 for those experienced in inspection & testing,before changing a consumer unit when a PIR is done to check for any faults first,which test would show a shared neutral ie the usual 1 when it was common practice of sharing a neutral on the upstairs & downstairs lights,I've got a vague idea of I & T but i cant think which test would show this,apart from when the new consumer unit is fitted & the RCD trips.Sorry if i've missed something simple just guide me in the right direction somebody please


jason

 

[saveloy]

On lighting I would turn the landing light on. Then isolate the lighting circuits that you suspect are sharing the neutral. Then take the Line from downstairs circuit and and test for continuity with the neutral from the upstairs circuit. If you have continuity then you know that the landing light is sharing the upstairs neutral.

 

[beanzntoast]

I appreciate your answer & see exactly what your saying saveloy but how would you suspect it has a shared neutral if its not your house.I thought it would show up on 1 of the PIR tests (not sure which 1)so you could tell the customer of the problems before the cu change,if not then when you fit the consumer unit the customers just going to say there wasn't any problems before an RCD was fitted.am i correct or missing something.

 

[ryanryan]

Electrical seperation.
before connecting the TandE ends for the lighting circuit,a simple IR test between Ps and Ns would highlight this fault!

 

[saveloy]

I appreciate your answer & see exactly what your saying saveloy but how would you suspect it has a shared neutral if its not your house.I thought it would show up on 1 of the PIR tests (not sure which 1)so you could tell the customer of the problems before the cu change,if not then when you fit the consumer unit the customers just going to say there wasn't any problems before an RCD was fitted.am i correct or missing something.

Unfortunatley thats the only way I know of proving it. Maybe someone else has another way. I personally would just suspect it anyway and inform the customer that it may be the case and so they are forewarned.

If the customer accepts the qoute and wants you to do the work, then I would do all the relavent tests before I'd even consider starting the board change. That way if there are any faults that will be chargeable you can inform the customer. It covers you against them blaming you for faults that were already there!

 

[wigan]

You could check continuity between neutrals whilst doing other checks. Test with switch operated incase polarity wrong too, should all be open except ring. I have known neutrals from ring to be connected with lights before now.

Kev

 

[beanzntoast]

Electrical seperation.
before connecting the TandE ends for the lighting circuit,a simple IR test between Ps and Ns would highlight this fault!

How would an insulation resistance show this up because if the insulation between line & neutral was ok the readings would come out fine,am i missing something here

 

[ryanryan]

yup sorry..
IR the two Ns for both Ns of the lighting...
not the P to Ns of each circuit!

 

[saveloy]

yup sorry..
IR the two Ns for both Ns of the lighting...
not the P to Ns of each circuit!

Sorry maybe I'm missing something here. The neutrals of the 2 lighting circuits will only be connected at the neutral bar in the board. the upstairs landing light has borrowed a neutral from upstairs. There is no link from the upstairs circuit to downstairs. (Other than at the board). Surely an IR test would indicate an open circuit if performed at the board.

 

[beanzntoast]

Sorry maybe I'm missing something here. The neutrals of the 2 lighting circuits will only be connected at the neutral bar in the board. the upstairs landing light has borrowed a neutral from upstairs. There is no link from the upstairs circuit to downstairs. (Other than at the board). Surely an IR test would indicate an open circuit if performed at the board.

It would show if the neutrals were taken at the ceiling rose or a junction box(upstairs to downstairs or vice versa)

 

[saveloy]

It would show if the neutrals where connected at the ceiling rose (upstairs to downstairs or vice versa)

This is my understanding, so please bear with me if I have it wrong.
Why would there be neutrals from 2 seperate circuits connected at a ceiling rose.As I understand it the situation of a shared neutral arises because power from the downstairs circuit is used to supply the upstairs landing light (via 2 way switching), and the neutral is borrowed/ shared from the upstairs circuit or vice vesa. There is no physical link of neutral conductors. Surely an IR test between neutrals would show an open circuit.

 

[ryanryan]

Sorry maybe I'm missing something here. The neutrals of the 2 lighting circuits will only be connected at the neutral bar in the board. the upstairs landing light has borrowed a neutral from upstairs. There is no link from the upstairs circuit to downstairs. (Other than at the board). Surely an IR test would indicate an open circuit if performed at the board.

i thought the question was a shared N in the wiring not the CU..
so disonnecting the Ns from the CU and either IR or Continuity test (on the disconnected ends) would highlight a N shared through the lighting circuitS.

 

[beanzntoast]

Could be your both right,2 different ways of pinching a neutral

 

[saveloy]

i thought the question was a shared N in the wiring not the CU..
so disonnecting the Ns from the CU and either IR or Continuity test (on the disconnected ends) would highlight a N shared through the lighting circuitS.

How? Where are the neutrals connected together? The only link there is, will be through the filament of the bulb between the Line of one circuit and the Neutral of the other. An IR test between the two neutrals will not indicate a short between the two neutrals. It will show an open circuit - which is what it would show if there was no shared neutral.

 

[electek]

Any more ideas on this , from what i can work out you would need to close all switches and ensure all bulb filaments are good then if you have low resistnce reading between disonnected lighting neutrals then you have a shared neutral...... I think.

 

[pushrod]

Hi all,Maybe a simple 1 for those experienced in inspection & testing,before changing a consumer unit when a PIR is done to check for any faults first,which test would show a shared neutral ie the usual 1 when it was common practice of sharing a neutral on the upstairs & downstairs lights,I've got a vague idea of I & T but i cant think which test would show this,apart from when the new consumer unit is fitted & the RCD trips.Sorry if i've missed something simple just guide me in the right direction somebody please


jason[/QU

If you get a fairly sensitive clamp meter and put it over line and neutral of the individual light circuits at the CU it should read zero as the current in the line should balance the current in the neutral. If however there is a shared neutral you will get a reading.

A very sensitive clamp meter can also be used for detecting earth leakage - very useful for working out what is causing nuisance tripping of rcds.

 

[Smugley]

Got a wander lead? Could you not just connect it to the neutral at the light, then go back to the board and see which circuit it shows continuity with?
Or check continuity to an adjacent upstairs or downstairs light?

I don't believe any of the standard PIR tests will show this up.

Saveloy's suggestion was good too, and I wouldn't have thought of pushrod's clamp meter idea. Learn something new every day

 

[baldsparkies]

How? Where are the neutrals connected together? The only link there is, will be through the filament of the bulb between the Line of one circuit and the Neutral of the other. An IR test between the two neutrals will not indicate a short between the two neutrals. It will show an open circuit - which is what it would show if there was no shared neutral.

Lamps in landing light on and you will get continuity across the neutrals if borrowed. The load is the link between the two. So if you have dual rcd's upstairs lighting on one, downstairs on the other. You wont have a problem until you switch the landing light on. Then click out goes one or both rcd's. Putting both circuits onto one rcd along with the neutrals onto one neutral bar and the problem goes away. Many will argue its not good electrical practice, however the real world is a different place.

 

[saveloy]

Lamps in landing light on and you will get continuity across the neutrals if borrowed. The load is the link between the two. So if you have dual rcd's upstairs lighting on one, downstairs on the other. You wont have a problem until you switch the landing light on. Then click out goes one or both rcd's. Putting both circuits onto one rcd along with the neutrals onto one neutral bar and the problem goes away. Many will argue its not good electrical practice, however the real world is a different place.

Totally agree mate. You have to have a lamp in place with the switch closed when doing an IR test between neutrals at the board. I was trying to explain, that, if you just do an IR test between neutrals,at the board, without the lamp on, then all you will get is an open circuit (the same result you get when there are no borrowed neutrals). You need the lamp/load in to complete the circuit/short/continuity.As you quite rightly pointed out, the problem then only becomes apparent when you energise the circuits and one of the RCD's pops.

 

[WayneL]

A very simple way (if your cu circuits are identified correctly) - and you don't even have to get your meter out

Open MCB/Pull fuse for upstairs lighting, disconnect neutral from neutral bar for downstairs lighting - if landing light still works, you have a borrowed neutral.

 

[WayneL]

A very simple way (if your cu circuits are identified correctly) - and you don't even have to get your meter out

Open MCB/Pull fuse for upstairs lighting, disconnect neutral from neutral bar for downstairs lighting - if landing light still works, you have a borrowed neutral.

On second thoughts, don't do this at all

I'm very naughty suggesting that you disconnect the neutral of an energised circuit

Safely Isolate whole board

Disconnect neutral for downstairs lighting circuit and safely terminate into connector block.

Re-energise downstairs lighting circuit and close landing-light switch.

If landing light comes on, you have a borrowed neutral.

That's better

 

[baldsparkies]

On second thoughts, don't do this at all

I'm very naughty suggesting that you disconnect the neutral of an energised circuit

Safely Isolate whole board

Disconnect neutral for downstairs lighting circuit and safely terminate into connector block.

Re-energise downstairs lighting circuit and close landing-light switch.

If landing light comes on, you have a borrowed neutral.

That's better

Reminds me of the sparky who did a pir on death row. He failed the electric chair.
Said the bloody thing was a death trap.