Lost neutral on PME consequences

[pc1966]

how can you test the supply to find out earthing type or maybe we just assume a ZE less than 0.35 is most likely some variation of TNC-S.

I don't think there is any easy test.

If the PFC & PSCC are very similar then all it tells you is the earth and neutrals have similar impedances, it is not explicitly telling you if they are common fairly close to the property in TN-C-S style.

With the power off at your installation looking the the (supply side) N to E voltage would give to an idea of how much current imbalance & impedance was present on any PEN cable, but that is also likely to pick up some volts from inductive coupling from other loads in the network.

However, if that voltage is very low, say less then 0.1V or so, you could probably do a resistance measurement N-E to get an idea of how far away any common point is if you have an idea of the type of cable feeding you (e.g. 16mm split concentric or whatever) so you know R per unit length.

 

[LastManOnline]

how can you test the supply to find out earthing type or maybe we just assume a ZE less than 0.35 is most likely some variation of TNC-S.

"pc1966" sums that up nicely. What I would add (or ask) is how could making such a fundamental change in the supply network go un noted? Afterall an electrician would feel duty bound to inform a homeowner that replacing a wooden shed with a metal one might necessitate a TT...ing of the electrical supply to it for safety reasons (I, m trying to think in UK terms here and may stand to be corrected).
How then could a service provider change the nature of the system supplying a whole street without considering the potential knock on effects for the various homeowners who carried out installation under a previous supply system? Am I making any sense here?

 

[pc1966]

What I would add (or ask) is how could making such a fundamental change in the supply network go un noted?

I suspect the move to TN-C-S was started many years ago when (a) most folk did not have significant outdoor power requirements like EV and hot-tubs, and (b) when the cables going in were nice and new and joint failures of the outer PEN of aluminium waveform cables was not an expected thing.

Fast-forward 30 odd years and here we are.

• Proper TN-S would be the safest/best system, but costs the DNO that earth extra conductor.
• Going TT like much of the EU avoids the DNO expense and the open-PEN risk. But in turn it pushes the extra cost on the home owner as they need an incoming RCD (unusual over two decades ago before the regs demanded them on most circuits) and an earth rod, which in an urban area has all sorts of practical problems. Also they become dependant on the electronics for disconnecting on practically any fault to earth, and if it fails they are in a worse position than TN-C-S as only one earth rod of fairly high impedance (hundred-ish ohms probably).
• TN-C-S offers the reliable disconnect on OCPD operation (just like TN-S) and so no modifications to most existing installations that going TT would demand, and saves the DNO money.

It is not hard to see why we got here!

As it stands the regs on caravan supplies have basically forced TT use there for some time now, but that is a small proportion of installs. Only now are the rapid rise in EV use is focusing attention on the risks of open-PEN faults once more.

 

[timhoward]

And on that very topic I found this paper interesting:
(source https://myenergi.com/wp-content/uploads/2019/09/Protection-against-damaged-PEN-Conductor-r1_0.pdf )

The number of reported open PEN faults is higher than I'd have guessed (and I can't help imagine there are plenty not reported)



The paper also goes on to discuss 3 pole automatic switch based upon RCD principles by measuring the earth fault current, with a focus on EV charging points and the inherent dangers on TN-C-S with broken PEN conductor.

 

[marcuswareham]

I suspect the move to TN-C-S was started many years ago when (a) most folk did not have significant outdoor power requirements like EV and hot-tubs, and (b) when the cables going in were nice and new and joint failures of the outer PEN of aluminium waveform cables was not an expected thing.

Fast-forward 30 odd years and here we are.

• Proper TN-S would be the safest/best system, but costs the DNO that earth extra conductor.
• Going TT like much of the EU avoids the DNO expense and the open-PEN risk. But in turn it pushes the extra cost on the home owner as they need an incoming RCD (unusual over two decades ago before the regs demanded them on most circuits) and an earth rod, which in an urban area has all sorts of practical problems. Also they become dependant on the electronics for disconnecting on practically any fault to earth, and if it fails they are in a worse position than TN-C-S as only one earth rod of fairly high impedance (hundred-ish ohms probably).
• TN-C-S offers the reliable disconnect on OCPD operation (just like TN-S) and so no modifications to most existing installations that going TT would demand, and saves the DNO money.

It is not hard to see why we got here!

As it stands the regs on caravan supplies have basically forced TT use there for some time now, but that is a small proportion of installs. Only now are the rapid rise in EV use is focusing attention on the risks of open-PEN faults once more.

1 vote here for PROPER mantained TNS

 

[pc1966]

The number of reported open PEN faults is higher than I'd have guessed (and I can't help imagine there are plenty not reported)

I think they will all be reported.

The DNOs have that duty, and the huge voltage swings and possible damage to home owner's equipment makes it hard for the home owners to ignore/not report and have fixed!

 

[pc1966]

Realistically these days you have to assume and design for TN-C-S or TT.

TN-S might not remain that way, nor can you know just how 'S' it really is.

 

[marcuswareham]

I have always thought regulation 722.411.4.1 (iv) where protection from a PEN fault is by measuring the utilization voltage between L & N and then disconnecting all conductors if it is outside the range of 207v - 253v, as a bit of a cop-out and only really solving half the problem as it is possible to have a PEN fault and the voltage to remain within that range

I have been thinking however the as a PEN conductor has multiple electrodes along its length so a break in the PEN conductor should still ensure you are connected to one or more electrodes, but I guess the weakest points are the joints and in particular the joints where individual houses etc connect to the PEN conductor, So would it be the case that most PEN faults occur at a joint where it branches off to feed a house also the section to the house is very unlikely to have any electrodes on, but in this case as it is only one house affected the above 722.411.4.1(iv) would work in most cases (subject to extraneous parts which are bonded), so is this why it is accepted as a way to mitigate PEN faults as in most cases it will protect due to the above

Attached is a crap picture to show the joints I am trying to convey

 

[timhoward]

the weakest points are the joints and in particular the joints where individual houses etc connect to the PEN conductor

I know exactly what you are saying, and I have no experience with the public distribution side but I wonder if it is three conductors coming together at that joint location in a resin joint, and a joint failure would also sever the ongoing PEN to the next joint.
On large private estates I've seen it done like this (and had to fix it after a mini-digger driver had a bad day!).

 

[snowhead]

50 or so houses were affected where my Son was living a few years ago, spread across all 3 phases.
It was a newish estate of around 50 or so houses and the Neutral connection failed where the estate was connected to the existing cable in the main road.
It cost Western Power tens of thousands to sort out the damage caused.

 

[marcuswareham]

I know exactly what you are saying, and I have no experience with the public distribution side but I wonder if it is three conductors coming together at that joint location in a resin joint, and a joint failure would also sever the ongoing PEN to the next joint.
On large private estates I've seen it done like this (and had to fix it after a mini-digger driver had a bad day!).

Yeah i hear ya, likewise I have no experience with anything on the DNO side etc.

I can see the joints to my house as its on the top of a pole at the end of my drive and I looks to be like i explained in my last post

 

[marcuswareham]

50 or so houses were affected where my Son was living a few years ago, spread across all 3 phases.
It was a newish estate of around 50 or so houses and the Neutral connection failed where the estate was connected to the existing cable in the main road.
It cost Western Power tens of thousands to sort out the damage caused.

Flipping heck

Knowbody received a shock then?

 

[snowhead]

Flipping heck

Knowbody received a shock then?

No deadly shocks.
But Daughter in law got a tingle from a socket screw when she unplugged her phone charger, whilst standing on a laminate wood floor laid on concrete.
It was the first indication of something being wrong, it was weekday during the morning and she was off on maternity leave.
She rang a few neighbours and they all said various items had stopped working during the morning, including someone's Plasma Tv that had stopped working with a puff of smoke out the top.

 

[DPG]

I have always thought regulation 722.411.4.1 (iv) where protection from a PEN fault is by measuring the utilization voltage between L & N and then disconnecting all conductors if it is outside the range of 207v - 253v, as a bit of a cop-out and only really solving half the problem as it is possible to have a PEN fault and the voltage to remain within that range

I have been thinking however the as a PEN conductor has multiple electrodes along its length so a break in the PEN conductor should still ensure you are connected to one or more electrodes, but I guess the weakest points are the joints and in particular the joints where individual houses etc connect to the PEN conductor, So would it be the case that most PEN faults occur at a joint where it branches off to feed a house also the section to the house is very unlikely to have any electrodes on, but in this case as it is only one house affected the above 722.411.4.1(iv) would work in most cases (subject to extraneous parts which are bonded), so is this why it is accepted as a way to mitigate PEN faults as in most cases it will protect due to the above

Attached is a crap picture to show the joints I am trying to convey

I think this is where a lot of the problems occur. It has happened on my estate more than twice. One was at my house where my cable is tapped off the cables running down the street under the pavement.

First we knew of it was when I used the hoover on the garage and it was struggling a bit. I put this down to it being a bit old initially. Then a couple of days later my wife got a tingle from the metal clad light switch in the garage. I got straight on to them and to be fair they ca.e out within a couple of hours from what I remember. They put a temporary feed across my drive from next door's cut-out until the jointers came and repaired things.

The jointers told me it's fairly common, especially with 1970s era housing.

 

[marcuswareham]

Iv just seen this video

so perhaps joint failure as i mentioned will be less of a concern in the future, although i exspect for now there there is plenty of old bad joints