When is TNS really PME and vice versa?

[Gavin John Hyde]

Had a interesting phonecall a moment ago with western power.
Have a customer wanting a ev charging point, so checking the supply and due to the old cut out appearing as a TNS and also having a PME sticker on it, I rang western power; they said that its now PME as according to the records there was an issue with the cable being damaged when the extension was built a few years back prior to this person having the property and the cable was replaced outside and turned into PME. They repaired cable but didnt replace the cut out. which given its age would have made sense!

Western Power said that the vast majority of properties now are really PME even if they dont appear as such given the cut out, earthing set up etc, as the amount of work thats gone on over the years in the roads and streets has meant very few can still be considered as proper TNS. The engineer told me that over the next few years they are discussing the idea of doing some work to identify remaining TNS systems within urban areas, not so much out in the sticks and change all over to PME and electricians should then unless a rod is present and clearly a TT system treat all systems as PME unless the DNO tell them otherwise.

It makes for an interesting world but it makes you wonder if the DNO really know whats on the network until it goes wrong...

 

[Wilko]

Yes probably safer to assume TNCS for design purposes nowadays.

 

[Midwest]

I had a similar story from my DNO. Every time they repair a cable, they convert it to PME.

 

[Deleted member 105166]

I rang western power; they said that its now PME as according to the records there was an issue with the cable being damaged when the extension was built a few years back prior to this person having the property and the cable was replaced outside and turned into PME. They repaired cable but didnt replace the cut out. which given its age would have made sense!

So, if I understand correctly, (correct me if I'm wrong), it's essentially TNS for just a few metres then TNCS from a join outside the property back to the transformer.

 

[Gavin John Hyde]

So, if I understand correctly, (correct me if I'm wrong), it's essentially TNS for just a few metres then TNCS from a join outside the property back to the transformer.

Effectively yes, hence why they put a pme sticker on the head. and why I had to ring them to clarify. its quite common these days apparently,

 

[Deleted member 105166]

So when the DNO do this modification, they're hoping for a Ze of <0.35Ω and where they don't achieve this they then have to go whole hog and extend the new cable inside the premises to the head?

 

[Ian1981]

Think there was an article in professional electrician magazine a few months back stating from the dno’s That all systems when in doubt should be assumed to be TN-C-S.
If I can dig it out I’ll upload it.

 

[ipf]

So when the DNO do this modification, they're hoping for a Ze of <0.35Ω and where they don't achieve this they then have to go whole hog and extend the new cable inside the premises to the head?

Nope......it's cheaper and less awkward to convert rather than repair to the original state.

 

[Midwest]

Effectively yes, hence why they put a pme sticker on the head. and why I had to ring them to clarify. its quite common these days apparently,

In my instance, the engineer demonstrated how to turn my TN-S into TN-C-S.

 

[Deleted member 105166]

Nope......it's cheaper and less awkward to convert rather than repair to the original state.

OK, I think I understand - as lead is 7% the conductivity of copper, irrespective of distance from the transformer and a small remaining section of lead, if the former Ze was <0.8Ω, the new Ze is going to always be well within <0.35Ω ?

 

[Percyprod]

As a retired dno fault technician I can assure you that certainly my dno's records are not always accurate! We had a laptop with the complete system drawings on, and most of the more up to date records were good, the older ones were copied from the old paper records. As some of the cables had been in from the year dot, it's no surprise that the records were either wrong or non existent.

 

[Delboybully]

I can't remember the last time I had a tns that had a ze that would be too high for a tncs.

A dno jointer once told me that at every joint that they attend to due to a fault is then pmed. He said in effect nearly all their supplies is now pme (except tt systems).

 

[westward10]

The earthing system is judged by the way it enters the building. If the neutral and earth are not combined it is TN-S regardless as to whether the external system is effectively now PME/TN-C-S.

 

[Deleted member 105166]

So where a DNO has placed a PME sticker on the head, it should be disregarded...

 

[Gavin John Hyde]

When installing ev charge points if any doubt about whether supply is tns all way back to transformer the rules on pme apply and you have to consider a rod for the charger. It was from calling dno due to having what appeared tns but also having a pme sticker that they told me practically everything is pme now as its been worked on somewhere along the line

 

[westward10]

If it enters as TN-S you have to consider it as such. But I agree in most cases now it is likely to be PME/TN-C-S outside.

 

[Delboybully]

The thing is the DNO call it pme. We call it tncs. As far as we are concerned as Westwood said, if it it tns as it enters a property that is what we should treat it as.

There is a difference between tncs (tera neutral combined seperate) and pme (protective multiple earthing). You can have tns and a pme system and that is the difference

 

[Ian1981]

If it’s tncs outside but separate earth entering the property then I’d imagine the dangers or an open circuit neutral in the road will still cause Potential for danger in the installation regarding exposed conductive parts rising to or near mains potential as any tncs installation would.

 

[Marvo]

I think it would be inadvisable to 'assume' the DNO supply to be PME if it appears TNS at the service head. If for some reason the supply is thoroughbred TNS then creating a N-E link anywhere would split the neutral current at best and could result in enormous current through that link in the case of an upstream DNO neutral fault in the future.

 

[mjb]

When I worked for DNO, if it did not have a PME label it was not PME regardless.