Lost neutral on PME consequences

[pc1966]

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

That bit of joint work was impressive, certainly gives some confidence in new installs. Full marks to the engineer who did that work for the video example.

But those are not the problem I suspect!

Same with the proposed building earth under the proposed 18th Amendment 2, great for now stuff, not really dealing with the last 30 odd years of deployment.

 

[marcuswareham]

Just seen a picture of an older PME joint

Apparently the copper "pigtail" in the left of the frame is a PME electrode, I wonder what kind of resistance that has, doesnt look like it has much surface area to be in contact with earth.

 

[marcuswareham]

I have been told by someone from the DNO fixing a power cut that this is a PME electrode at the bottom of a service pole in my village, At least my PME looks to have a much better electrodes that the last post with the underground joint. It seems as if every pole with a joint in my village has an electrode (not a joint to someones house, A joint in the main run)

 

[LastManOnline]

I have been told by someone from the DNO fixing a power cut that this is a PME electrode at the bottom of a service pole in my village, At least my PME looks to have a much better electrodes that the last post with the underground joint. It seems as if every pole with a joint in my village has an electrode (not a joint to someones house, A joint in the main run)

View attachment 83493

Well exhibit "B" (from your village) definitely inspires more confidence than exhibit "A" (pigtail). But regardless TNC-S remains the black sheep of the electrical family. There is still no practical means of testing the effectiveness of the DNO,s PME rods. If a homeowner was to ask me tomorrow "How protected am I under an open PEN fault?", probably the best indicator you can give is the physical distance the home is located in relation to the DNO, s distribution trafo

 

[marcuswareham]

Another interesting PME electrode, rod just chucked in the hole

 

[LastManOnline]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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


I also wonder how street lights are allowed on PME