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Topic Title: Borrowed Neutral Detection
Topic Summary: Test description required?
Created On: 13 March 2019 05:47 pm
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 13 March 2019 05:47 pm
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PG

Posts: 204
Joined: 17 October 2011

Whilst the danger of "borrowed neutrals" is recognised within the IET documents, I haven't traced a description of inspections or tests that would detect the presence of the connection in new or existing installations.
Would there be any support for recommending to the IET that something on the lines described below should be included in the Wiring Regulations, GN3 and OSG?
EIC - Add a procedure similar to R1+R2 loop test but form the loop from Rn +R2 at the Consumer Unit. Where an item of apparatus was connected to the wrong neutral then the loop would be absent.
EICR - All lights/loads on and final circuits identified.
Sequentially (and safely!) isolate the circuit neutral, re-energise the circuit and check that only the correct loads are switched off.
Reinstate the circuit.

Perhaps some of you do something on these lines? Previously, I have never bothered and have only discovered a "borrowed neutral" when I got a "belt" off an "isolated" ceiling light connection. In the time-honoured fashion, I shifted the lighting circuit phase conductor onto the same circuit that had the offending neutral connection. I suppose that I had not got rid of an induction loop but it was safer and didn't need new cabling (Code 2 to Code 3).

Regards
 13 March 2019 05:58 pm
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daveparry1

Posts: 8020
Joined: 04 July 2007

I've only come across them when doing a c/unit change and found that switching on the hall light or landing light trips the rcd. The only practicable "cure" in this situation is usually to put both circuits on the same mcb. In cases where I have suspicions I have sometimes tested for continuity between the neutrals whilst operating the hall and landing switches whilst doing the dead tests prior to wiring up the new c/unit.
 13 March 2019 06:45 pm
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Zoomup

Posts: 6117
Joined: 20 February 2014

I have never found a borrowed neutral after a rewire or new house wiring carried out by myself. It is always the other fella's fault on existing installations. In old non-R.C.D. protected installations with no split load consumer units, borrowed neutrals were not usually noticed until changes or modernization. With correct installation and testing of an installation initially, no incorrect wiring should be evident. But if several electricians are involved in a large installation then perhaps correct neutral termination confirmation may be required, as mistakes may occur.

Z.
 13 March 2019 07:49 pm
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Legh

Posts: 4427
Joined: 17 December 2004

I had one of several 'borrowed' neutrals discovered at various times One of which where a master clown had linked twelve banks of 3 lamp fittings into pairs then the neutrals were linked in pairs again so line 1 and 2, lines 3 and 5 then lines 4 and 6. are linked and then switched across a three ganged switched. I suppose there is nothing really wrong with that until you attempt to separate the switching lines out so that each pair of six 3lamp fittings can be switched separately. Got the picture?
Insulation resistance testing across each of the lives then each of the neutrals allowed us first to find the lost switch lines and then the borrowed neutrals.
Note this situation only came about through imaginative wiring....

Legh

-------------------------

http://www.leghrichardson.co.uk

de-avatared
 14 March 2019 01:01 am
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RB1981

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Joined: 16 September 2007

The National Rules for Electrical Installations (ET101) published by the ETCI (Electro-Technical Council of Ireland) - now taken over by the NSAI and to be updated as I.S. 10101 - prescribe a test for erroneous connections between phase conductors.

A test for erroneous connections between neutral conductors could be a possibility to help locate this fault I suppose.

-------------------------
Walsh Electrical Services
http://www.walshelectrical.ie/
RECI REC & NICEIC Approved Contractor
 14 March 2019 10:12 am
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AJJewsbury

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EIC - Add a procedure similar to R1+R2 loop test but form the loop from Rn +R2 at the Consumer Unit. Where an item of apparatus was connected to the wrong neutral then the loop would be absent.

While I'm all in favour of some kind of test on the N conductors anyway (currently our tests wouldn't spot a bad/loose N connection on a radial), I'm a bit wary of using a test like this to "prove" that no borrowed N exists. I suspect that unless you can be absolutely sure that all loads are disconnected/switched off there's a possibility of a 'false positive' continuity from one circuit's N to another circuit's N via the L of one of the circuits and connected loads (e.g. lamps). Probably less of a worry on an EIC than on an EICR where hopefully there should be fewer unknowns lurking - but as most borrowed Ns are (in my experience) a result of a deliberate bodge rather than honest mistake, I'm not sure how many occurrences this test would ever catch on an EIC.

EICR - All lights/loads on and final circuits identified.
Sequentially (and safely!) isolate the circuit neutral, re-energise the circuit and check that only the correct loads are switched off.
Reinstate the circuit.

OK I admit to feeling uncomfortable about using "live tests" in this way, especially on an unknown installation. Maybe I'm being overly cautious, and worrying about things like the old East German trick of connecting socket PE contacts to N which are very unlikely in the UK. I'll have t give that approach a bit more thought. It does seem very time consuming as well - as described if I had four lighting circuits with 10 lights on each, it seems I'd have to walk around the whole house 4 times checking at least 30 lights each time - so 120 checks. And that's before I start to think about wall lights's N being connected to the ring or other power circuit.

It is a problem worth considering though.

- Andy.
 14 March 2019 11:33 am
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mapj1

Posts: 12039
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This is a live test, but does not involve any wires being disconnected while the power is on.
IF the tails in the CU end are long enough, live bus bars etc are not exposed to easy touch, and the expected pairing of L-N is visible, turn everything on and go round each final circuit in turn for a quick flick with the clamp meter, and compare currents. If amps going out does on L not equal amps coming back on N, there is a problem that needs investigating. (putting both side by side in the meter clamp will do the subtraction for you.)
In general in terms of shock risks a shared neutral (by which I mean there is a link between the neutrals of 2 independant final circuits)
is not quite as dangerous as the borrowed neutral - where a load is wired between the Live of one circuit, and the Neutral of the other.
Both will trip an RCD, and fail on the test above.

-------------------------
regards Mike


Edited: 26 March 2019 at 11:37 am by mapj1
 18 March 2019 04:14 pm
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PG

Posts: 204
Joined: 17 October 2011

Thanks very much for the comments. It sounds as if the concept of instigating tests to discover the presence of borrowed neutrals has some, conditional, support. Perhaps the Forum Participants that sit on the IET Regulations committees could bring this up for consideration.
I appreciate the doubt of a test for a new Installation. On the other-hand it wouldn't take longer than the dead circuit R1+R2 loop test. I've never heard of a neutral being borrowed for a power circuit so perhaps the test would be limited to lighting?
I can see the advantage of Mike's clamp meter live/neutral imbalance test during a Periodic Inspection. I think that one could get a clamp meter on all (or most) neutral connections. However, some individual LED lamps could draw as little as 0.1A at 230V. I don't know the repeatability of my clamp meter but it has a range of 0.5A to 400A and an accuracy of [3.5%+0.5A]. Thus, looking for a difference of 0.1A looks a bit unreliable when (say) having a base load of 5A - i.e. a difference of only 2%.
To address Andy's concern on the extra time required to do the live, EICR test -
I walk around an installation with a sketch-pad. Recording the electrical layout and which circuit the apparatus is connected to. The extra time for isolating, disconnecting the final circuit neutral, deisolating, walking around the house, isolating the CU and reconnecting the neutral on (say) 12 circuits shouldn't take more than about an hour.
Worth it for a Code 2 fault?
I'll keep an eye out on future amendments. Perhaps RB1981 could keep us posted on I.S. 10101?
Regards
 18 March 2019 06:52 pm
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wallywombat

Posts: 96
Joined: 19 October 2018

Originally posted by: PG
IHowever, some individual LED lamps could draw as little as 0.1A at 230V. I don't know the repeatability of my clamp meter but it has a range of 0.5A to 400A and an accuracy of [3.5%+0.5A].
Regards

That's why it's handy to have an earth leakage clamp meter - like a normal one, but it has a resolution in sub-mA.
 18 March 2019 08:01 pm
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sparkingchip

Posts: 11682
Joined: 18 January 2003

I have been testing for borrowed neutrals on lighting circuits this afternoon.

Test the dead and disconnected circuits with a insulation tester on the 250 volt range with the loads connected.
 19 March 2019 12:20 pm
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

I've never heard of a neutral being borrowed for a power circuit so perhaps the test would be limited to lighting?

I have. Typically retro-fitted wall lights. L picked up from the switch by the door (replacing a 1G switch with a 2G), but N taken to a convenient socket in the same room.

Lighting-lighting is usually the landing light fed from the downstairs lighting circuit - usually easily detected by observing that it doesn't light when the downstairs circuit is off and a quick peek in the loft to confirm that N is connected into the other upstairs lights.

- Andy.
 19 March 2019 06:22 pm
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RB1981

Posts: 513
Joined: 16 September 2007

Originally posted by: PG

Perhaps RB1981 could keep us posted on I.S. 10101?


The current situation is as follows:

The 4th Edition of the Electro-Technical Council of Ireland (ETCI) National Rules for Electrical Installations (ET101:2008) is to be replaced with the 5th Edition. The ETCI has since ceased trading and this has been taken over by the National Standards Authority of Ireland (NSAI).

This is to be published as I.S. 10101:20xx - the Draft for Public Enquiry was available until 15th March - so comments on the Draft are now closed. I will certainly let you know once the final text has been published and once the implementation date is known.

-------------------------
Walsh Electrical Services
http://www.walshelectrical.ie/
RECI REC & NICEIC Approved Contractor
 26 March 2019 10:43 am
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PG

Posts: 204
Joined: 17 October 2011



That's why it's handy to have an earth leakage clamp meter - like a normal one, but it has a resolution in sub-mA.

Thanks Wallywombat. I found an example of a meter that would enable the test procedure described by Mike (Dilog DL6518). I'll save up for one

I think that we have enough interest for the test to become prescribed by the IET. I look forward to RB1981's feedback on the progress in Ireland.

Forum members name corrected

Edited: 26 March 2019 at 01:13 pm by PG
 26 March 2019 11:00 am
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PG

Posts: 204
Joined: 17 October 2011

As a correction to my figure of a single LED lamp drawing as little as 0.1A at 230V - Megaman have advised that a 4.8W downlighter can take as little as 35mA. Still detectable by the earth leakage clamp meter.
 26 March 2019 11:35 am
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mapj1

Posts: 12039
Joined: 22 July 2004

Typically retro-fitted wall lights. L picked up from the switch by the door (replacing a 1G switch with a 2G), but N taken to a convenient socket in the same room.


Wall lights also seen (mis)using the lighting CPC as neutral, not sure if the chap who did it had an industrial or DNO background looking at the sort of fittings used in the rest of the job. Such a thing still fires the RCD and shows up in the L-N balance test of course. Same installation had 2 way lights using T& E pressed into use as 3 core by oversleeving the core intended as the CPC in red, which also caused some head scratching for a minute or two.

-------------------------
regards Mike
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