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Topic Title: Establising if a borrowd neutral exists between 2 light circuits
Topic Summary: How to Check / Test
Created On: 25 November 2011 08:49 PM
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 25 November 2011 08:49 PM
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12216

Posts: 45
Joined: 22 August 2010

good evening
I am going to visit a domectic installation tomorrow and need to establish if the lighting circuits share the same neutral.

Can anyone give us any advice on the most practical way of establising if it has.

does it always follow that this type of arrangment is always wired in pvc singles only?

Also would i prove it if i carried out a continuity test between the neutrals between the 2 light circuits.

Regards

Martin
 25 November 2011 08:55 PM
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daveparry1

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Switch all the lights on, switch the breakers off, disconnect the neutrals and then measure between them with your continuity tester, it's very fresh in my mind, i've done one today!

Dave.
 25 November 2011 08:59 PM
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tillie

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Dave , would this method work if there were compact fluorescent led or low energy lamps installed in the fittings ?.

Regards
 25 November 2011 09:02 PM
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alancapon

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Possibly not.

Regards,

Alan.
 25 November 2011 09:03 PM
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daveparry1

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Probably not Tillie although an insulation tester would probably give a reading of a few hundred K ohms

Dave.
 25 November 2011 09:07 PM
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12216

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Dave
many thanks for your reply, i will do what you said but can you tell me the purpose of having to close all switches (lights on) as these are in the line conductor and what effect it may have if the light switches were left open, as i am only testing between the 2 neutral conductors.

this question is no way intended to question you as i appreciate your advice, it is purely intended to assit me in achieving a comprehensive understading of the borrowed neutral scenario.

many thanks

Martin

Switch all the lights on, switch the breakers off, disconnect the neutrals and then measure between them with your continuity tester, it's very fresh in my mind, i've done one today!



Dave.
 25 November 2011 09:10 PM
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daveparry1

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When I come across the shared neutral problem on a c/unit change I usually put both lighting circuits on the same breaker, although there could be tripping problems if too many lights of course. (10 amp breaker then!) The one I done today was slightly different, there were two feeds on each circuit so I put the pair with the shared neutrals on one brealer and rcd and the other pair on the other side of the board,

Dave.
 25 November 2011 09:17 PM
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daveparry1

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Martin, the neutrals only become connected together through the load, when current flows through the lamp fillament into the neutral, that's why the rcd's only trip when the offending light is switched on,

Dave.
 25 November 2011 09:27 PM
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dickllewellyn

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Switch one breaker off, disconnect the neutral with some VDE long nosed pliers and see if it sparks and other lights go out?! (please don't actually adopt this method!)

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Richard (Dick)

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 25 November 2011 09:43 PM
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12216

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Originally posted by: daveparry1

Martin, the neutrals only become connected together through the load, when current flows through the lamp fillament into the neutral, that's why the rcd's only trip when the offending light is switched on,



Dave.


Dave
Thanks for all your help. scenario then 2 circuits with borrow neutral, one circuit on 1 rcd & other on other rcd (split CU), lets say when landin light is swithed on current flow from mcb to light, through filament and back through the neutal however some of the returning current in the neutral is also is going to pass through the neutral of the other light circuit so the rcd of the landing light now sees that imbalnce and trip occurs.

Im gonna go a draw it out,

Regards Martin
 25 November 2011 10:05 PM
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12216

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Originally posted by: dickllewellyn

Switch one breaker off, disconnect the neutral with some VDE long nosed pliers and see if it sparks and other lights go out?! (please don't actually adopt this method!)


Thanks for your reply richard

Would i be right in saying that using this method, as long as i keep the light switches that are being supplied by the MCB i have turned off in the closed position then when the neutral is removed of that circuit there would also be a voltage not only on this neutral but by leavin the switches closed the voltage would also flow through the fiament of the lamp & down the line to the load side of the mcb i have turned off.

Many Thanks for your help

Regards
Martin
 26 November 2011 08:56 AM
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Legh

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Originally posted by: 12216

Originally posted by: dickllewellyn

Switch one breaker off, disconnect the neutral with some VDE long nosed pliers and see if it sparks and other lights go out?! (please don't actually adopt this method!)


Thanks for your reply richard

Would i be right in saying that using this method, as long as i keep the light switches that are being supplied by the MCB i have turned off in the closed position then when the neutral is removed of that circuit there would also be a voltage not only on this neutral but by leavin the switches closed the voltage would also flow through the fiament of the lamp & down the line to the load side of the mcb i have turned off.

Many Thanks for your help

Regards

Martin


Where there are two or more lighting circuits being supplied from a DB (domestic only)
Then leaving one MCB on and live and removing each of the neutrals for the other lighting circuits in turn (as Dick has stated) to look for sparking is one way of doing things.

It might be pertinent to mention that you are working live, when you could achieve the same 'test' when the circuits are dead, albeit you might have to put some dummy loads across certain types of lights.
Very time consuming one suspects.

Legh

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 27 November 2011 11:16 PM
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AJJewsbury

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does it always follow that this type of arrangment is always wired in pvc singles only?

More common when in sheathed singles, but can still happen with T&E - e.g. line for landing light "borrowed" from hall light L at switch (i.e. a link between the 2 gangs) then twin for strappers up to landing light switch (where L in drop from landing light might be used to start another 2-way circuit for corridor on next staircase...).

If you want a solid test, try R1+Rn. Like the R1+R2 test (where you bridge L and PE at the CU and go around with a continuity meter), but bridge L and N at the CU instead (disconnecting N from the N bar, so it's connected ONLY to it's own circuit's L). If you get open circuit at a light fitting (with switch closed) then the N has gone astray.

- Andy.
 28 November 2011 01:58 PM
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sparxxxx

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I've seen 3 or 4 domestic borrowed neutrals and they have always been at the landing light. On this basis, one very quick test would be to fit a tungsten lamp in the landing light and switch on all upstairs lighting. Then switch lighting mcb's off and back on if the there's a borrowed neutral you'll see the landing light dim.
 28 November 2011 06:21 PM
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brian1962

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Usually like you said its the landing light on a domestic, fed from the hall. So if you pull the upstairs fuse, the landing will remain on. Fed from the downstairs fuse and returning thru the neutral of upstairs.
 28 November 2011 08:36 PM
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kj scott

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When the neutral is borrowed for the two way lighting in a domestic, you can see it by looking at the wiring of the upstairs 2 way switch; there will be two strappers from downstairs and a single switch line to the lighting point; no test required.
Alternatively a clamp meter can be used line and neutral currents should be equal when loaded, if there is a difference, then a borrowed neutral is likely.

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