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Topic Title: Open PEN conductors.
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Created On: 21 April 2017 12:12 PM
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 21 April 2017 12:12 PM
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Zoomup

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There are concerns about open PEN conductors according to a recent report. Charity's Roundtable Seeks Neutral/Pen Conductor Risk Reduction
Electrical Safety First's recent electrical installation safety roundtable focused on open-circuit PEN conductors, which serve as both a protective earthing and neutral conductor. Although PEN conductors within installations have been prohibited for some time now, older ones still remain in, for example, the rising mains in blocks of flats and military camps.
When the PEN conductor fails, not only does this present a shock hazard but the diverted neutral current can also create a significant heat build-up, as it typically makes a circuit via exposed metalwork (such as gas, water and oil pipes, etc.) - which can then lead to fire.
Data obtained from the Health and Safety Executive shows at least one incident of an open-circuit PEN conductor is reported every day - with around 10% of them leading to injury. But as many incidents go unreported, it's believed the real figure could be significantly higher.
"Anecdotal evidence suggests that open-circuit PEN conductors might be a growing problem due to a range of factors - including our ageing distribution system, metallic service pipes being replaced and the theft of copper from cables", explains Martyn Allen, Technical Director at Electrical Safety First.
"The Charity is committed to promoting collaborative working and, while electrical distributors are legally required to ensure the safety of the electrical distribution network, we believe that the electrical installation sector can also help reduce the risk that open-circuit PEN conductors can present."
This opportunity to help mitigate the risk has also been recognised and considered by the Committees responsible for developing the 18th Edition of the IET Wiring Regulations. Requirements for foundation earthing, supplementary earth electrodes and over-voltage detection/protection devices may be included in the draft for public comment (DPC) to BS 7671: 2018 (18th edition of the Wiring Regulations). The DPC availability and commenting period is scheduled for June to August 2017 and will be accessible via the BSI Draft Review Portal https://standardsdevelopment.bsigroup.com/.

What are your views?

Z.
 21 April 2017 01:27 PM
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scottseedell

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Rather than the risks associated with diverted neutral currents, I'd suggest the fundamental risk would be accidentally putting 415V across all your single phase supplies if a PEN conductor (or just the incoming neutral) became disconnected on a three phase installation. Blowing up and/or significantly overloading any single phase equipment would present far more of an immediate shock/fire risk.

-------------------------
Scott Seedell - MSc IEng MIET
 21 April 2017 01:28 PM
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AJJewsbury

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This opportunity to help mitigate the risk has also been recognised and considered by the Committees responsible for developing the 18th Edition of the IET Wiring Regulations. Requirements for foundation earthing, supplementary earth electrodes and over-voltage detection/protection devices may be included in the draft for public comment (DPC) to BS 7671: 2018 (18th edition of the Wiring Regulations).

Sticking plaster solutions that don't address the underlying problem. Join my Campaign for Real Earthing - bring back true TN-S!
- Andy.
 21 April 2017 02:32 PM
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broadgage

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I rather like the idea of a society for real earthing. As someone said on these forums " on a PME system the green/yellow wire is not really the earth but is in fact the neutral in disguise"

Look at all the requirements and concerns regarding PME being exported to outbuildings or electric vehicles, and the problems with petrol filling stations and livestock barns.

All to save the supply industry providing mains with 4 insulated conductors and an earthed sheath.
 21 April 2017 03:09 PM
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Zoomup

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Originally posted by: AJJewsbury

This opportunity to help mitigate the risk has also been recognised and considered by the Committees responsible for developing the 18th Edition of the IET Wiring Regulations. Requirements for foundation earthing, supplementary earth electrodes and over-voltage detection/protection devices may be included in the draft for public comment (DPC) to BS 7671: 2018 (18th edition of the Wiring Regulations).


Sticking plaster solutions that don't address the underlying problem. Join my Campaign for Real Earthing - bring back true TN-S!

- Andy.



I'll join Andy.

Z.
 21 April 2017 07:05 PM
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mapj1

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Join
CAMReE

eh - probably not possible without a change to the maths syllabus, and blaming the previous administration to encourage youngsters to learn counting beyond four, which I assume is the basic problem.
In the mean time, there is always TT - no questions of step voltage or large diverted currents but a need for fast moving RCDs.

Joking aside, I have seen figures that suggest for the circuits fed by any one substation my go off-neutral has a probability of once every thousand years or so but of course thats not fair, as some parts of the country are wired in ways far more susceptible than others.
So it happens more often if the wiring is aluminium and in wet ground.

-------------------------
regards Mike
 21 April 2017 07:28 PM
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sparkingchip

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Or we could earth the installations the same way as New Zealand and Australia.

Andy B.
 21 April 2017 10:30 PM
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mapj1

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MEN you mean, more like PME or PNB when there is only a few users. The only significant addition in AS3000 is more end of line electrodes than we might have here.

-------------------------
regards Mike
 22 April 2017 10:35 AM
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lyledunn

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Originally posted by: mapj1

Joking aside, I have seen figures that suggest for the circuits fed by any one substation my go off-neutral has a probability of once every thousand years or so but of course thats not fair, as some parts of the country are wired in ways far more susceptible than others.

So it happens more often if the wiring is aluminium and in wet ground.


I think it is fair Mike. From reading your posts it is clear that you have a rational approach to risk which is somewhat unusual for our industry. Engineers are good at pointing out the potential technical consideration that might give rise to injury but are not so good at evaluating the risk. The reason that they are not good is not because of any failing on their part. A lifetime relationship with the regulatory confines of the Wiring Regulations and the lack of detailed incident figures cause an aversion to risk that results in unnecessary intervention.
I see the potential issues with TNCS and I am aware of a few real incidents that have had bad effects but without the figures to address the likelihood, I am in no position to make an evaluation of the risk.
When I lifted the post this morning a transit van flew past with the driver obviously looking for something on the passenger seat while cradling his mobile phone between chin and shoulder. As much as I admire AJ, if I am going to sign up to a campaign, I don't think that it will be his!


-------------------------
Regards,

Lyle Dunn
 22 April 2017 12:17 PM
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sparkingchip

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Originally posted by: mapj1

MEN you mean, more like PME or PNB when there is only a few users. The only significant addition in AS3000 is more end of line electrodes than we might have here.



The benefit may be between one and several dozen electrodes added to the network, one may not make much difference, but dozen may.

But they would condemn a TT installation, as MEN provides excellent earth path under installation fault conditions without so much reliance on a RCD.

Andy B

Edited: 22 April 2017 at 12:44 PM by sparkingchip
 22 April 2017 01:17 PM
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davezawadi

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I simply don't believe that there is a 10% risk of injury (whatever that means, severity undefined) but there are probably zero deaths. There may be the odd fire in appliances, but otherwise not much really, assuming that equipotential zones work as we understand. This is another attempt to deal with another non-problem, and is nothing to do with installations, only supply authorities! Assuming all our careful suggestions following the DPC are ignored again, why bother even thinking about it? They already have a statutory duty, what else can one reasonably do?

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 22 April 2017 01:52 PM
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sparkingchip

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Just imagine all the whinging there would be if we were told that we had to install a rod at every PME earthed installation with an overhead supply, before we could undertake and modifications or additions to the existing installation.

Andy
 22 April 2017 06:58 PM
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mapj1

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Originally posted by: sparkingchip

Just imagine all the whinging there would be if we were told that we had to install a rod at every PME earthed installation with an overhead supply, before we could undertake and modifications or additions to the existing installation.
Andy


Ooh. If it did happen, then give it a year or two and there would be weird questions from those too young to know the history.
"I've installed local electrodes and an RCD, so do I have to put an insulating junction in the wire to the DNO earth terminal, like we do for the gas main, as I think it is a metallic service that may introduce an external potential ?"

Many farms are of course wired like this already.

I'm with DZ and Lyle on this one - an open PEN is rare on any one location, and we know it is, from the fact it
sometimes makes the national news
or the local
Sometimes the victim more or less asks for trouble.

and as the phase voltages go all over the place it is normally pretty obvious, and folk tend to report their lights flashing and equipment smoking, so likely to be fixed quite fast.

Meanwhile there are hundreds of thousands of exposed metal thngs like lampposts on PME supplies, so nasty accidents as a fraction of the total are pretty rare.

There is a significant number of events country wide, and in high risk places like caravans, marinas and so on, where the risn of open PEN is probably unchanged, but the consequences may be less funny, then the current approach of TT islands is the correct one.

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


Edited: 22 April 2017 at 07:09 PM by mapj1
 22 April 2017 09:24 PM
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sparkingchip

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Going back to the original post:

"This opportunity to help mitigate the risk has also been recognised and considered by the Committees responsible for developing the 18th Edition of the IET Wiring Regulations. Requirements for foundation earthing, supplementary earth electrodes and over-voltage detection/protection devices may be included in the draft for public comment (DPC) to BS 7671: 2018 (18th edition of the Wiring Regulations)."

There isn't a suitable over-voltage device available at the moment is there? So that leaves supplementary earth electrodes as the option for most installations, which is the Antipodean way of doing things, MEN.

Andy B
 22 April 2017 09:27 PM
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AJJewsbury

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if I am going to sign up to a campaign, I don't think that it will be his!

Good - it wouldn't be much of a campaign if everyone just agreed!

assuming that equipotential zones work as we understand

I think that's part of what's getting more difficult - even though equipotential zones never were really equipotential (as true extraneous-conductive-parts will drag down 'corners' of the zone by differing amounts) it seems to be getting harder even to define the boundary of the zone as installations move further and further outdoors. When PME first arrived I think most homes' external electrical installation at most consisted of a single light above the front door - nowadays most have outdoor sockets, lights across the patio/decking, decorative cast-iron-effect 'lamp posts', outbuildings and soon enough EV charge points; and given the size of the typical building plot, hardly any of those will be entirely out of reach of one another.

So it seems that the goal posts are moving at both ends of the field - consumers' installations are moving outdoors, metallic piped services are being replaced by plastics, and (allegedly) the DNOs networks are deteriorating - so all in all past statistics might well not be a good guide to the extent or severity of future problems.

It would be interesting to wind the clock back and understand why we adopted PME in the first place - was it just a technique to keep corroding cables in service and save money on copper, or were there some other perceived benefits?
- Andy.
 22 April 2017 09:36 PM
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AJJewsbury

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"This opportunity to help mitigate the risk has also been recognised and considered by the Committees responsible for developing the 18th Edition of the IET Wiring Regulations. Requirements for foundation earthing, supplementary earth electrodes and over-voltage detection/protection devices may be included in the draft for public comment (DPC) to BS 7671: 2018 (18th edition of the Wiring Regulations)."

There isn't a suitable over-voltage device available at the moment is there?

There are such devices - a poster from Malta I think mentioned using one quite recently. But it occurs to me that it's another extra cost to the consumer - not just in initial purchase but in running costs too (even if it's only 5 or 10W for the OVR and contactor it'll add up). Really it's a supplier's problem and the supplier should sort it - so why not use smart meters to do it? They presumably already measure voltage (as well as current) and have a contactor, so it might even only be a software patch to add over-voltage protection functionality and no extra power consumption.
- Andy.
 22 April 2017 10:02 PM
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mapj1

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Actually much of the equipment damage of dropped neutrals can be avoided in a more or less regs compliant way, by having an RCD, and connecting a capacitor between the outgoing neutral and a local electrode - if the NE voltage rises above some preset value, such as that which would pass 30mA at an NE offset of 50V then the RCD will disconnect the load . Of course disconnecting L and N does not mitigate against the rise in CPC potential from other users per se, but removing load generally until the fault is corrected protects the loads, and if everyone does it reduces the risk of a really wild neutral behaviour.
I don't know for sure, but I suspect most if not all smart meters interrupt just L not both L and N, and even if they turned off on wildly out of spec voltages, and in some ways DP isolation in the manner of an RCD would be better.

-------------------------
regards Mike
 22 April 2017 10:30 PM
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sparkingchip

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They are sounding like home spun devices rather than a off the shelf solution, it might be time to go into design and production.

You get the ball rolling and I'll buy a few quids worth of shares in the company if it looks like a change in the regs is imminent.

Andy B
 22 April 2017 10:48 PM
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mapj1

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Not my idea - the original suggestion came from South Africa, where apparently copper theft commonly removes the neutral from the overhead lines and leaves the phases connected.
Other places solve this other ways Brazil for example puts the neutral above the phase wires, so that the first wires cut are the phases, and being live may act as a disincentive
A variation on the theme of the RCD and capacitor can be used to detect L-N reversal when using adventitious supplies at remote camps and similar locations. Not an issue on single phase, but on 3 phase quite serious, not as bad as the PEN, because such things are TN-S

-------------------------
regards Mike
 23 April 2017 10:59 AM
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AJJewsbury

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One problem with the RCD approach is that it latches off - so the home can remain blacked out, freezer defrosting etc long after the fault has been corrected.

Like I said, voltage monitoring relays are already available - e.g. http://uk.rs-online.com/web/p/...toring-relays/0146171/ (or for 3-phase http://uk.rs-online.com/web/p/...toring-relays/0146480/ ) - just team it up with a suitable contactor to switch off an entire installation while the supply fault remains.

Although if such devices become common it begs the question of what happens to grid stability if large numbers of installations start switching themselves off or on at the same time...

- Andy.
IET » Wiring and the regulations » Open PEN conductors.

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