Lost neutral on PME consequences

[LastManOnline]

I've been in both situations (both years ago)- surprised to not get a jolt standing on soil when it had just rained after I messed up isolating an outside light, and surprised to get a jolt on a wooden step ladder on ary concrete floor.

While we all understandably conclude that standing "outside" is very dangerous and standing "inside" is very safe, it is, nt always, as clearcut as that. "outside" earth will be very dry and very unconductive if you have a few weeks of good dry summer days. Add on to that someone wearing good shoes and sockets (not after a hard day's work and perspiration) and you will certainly have a high resistance path. On the "inside" as mentioned by other posters, if you have an older home with a concrete floor with no damp coarse you will certainly feel a tingle and perhaps something more. I recently "felt" 27 volts due to a faulty shower while wearing shoes and socks and my feeling afterwards was "Well I can understand the homeowners panicky phonecall now" as he received the same shock with water flowing down around his feet.
I must also say that regardless of any situation I encountered there was never any mystery to the shock felt. It could all be explained by Ohms law.

 

[LastManOnline]

4) What, if anything, is in place to protect against these sorts of dangerous situations occurring?

I, ve just read the detail regarding that very, very sad case of the 7 year old electrocuted in a pub garden due to a seriously underspec electrical intallation.
Its sobering to consider that the exact same problem could occur with the same fatal consequences in a PME installation considered up to spec.

 

[marcuswareham]

I, ve just read the detail regarding that very, very sad case of the 7 year old electrocuted in a pub garden due to a seriously underspec electrical intallation.
Its sobering to consider that the exact same problem could occur with the same fatal consequences in a PME installation considered up to spec.

Quite! although rare, it seems strange too me so many people accept that risk !

 

[LastManOnline]

Quite! although rare, it seems strange too me so many people accept that risk !

Yes.I think though the tide of opinion is gradually changing towards "accepting that risk" within our industry.

 

[timhoward]

I, ve just read the detail regarding that very, very sad case of the 7 year old electrocuted in a pub garden due to a seriously underspec electrical intallation.
Its sobering to consider that the exact same problem could occur with the same fatal consequences in a PME installation considered up to spec.

Very tragic case. Unless I'm missing something I guess you mean with broken upstream Neutral as per this thread or exposure to live parts (as was case in the water filled light you referred to).

Also sobering, this isn't exactly a new debate - I spotted the following discussion elsewhere several years ago which is rather poignant:

 

[marcuswareham]

Very tragic case. Unless I'm missing something I guess you mean with broken upstream Neutral as per this thread or exposure to live parts (as was case in the water filled light you referred to).

Also sobering, this isn't exactly a new debate - I spotted the following discussion elsewhere several years ago which is rather poignant:
View attachment 65376

Exsactly whey my outside lights are SELV or double insulated, they are in easy reach and one of them is likely to be touched

 

[LastManOnline]

Also sobering, this isn't exactly a new debate -

Correct. It's an old debate. However, far from being yesterday's news it has the potential to become very current with one single high profile incident. The example below (outside light) would probably have constituted the vast bulk of "at risk" metalwork under open PEN , 25 years ago.
Today the level of "at risk" exposed metal parts outside the home has changed enormously (electric gates, heat pumps to mention but a few). Add to that the DSO, s approach of employing largely TNC-S for new installs plus converting existing TNS cables to TNC-S when repairs are required.The electrical landscape is starting to look very different and with that change the potential for open PEN issues can only increase.

View attachment 65376

 

[marcuswareham]

Correct. It's an old debate. However, far from being yesterday's news it has the potential to become very current with one single high profile incident. The example below (outside light) would probably have constituted the vast bulk of "at risk" metalwork under open PEN , 25 years ago.
Today the level of "at risk" exposed metal parts outside the home has changed enormously (electric gates, heat pumps to mention but a few). Add to that the DSO, s approach of employing largely TNC-S for new installs plus converting existing TNS cables to TNC-S when repairs are required.The electrical landscape is starting to look very different and with that change the potential for open PEN issues can only increase.

The conversion of old TNS to TNCS in the street and the head staying the same inside the property I feel is a concern as a spark could install something (hot tub, heat pump, etc etc) outside believing the loss of PEN conductor risk is not an issue due to the fact it is TNS, him being unaware it is really now TNCS

 

[LastManOnline]

The conversion of old TNS to TNCS in the street and the head staying the same inside the property I feel is a concern as a spark could install something (hot tub, heat pump, etc etc) outside believing the loss of PEN conductor risk is not an issue due to the fact it is TNS, him being unaware it is really now TNCS

I suspect that an experienced spark will likely test the supply and be pretty sure what he is dealing (or perhaps ring the DSO) but, yea to make a major change in the supply system by converting a TNS to a TNC-S and not leaving a very clearly marked label or"record " of it behind for the homeowner (and future electrician) is I feel very, very poor practice.

 

[newfutile]

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]

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.