Lost neutral on PME consequences

LastManOnline · Mar 30, 2021

marcuswareham said:
Another interesting PME electrode, rod just chucked in the hole

View attachment 83965

Good photo. We are not used to see rods " horizontal". The hole its in though looks reasonably deep so when the backfilling is done and earth compacted it should be as effective as when hammered in vertically. Still a lot better than the "pigtail" in one of your other previous(and very interesting) photos. And I noticed its all "denzoed up" too.

pc1966 · Mar 30, 2021

A horizontal rod at that depth is probably better than the same rod driven in vertically as you won't have much risk of the top section drying out or freezing and going high-impedance.

But equally I would be surprised it that rod was much below 50 ohms.

marcuswareham · Mar 30, 2021

I guess it shows they don't test them, as the joint is already filled with resin and they cant access it or disconnect the cable joining it to the PEN once the earth is filled back in, same for the pig tails i suspect

LastManOnline · Mar 30, 2021

pc1966 said:
A horizontal rod at that depth is probably better than the same rod driven in vertically as you won't have much risk of the top section drying out or freezing and going high-impedance.

But equally I would be surprised it that rod was much below 50 ohms.

Agreed. It begs the question as to why they don't stick in several rods rather than one 4 foot rod.And do it at every joint or pole. The cost would be minimal yet the effect on reducing touch voltage under open PEN could be significant.

LastManOnline · Mar 30, 2021

marcuswareham said:
I guess it shows they don't test them, as the joint is already filled with resin and they cant access it or disconnect the cable joining it to the PEN once the earth is filled back in, same for the pig tails i suspect

Agreed. The "hit and hope" aspect of PME is more than a little baffling

marcuswareham · Mar 30, 2021

So my village has 5 PME electrodes at the bottom of the service poles. If I assume each one has a resistance of 30ohms (which is probably optimistic) the parallels resistance of all of them is 6.7ohm. If there was a PEN fault between the first pole with a rod and the transformer using the calculation from A722.3 around 12A would be the max amount of current to keep the touch voltage on the broken PEN below 70v, The transformer and cable route in question supply around 90 houses so at any given time I would imagine the load to be considerably more than 12A

So looking at this it seems the mitigation of using PME on a TN-C-S supply is a poor solution (almost pointless), perhaps that's why the DNOs often don't put PME electrodes in as mentioned in the video from GSH

Please tell me if I have calculated this wrong,

LastManOnline · Mar 30, 2021

marcuswareham said:
So my village has 5 PME electrodes at the bottom of the service poles. If I assume each one has a resistance of 30ohms (which is probably optimistic) the parallels resistance of all of them is 6.7ohm. If there was a PEN fault between the first pole with a rod and the transformer using the calculation from A722.3 around 12A would be the max amount of current to keep the touch voltage on the broken PEN below 70v, The transformer and cable route in question supply around 90 houses so at any given time I would imagine the load to be considerably more than 12A

So looking at this it seems the mitigation of using PME on a TN-C-S supply is a poor solution (almost pointless), perhaps that's why the DNOs often don't put PME electrodes in as mentioned in the video from GSH

Please tell me if I have calculated this wrong,

Looks like a very interesting calculation you have going on there and an even more interesting outcome. Sounds good to me but I,m not confident I can corroborate it.I think "pc1966" will look forward to getting his teeth in to it though?

marcuswareham · Apr 1, 2021

So historically true TN-S with the old bitumen type cable was most common, then PME came along and now most of the UK is PME and alot new installations

So what changed? , I guess the DNOs figured they could save some money in losing an extra conductor,

As mentioned before in the thread repairs to older TN-S networks are being made as PME as the DNO can no longer get the cables etc,
Is this because the cable manufacture stopped making the cable so the DNOs had to stop using it?
OR is it because the DNOs stopped using it that the cable manufacture then decided to stop making it due to low demand

Also what is the DNOs excuse for using a less safe system when the availability of the TN-S type of cable is not an issue: Just like where I live we are supplied overhead and they are separated cables on the poles, so each cable is a single conductor, no reason they cannot add another and make it TN-S rather than TN-C-S

If more people started to get injured or worse killed due to PEN faults with a bigger uptake of electric cars and car chargers on PME without good enough PEN fault mitigation would the DNOs be held responsible in court, would there be an inquiry as to why they didn't use TN-S. I guess not and instead it would be the electrician who installed it in court (even if he complied with reg 722.411.4.1(iv) & that reg itself didn't provide enough mitigation for a PEN fault)

LastManOnline · Apr 1, 2021

marcuswareham said:
So historically true TN-S with the old bitumen type cable was most common, then PME came along and now most of the UK is PME and alot new installations

So what changed? , I guess the DNOs figured they could save some money in losing an extra conductor,

As mentioned before in the thread repairs to older TN-S networks are being made as PME as the DNO can no longer get the cables etc,
Is this because the cable manufacture stopped making the cable so the DNOs had to stop using it?
OR is it because the DNOs stopped using it that the cable manufacture then decided to stop making it due to low demand

Also what is the DNOs excuse for using a less safe system when the availability of the TN-S type of cable is not an issue: Just like where I live we are supplied overhead and they are separated cables on the poles, so each cable is a single conductor, no reason they cannot add another and make it TN-S rather than TN-C-S

If more people started to get injured or worse killed due to PEN faults with a bigger uptake of electric cars and car chargers on PME without good enough PEN fault mitigation would the DNOs be held responsible in court, would there be an inquiry as to why they didn't use TN-S. I guess not and instead it would be the electrician who installed it in court (even if he complied with reg 722.411.4.1(iv) & that reg itself didn't provide enough mitigation for a PEN fault)

From any DNO, s point of view TNC-S is cost wise the most attractive supply system. However, PR wise it would, nt look great to highlight that aspect of it. All countries faced the same issues in the 90,s due to the rapid transformation of metallic services to plastic. Most countries had up until that point TT systems, using the metallic services to create a low resistance fault path. Very few countries had a TN-S supply system. I have some sympathy for the situation the DNO, s found themselves in at the time. The solution to convert to TNC-S, from a cost and convenience point of view must have been very hard to resist. Two feet of 10mm (or 16mm) earth and job done.
Historically, TNC-S was obviously avoided for reasons that have been discussed all too often on this forum.
Your other questions such as "why not run an extra conductor along with the other overhead conductors?" is one I can't answer but I think it's an opportunity missed. In my housing estate of 50 homes, imagine just dividing up the cost of that extra conductor between 50 households to create a TN-S system?. At time of installation it might have been an extra 100 euro on to the cost of the house. A great investment.
Obviously due to EV chargers and the increase in metalwork outside the home, this issue will become more rather than less prominent.
I can, t comment on who would be most likely to be held responsible in the eve t of a unfortunate incident

marcuswareham · Apr 5, 2021

Was round my mums building some raised beds, and on her road UKPN were putting a new supply in for a house down the road, on my way back to the car UKPN had gone home so I had a little look in the whole, someone also left the meter box open on the side of the house and it has a new PME service head, here is the joint, doesn't look to have an earth electrode to me

[ElectriciansForums.net] Lost neutral on PME consequences

I also wonder how street lights are allowed on PME

LastManOnline · Apr 5, 2021

marcuswareham said:
Was round my mums building some raised beds, and on her road UKPN were putting a new supply in for a house down the road, on my way back to the car UKPN had gone home so I had a little look in the whole, someone also left the meter box open on the side of the house and it has a new PME service head, here is the joint, doesn't look to have an earth electrode to me

View attachment 84259
I also wonder how street lights are allowed on PME

I wonder if that black cable running out of the cable joint is, nt going to an earth electrode?

LastManOnline · Apr 5, 2021

marcuswareham said:
So looking at this it seems the mitigation of using PME on a TN-C-S supply is a poor solution (almost pointless), perhaps that's why the DNOs often don't put PME electrodes in as mentioned in the video from GSH

I have been "walk checking" TNC-S supplies to "stand alone" houses in the countryside in recent times. Often a house will have its own pole mounted trafo. It's all TNC-S here so I, m really curious to know what type of earth electrode the DNO is installing. As they (pole trafo, s) are all installed on private property,I have, nt been able to get up close enough yet to inspect one. The other difficulty is that even in getting up close to inspect one, you still cannot know what kind of electrode has been installed in the ground. But bearing in mind modern houses have no alternative earth paths, it would have to be a very, very significant electrode to be of any use.

marcuswareham · Apr 5, 2021

LastManOnline said:
I wonder if that black cable running out of the cable joint is, nt going to an earth electrode?

The black cable running out of the joint on the left of the image is the feed to the new suppy head,

I am not sure what the black cable in the middle is, it looks like it runs under the joint, but I would have thought any cables would have to come out of the ends, I think when i looked it thought the black one in the middle was a bit or PVC tape or cable tie, but it does look like a cable on the image maybe its an electrode cable

marcuswareham · Apr 5, 2021

LastManOnline said:
I have been "walk checking" TNC-S supplies to "stand alone" houses in the countryside in recent times. Often a house will have its own pole mounted trafo. It's all TNC-S here so I, m really curious to know what type of earth electrode the DNO is installing. As they (pole trafo, s) are all installed on private property,I have, nt been able to get up close enough yet to inspect one. The other difficulty is that even in getting up close to inspect one, you still cannot know what kind of electrode has been installed in the ground. But bearing in mind modern houses have no alternative earth paths, it would have to be a very, very significant electrode to be of any use.

Interesting, my village has a transformer mounted on 2 poles, its very easy to get to as its on a public footpath, this seems to have a rod much like the ones at the bottom of the services poles, the dog keeps the resistance down by keeping it watered ?

There is another very small fenced in sub station on the sane path with a transformer of a similar size of a pole mounted one all cables to this are underground, I guess this area could have an earth mat, there is no evidence of a rod from what you can see

mattg4321 · Apr 5, 2021

Not normally electrodes but big copper plates I've seen DNO use round here on properties with their own transformer

LastManOnline · Apr 5, 2021

mattg4321 said:
Not normally electrodes but big copper plates I've seen DNO use round here on properties with their own transformer

I appreciate you meant to draw a distinction between an "earth rod" and a "copper plate". In principle an electrode covers any type of conductive material designed to make contact with the ground. I suspect (but cannot confirm yet) that the electrodes (whatever type) used over here in TNC-S systems are not adequate in "stand alone" electrical installations.

marcuswareham · Apr 22, 2021

Walked past another bit of pavement UKPN have dug up tonight

They have been repairing some cable, they have put 2 new sections of cable in with 4 joints, none of which have electrodes. There is also a smaller joint that has new TN-C-S cable on one side of the joint and an old TNS bitumen covered style cable on the otherside, I guess this was to connect a house to the new cable. Again no earth electrode and goes to show TN-S is almost always TN-C-S

LastManOnline · Apr 22, 2021

Again very interesting photo, s. One thing that strikes me as odd is the conversion of the supply system does not appear to be followed up in an orderly manner by informing the homeowner and posting a notice somewhere, like on the DNO, s website, confirming a significant change has taken place and that new safety measures need to be considered (open PEN sensors etc)