Borrowed Neutral

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davetheglitz

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I understand that this was quite common years ago - particularly with regard to 2 way switching on stairs.

Does anyone have any quick and easy way of detecting this has been done? It's probably something that should be picked up on a PIR - but without stripping down the circuit completely - all at a cost - I can't see and easy way out.

Cheers

Dave

 

extension15

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I understand that this was quite common years ago - particularly with regard to 2 way switching on stairs.Does anyone have any quick and easy way of detecting this has been done? It's probably something that should be picked up on a PIR - but without stripping down the circuit completely - all at a cost - I can't see and easy way out.

Cheers

Dave
Disconnect neutrals at board and check for continuity, place both circuits on same MCB, problem solved.. :D

 

davetheglitz

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Thanks Ext - yes I did post on the other place as well - sometimes it can be useful but often it turns into a playground! In this case it was useful - as was your reply.

Just breaking down the method:-

1) Switch all lights

2) Switch off all breakers off/remove fuses.

3) Remove neutrals one group at a time and check continuity to neutral bus bar. If there is continuity there is a borrowed neutral. Mark neutral.

4) Continue until all checked.

Need to do this with no power on board. Only problem is that in many offices the workers object to losing all power for any length of time - so if this is a problem put the fact that this is not tested as a limitation.

Does that sound about right to you?

Incidentally none of this would show up from the recommended tests as per 2391. How many inspectors actually check for borrowed neutrals?

Cheers

Dave

 

extension15

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Thanks Ext - yes I did post on the other place as well - sometimes it can be useful but often it turns into a playground! In this case it was useful - as was your reply. Just breaking down the method:-

1) Switch all lights

2) Switch off all breakers off/remove fuses.

3) Remove neutrals one group at a time and check continuity to neutral bus bar. If there is continuity there is a borrowed neutral. Mark neutral.

4) Continue until all checked.

Need to do this with no power on board. Only problem is that in many offices the workers object to losing all power for any length of time - so if this is a problem put the fact that this is not tested as a limitation.

Does that sound about right to you?

Incidentally none of this would show up from the recommended tests as per 2391. How many inspectors actually check for borrowed neutrals?

Cheers

Dave
I must confess I don't check for borrowed neutrals unless it's a FULL PIR..

However, for SAFE practice when working on any installation (usually domestic in my case), I switch off/isolate ALL lighting circuits when doing any remedial work (just in case)..

B)

 
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1) Switch all lights

2) Switch off all breakers off/remove fuses.

3) Remove neutrals one group at a time and check continuity to neutral bus bar. If there is continuity there is a borrowed neutral. Mark neutral.

4) Continue until all checked.

Hi guys...

not sure these 4 steps are strictly correct...

A borrowed neutral actually means that two phase (or live supplies)

connected to two independant loads

then make the return path to the CU via one neutral wire.

(more a shared neutral really)

so step (3) checking continuity between neutrals and the N bar would not neccesarily show a reading!

With the lights switched ON, (all bulbs known to be working).

all breakers & power off

and all neutrals diss'd at CU.

the continuity would be between the two lighting circuit outgoing phase conductors.

 
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Just to add a bit more

re previous post..

they say a picture speaks a thousand words...

this little extract from our NICEIC friends makes it clear where the sharing and continuty can be tested to check for borrowed/sharred netrals.

http://i291.photobucket.com/albums/ll306/SPECIALLOCATION/borrowedneutral-1.jpg

The continuity path is;

supply 1,

load 1

combined neutral

load 2

supply 2.

(assuming the picture link thingy worked? :| )

 
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Specloc.....

"With the lights switched ON, (all bulbs known to be working)."

My understanding is that the bulbs should have been removed at this point?

Or have I had more rum than was good for me (n.b. it IS half past one in the morning down here in welsh wales!!!)

 
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Specloc....."With the lights switched ON, (all bulbs known to be working)."

My understanding is that the bulbs should have been removed at this point?

Or have I had more rum than was good for me (n.b. it IS half past one in the morning down here in welsh wales!!!)
I think the second point here..... bit too much rum;\

We need to read a continuty bridging two circuits through the loads.

The fundimental problem is that;

Two seperate circuits, supplied from seperate MCBs/Fuses,

Through two seperate functional switches & loads

are combined together on the neutral side

"before the neutrals are back at the CU".

The reading you typicaly get will be 50 -70ohm through the bulb(s) that are actually bridging the two circuits.

You can read it though the neutral side or live side....

But I tend to find this problem is on older installtions wired in singles.

where the neutrals are all bunched into a handfull of terminations on a small neutral bar in an old 6 or 8 way wylex type box.

Where it can be hard to work out which neutral belongs with which live...

so I just find it easier to test on the phase side where I know the two outgoing light wires from the fuses they connect onto!

again I think the picture on my previous post makes it clearer...

It is one of those subjcet that drawn out is easiser to see than explained in writing. :)

 
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Nice input on the 'Borrowed Neutral' there Special..The Don is happy.
Cheers M8:)

Had a couple of nasties with 'Borrowed(bodged)' neutrals.

House had modern split load CU.

Im doing bathroom electrics alterations..

upstairs lights OFF!!!

left downstairs light on cuz floor fitters are doing the lounge

Then someone presses landing light on because its getting a bit dark... over cast day....

Landing light comes on and so do MY FINGERS whilst I am holding the new bathroom wires :eek: :O:_|

(typical! not doing safe isolation!!!

made assumtion modern box circuits would have been done right.... Doh:()

Another one had a conservatory light wired from downstairs lights... neutral to downstairs sockets!!

that made me jump a bit as well when had socket open, sockets off at CU...

but then someone switches the conservatory light on!! :eek: :O

Not Good when it happens!

as they say you learn by your mistakes...

More than happy to try and help others not get caugth by bad practice! :)

SL

 

davetheglitz

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Thanks SL - my terminology was definitely at fault there. I was thinking of the situation where there is 2 way switching for downstairs and upstairs lights - downstairs fed by one fuse - so at the upstairs circuit a neutral is pinched from the upstairs lighting circuit to avoid running another cable. More of a case of borrowed live - so only a hazard if downstairs lighting circuit left on and downstairs 2 way light is switched on whilst isolating and working on the upstairs - but potentially a problem with 17th edition if trying to upgrade and putting seperate circuits on RCDs. In this case the method of lights on and checking between neutrals makes sense.

Regarding the borrowed neutral, the only certain way of detecting as far as I can see is to remove all loads from the installation (all lights off and switch off everything) so no L-N continuity, then remove all fuses and check all combinations of L-L.

How many inspectors do this on a PIR - and should it be done to cover your back?

Hey we haven't deviated from the original post yet!

Remember Fireball XL5 and Supercar - Supermarianation - Gerry Anderson? That should get a few oldies out!

Cheers

Dave

 

steptoe

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EASY way, disconnect the N you suspect to be borrowed, switch on a light on other circuit, stick on your neon screwdriver and if its borrowed it will glow.!

or do it at the DB

ie,

turn off the MCB for the circuit not being used,

remove the N for the circuit being used,

turn on a light, if it works then the N's are doubled up somewhere,

 
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"How many inspectors do this on a PIR - and should it be done to cover your back?"

I do! & yes!

Well it would be a deviation from wiring regs '314' if it was on an installtion...

Each final circuits should be electrically seperate from all other final circuits!

(314-01-04) ???

"Hey we haven't deviated from the original post yet! "

Beer corection factor [bCf] not kicked in yet...

everyone needs to go and have another drink!!! :^O :^O:^O

 

extension15

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I've yet to come across a borrowed neutral, but I'm sure it's waiting for me..

:|

Unless I've already missed it...................... :eek: :^O

 

steptoe

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u should deffo always check for this on PIR, and a proper IR test will show it up.

but only if you IR on individual circuits, (sometimes dont happen!),

P/P, P/N, N/N, P/E, N/E, E/E.

TBH I dont always test like this, but I do check for mixed/borrowed Ns

 

The Godfather

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u should deffo always check for this on PIR, and a proper IR test will show it up.but only if you IR on individual circuits, (sometimes dont happen!),

P/P, P/N, N/N, P/E, N/E, E/E.

TBH I dont always test like this, but I do check for mixed/borrowed Ns
Steptoe, what age (installation) are we talking about, in particular here?

Don

 
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Hey we haven't deviated from the original post yet!

Remember Fireball XL5 and Supercar - Supermarianation - Gerry Anderson? That should get a few oldies out!

Cheers

Dave
Actually, that wouldnt be a thread deviation as such.......

More a 'forum deviation'!!!! :^O

perhaps to here? :|

http://www.tvcentury21.com/component/option,com_fireboard/Itemid,170/

Now forum deviation is a much more complicated formula!!

which I think you need a slide-rule and log tables to calculate correctley!!

 

davetheglitz

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Now forum deviation is a much more complicated formula!!

which I think you need a slide-rule and log tables to calculate correctley!!

I see the Bcf has kicked in!!

 
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