Diverted Neutral Current

Question

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&rsquo;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&rsquo;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..

mapj1 · Accepted Answer

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

mapj1 · Answer

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

gkenyon · Answer

Paul Meenan said: the subject has been kicking about since 2014 
 If you look back in history, it's been going on a lot longer than that. The DTI guidance on ESQCR 2002 acknowledges the issue, but it's been discussed much earlier. CP 1013:1965 has discussion on PME, There are some interesting snippets, such as recommendations for consumer's earth electrodes, but most telling. Well worth a read. But issues must have been known at the time the Electricity Act 1947 was drafted, otherwise the provisions in the legislation would not have been put in place. 
 On and off over the years, since at least as long as I have held C&G "wiring regs" qualifications, i.e. > 30 years, there have been various debates regarding "exporting PME" outside what we used to call the "equipotential zone". The discussions on EV and PME are a continuation of this particular issue, to be honest. 
 "Neutral current diversion" is not new either, although perhaps we are seeing more effects due to the prevalence in plastic pipes, and the age of the network? 
 Finally, if we think that TN-S would resolve the issue ... there are such things as protective conductor currents. May only be mA from a single dwelling, but in other premises, and parts of distribution networks, we are looking at many amperes, perhaps 10s of A. And of course, these currents have the same capability for wreaking havoc if the protective conductor breaks (just like the CNE conductor in PME systems - at least with PME the lights go dim rather than relying on a person alone to find out by touching an exposed metal part connected to the installation earthing system). 
 
 Paul Meenan said: I personally would like to see more transparency on such a important safety matter, even if it is a work in progress. 
 Agreed, although coming to agreement on the seriousness of the issue is quite difficult - particularly, as I pointed out in the immediate para above, protective conductors are not necessarily "safe to touch", and never were - especially if you disconnect them in a live system.

Paul Meenan · Answer

Now Graham, you know I have read CP 1013 and BS 842, plus every edition of G12 and associated docs. 
 The term throwing ones rubbish over the fence comes to mind (DSO). 
 
 Neutral current diversion" is not new either, although perhaps we are seeing more effects due to the prevalence in plastic pipes, and the age of the network? 
 
 Now that I totally agree has possibly exaserbated the issue. I am bemused the ENA have been able to influence the new electrode recommendation as per told by IET a colleague who we both know well. cautions were always supposed to be taken when installing electrodes as to the effects on the network currents, but this seems to have gone out the window. 
 
 In the old old days, stuff was debate at the institute with peer feedback, this is the only place we now have to attempt to recreate that transparency.

Lewiswynd · Answer

HI, 
 
 Not sure if it has already been mentioned but what would be your thoughts on the installation of a 1:1 isolating transformer at the origin, effectively creating a TNS system. I know this would be an expensive option. 
 
 Thanks

Diverted Neutral Current

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