Diverted Neutral Current

So for the past 5 years if so I have been seeing and experienced a phenomena known as diverted neutral current or diverted networks current.

It has a few names. Much like the incredibly slow realisation that type AC RCDs have had their day. Even though type A has been about over twenty years.

I get at least two / three messages a week from electricians who are now using clamp meters to check the earth conductors of LV installations. Low and behold they find anything from 2A up to 169A in a few cases.

I have research this at work for the last few years and some of the work I have ensured is published via the following link.

http://tangle-tamers.com/page21.html

We have also managed to produce a visual guide for measuring this issue. This work also helped with the Broken PEN IET article. 

It appears the aged life expired state of the LV network is a bigger problem and will only continue to grow. The use of CNE cable has been a tad o IMO. Years ago the RECs admitted cost was the key factor, but made the decision based upon a REC. a government owned network. Since privatisation the DNOs have fixed on fail, leaving the LV side unmonitored to a large extent. This is now slowly changing. But we assume a broken PEN conductor or failure of this can cause hazardous over voltages but also large amounts of network currents bypassing the broken PEN and importing and exporting on domestic earthing in some cases.

whilst the number are small in the scheme or things. It’s a growing issue. The network know about it. But the industry appears to not be discussing it. Why do we not get a safety alert for all electricians nationally. If we find this why are the DNOs and yes some are taking horrific decisions. And in some cases charging. I’ve issued a safety alert for the Railway where I work. 

be good to gauge folks thoughts as a broken pen gives you CNE consideration in a home, not allowed by BS 7671 and also brings up the whole what is safe isolation with this in mind. We never consider earth as a energised conductor and only ever consider rise of voltage. Maybe with changing and ageing networks, non linear loads etc. We need to change the way we see things.

I live in hope the networks will be more open on this.

thoughts..

  • Posted via phone on a night shift.
    Go easy on me will ya.

  • I agree.

    I assume you are already aware of the HSL report for the IET looking at the dangers of electric car charging -it all relates to this and is  attached for anyone who is not.

    Aluminium concentric cables are a know problem to many electricians  who work in areas that have them, but part of the problem is that BS7671 which some folk treat as the last word, rather than just good advice for most cases, in effect assumes that anything before the meter tails is someone else's problem and therefore invisible.

    The move to all plastic gas mains and water supplies changes the risk somewhat as well.

    It would be possible to break all poles LNE at the incomer based on neutral lift, or indeed a current clamp in the CPC, but interrupting CPCs and certainly adding series transformers is so far off the radar that a step change in thinking would be needed to get folk to not consider it so wrong as beyond the pale - almost like a fuse in the earth path....

    Consider some of the ideas in the 1987 article The Killing Ground by G3RZP

    part 1

    part 2

    What would most sparks do if they saw this sort of wiring?  Probably decide it was not right as that is not  in the OSG ;-)

    Mike.

    PDF

  • Great response, no i was not aware of the report. Thank you for sharing.

    Given the safety related nature of this it is a shame its not wider known, I have spent a few years and a lot of money analyzing this and the standards around earthing, low and behold they are out of sync, contradicting and poor at best, I hope to share this soon as it will shift a huge re-write of railway standards.

    Interesting the assumptions couldn't be further from the 'real world' (in my experience) in the report and its ten years old!

  • I've read the report. this desperately needs to be revised, the assumptions are flawed IMO.

  • If you are happy to do so, it would be interesting to discuss what you think needs to be updated - the report despite being for public release, has not really been in wide circulation, and pre-dates things like the O-PEN products for charge points becoming available, neither the sort that needs an electrode nor the kind based on L-N voltages out of spec (easier to spot an open neutral on 3 phase, harder on single)

    The topic of earthing vs bonding, step voltages and the true probability of various dangerous or possibly dangerous conditions happening but not being detected and fixed is not very well propagated, at least as far as the folk installing car chargers as far as  I can see.

    We probably need to be careful not to be too rude about the authors, who were doing their best with the information available at that time, things like historical incident databases are not really in a format that allows floating neutral events to be counted separately  from other things like tree falling through overhead supplies. Nor can we be sure how many are neutrals  fixed as part of something else going on.

    Mike.

  • Agree Mike, the authors have done their best. At the time was a amazing piece of work. Moving on from it. Now we can see it needs revisiting. Happy to chat on it DM me your contacts on here anytime. Or paulmeenan@me.com

  • But the industry appears to not be discussing it.

    Well, discussions are being held. As Mike says, lots of discussion driven by EV charging equipment installation.

    The 9th Edition (2022) of IET Guidance Note 3, published last week, includes a safety check for diverted neutral currents, in Appendix D.

  • Nice video Paul, if a bit slow paced for me, I watched it at time and a half ;-)  - but then I do talk quite fast.

    some thoughts that may augment the discussion that you may care to consider to either include in any update (or reject entirely of course)

    Admin thoughts

    Some links to the various docs flashed up and flicked through would be good - I suspect most folk do not know how to get to the DNO or ENA archives for example.

    Technical thoughts

    First thought,

    There is not, and perhaps there should be, a distinction drawn in discussion, regulations and descriptions, made between what I see as two sorts of earthed object.

    1) metal objects that are in the ground, that are in effect connected to a wire at one point, and the only other contact is via terra-firma - like a rod electrode or a lamp post, with one earthing lug and and a bare surface. Current is limited by the resistance of the soil.

    2) similarly conductive things things in the ground that are connected in two places able to perform more like a length of wire, that just happens not to be insulated and in intimate contact with terra-firma - such as a water pipe bonded to a PME earth at two or more properties. Current is not limited except by the metal resistance.

    No need to have really chunky bonds to type 1, but type 2 are the potential fire starters as your film describes.

    Second thought

    Arc energy  and possible burns.  As sort of alluded, I agree here is a world of difference between opening a bolted contact carrying 20A, if that is in effect a source of 230V in series with a touch over 10 ohms, or if it is half the current in a pair of conductors of 100milliohms each, and all you do is now increase the current in the other parallel path, but the open circuit voltage is a fraction of a volt. It is salutatory to realise that even a 15mm copper water pipe has a X-section of ~ 20-30mm2 equivalent so compares to a house service cable PEN, and when all is well, they could share more or less half each if the distances compare

    Third thought

    Now at the moment there is no approved instrument for electricians to use that I know, that can tell the difference, but there are several techniques  that do exist and are used in places where cutting the wrong wire first would be 'most disappointing'. For those unaware I shall describe the principle.
    Two current transformer like cores are clamped around the cable that may or may not be part of a low impedance loop, one is connected to a generator of an audio tone or complex spread spectrum signal unrelated to the local mains frequency. By dint of carefully termination impedance, turns ratios  and core cross-sections, a known fraction of a volt is induced in series with the clamped conductor.

    The second core is connected to what is in effect a detector tuned to recognize that signal (or in the earlier forms of the same gear, a pair of headphones and a highly strung tuned operator) . When the round loop impedance is low (safe to cut the wire) the receiver detects a lot of mA flowing from that fraction of a volt signal, on the other hand when there is little or no current flow in response to the driving voltage or no tone in the 'phones there is no low impedance parallel path, and so beware of the dragon, and  fit a bypass and then verify the OK light comes on before reaching for the big snips.

    Fourth  thought
    Do not get to hung up on the lack of a standard for the O-PEN detector - I think one will not be written until there is some evidence of how well they work and what they do or do not detect  from use in the field, and if it is written before the fact, it will have to have a lot of edits. Waiting for the perfect should not be the enemy of the at least adequate.

    Fifth thought

    Are you aware of the South African stolen neutral detection  of using an RCD before the N-E split, but putting  an electrode, if need be  via a shock current limiting impedance at the point of N-E split so that the RCD fires if the N-E current rises appreciably ?

    a similar idea would work here. But would likely need a rules change to ESCQR to allow equipment before the N-E split, or rather the NE link consumer side of the meter. But a current trip to an electrode in the manner of the old VOELCB would not.

    I did have a drawing that I did of it when this was first mentioned on here (post timestamp 2.49pm onwards here ) Sadly that website and the image on it is lost now but at some point when I have more time I will re-draw.

    Caveats.

    I do not do domestic wiring for a living, except my own and close family.
    My work is more electronics design, and related activities.  I do spend far too much time 'playing' at my job as my wife sees it.

    Mike

    edited to clarify

  • Agree Graham, but the subject has been kicking about since 2014 at least from my memeory, and im well chuffed that GN3 finally acknowledges it. I personally would like to see more transparency on such a important safety matter, even if it is a work in progress.