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

 

[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.