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Topic Title: lighting wiring diagram with borrowed neutral
Topic Summary: Help please
Created On: 21 November 2011 09:18 PM
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 21 November 2011 09:18 PM
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12216

Posts: 45
Joined: 22 August 2010

Good Evening All
please could anyone tell me where i can find a wiring diagram that details a lighting circuit with a borrowed neutral as i wish to gain a comprehensive understanding to how this would be actually wired & the relationship it would have in relation to the strapper cable.

I have searched the net & this forum and found a No of topics relating to this issue but none describes how it is wired & can only find circuit diagram drawings

any help would be greatly appreciate.

Many Thanks

regards

Martin
 21 November 2011 09:42 PM
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chunk522

Posts: 126
Joined: 12 December 2010

Martin
I have only ever come across the borrowed neutral situation in a domestic building and its always on the landing light
When the old sparks in there day used singles from the DB straight to the light switch with The line conductor and a switch wire straight to the fitting then another line condutor to the next switch and so on. However the neutral went directly to the light fittings and instead of running a single neutral to the landing light they would borrow the upstairs lighting circuits neutral lazy gits lol well this was ok as theneutrals were all commoned up untill you introduce the RCD lol then you have a problem.
Sorry though mate the wiring diagram is all in my head not omn paper
 21 November 2011 10:19 PM
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12216

Posts: 45
Joined: 22 August 2010

many thanks for your reply Chunk
i am on the right track with the following.

grd fl light circuit wired in pvc singles.
line conductor from mcb toall grd fl fitting then a drop to each sw then when we get to hall waygrd fl light fitting take line conductor from the loop up to the 1st fl landing light fitting.

although all ground fl light fittings are supplied with a neutral supposedly run with the ground fl line conductor, the 1st fl landing light fitting would have got its neutral from the neutral that was run with the 1st floor line conductor.

many thanks for your time with this one

regards
Martin
 21 November 2011 11:51 PM
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alancapon

Posts: 5844
Joined: 27 December 2005

Originally posted by: chunk522
I have only ever come across the borrowed neutral situation in a domestic building and its always on the landing light . . .

. . . well this was ok as theneutrals were all commoned up untill you introduce the RCD lol then you have a problem.

No, you have a problem whether you have an rcd or not. The sharing of neutrals between circuits is inherently dangerous and should be rectified where it is discovered.

Regards,

Alan.
 22 November 2011 12:04 AM
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Legh

Posts: 3538
Joined: 17 December 2004

I agree with Alan,
disconnecting a neutral that is still live from another circuit, even though its covered by an RCD will provide you with mains voltage. You touch it and you are now in series with the load its attached to. You then have approximately 10ms to let go.... I would say that's more or less impossible!

Legh

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 22 November 2011 06:37 AM
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jumpinjax

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Have i missed the fact that no one has mentioned to the OP that it is only a borrowed neutral when the 2 supplys are on different mcb, ie when you do a board change and split upstairs lights from downstairs lights, that is when it is dangerous if not rectified besides causing the rcd to trip with switching of the 2way.
You can have a socket radial with a resistive load plug in, then disconnect the neutral near the board and it will test live if isolation has not taken place.
 22 November 2011 09:01 AM
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gkenyon

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Joined: 06 May 2002

Originally posted by: jumpinjax

Have i missed the fact that no one has mentioned to the OP that it is only a borrowed neutral when the 2 supplys are on different mcb,
Although, new Section 444 does contain a Reg that effectively states that "borrowing" a neutral from the same circuit constitutes bad EMC practice, and therefore not conformant with EMC Regulations, which require good EMC practice in installations.

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Eur Ing Graham Kenyon CEng MIET TechIOSH
 22 November 2011 09:49 AM
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OMS

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Have i missed the fact that no one has mentioned to the OP that it is only a borrowed neutral when the 2 supplys are on different mcb


Well if it was on the same MCB it would be the same circuit by definition and thus cannot be "borrowed" - notwithstanding Grahams comments regarding how the circuit neutral is physically arranged for EMC compliance.

Personally, I'm a fan of any circuit leaving a DB and looping in and out of accessories etc without the use of "Spurs", "Tees" or "Branches" - Old Skool I guess.

Regards

OMS

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 22 November 2011 11:53 AM
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rougediablo

Posts: 112
Joined: 09 November 2011

There's a good explaination with diagrams of a "borrowed" or "robbed" neutral in Alan Lynch's book The Part P Doctor.
 22 November 2011 10:53 PM
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mistercohen

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Joined: 28 October 2011

How about........ outside light to the side of front door complete with PIR.

Live supplied from the hall light switch in single double insulated wire burried in a shallow chase horizontal to the front door frame then vertical up the edge of the frame and then drilled hole to the outside. The neutral was borrowed from a GF ring supplied twin socket in the lounge by dropping a sinlge double insulated down the cavety into the back of the ko box.

After a CU change where everything tested ok. Customer reported spurious RCD tripping, always reset no trouble. It took quite a while to find it, as it only tripped when the PIR opperated the light, always after dusk and only if someone passed or called.
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