You can see one of them in the picture!
is this TN-S or PME?
- Thread starter THRIPSTER
- Start date
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Multiple Earths
On a newly installed PME system the neutral and steel work is earthed at the substation, either with values of below 20ohm for the LV & 40ohm for the HV steelwork. More usually it will be a combined earth of a value below 1 ohm.
At the end of each main we would install a single earth rod connected to the CNE.
If there is any SNE cable involved of the older lead sheath type, that sheath (and any armouring) is also classed as an earth electrode as well as providing the earth continuity and connection.
If we do service connections or joints from SNE to CNE we would also install a single rod
(Overhead networks tend to have a lot more earth rods at intervals along the line)
On a newly installed PME system the neutral and steel work is earthed at the substation, either with values of below 20ohm for the LV & 40ohm for the HV steelwork. More usually it will be a combined earth of a value below 1 ohm.
At the end of each main we would install a single earth rod connected to the CNE.
If there is any SNE cable involved of the older lead sheath type, that sheath (and any armouring) is also classed as an earth electrode as well as providing the earth continuity and connection.
If we do service connections or joints from SNE to CNE we would also install a single rod
(Overhead networks tend to have a lot more earth rods at intervals along the line)
Indeed I can, but I didn't think that one counted as 'multiple'!
Kind Regards, John
Fair enough. Does that mean that , in an underground 'fully CNE' situation (CNE all the way from transformer to consumers), the 'multiple' may just be one earth at the substation and one at the end of the main? I must say that I had thought/assumed that there would be a lot more than that.
Kind Regards, John
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If it was a single main from the substation with only one end (i.e, no branches), yes
Thanks. When you say 'no branches', I presume it is accepted that there will be consumers'services connected to the main along its length? I think I had previously assumed that there would probably be an earth at each of those connection points.
Kind Regards, John
Yes that is correct, A branch would be another main jointed off the original (think of a tree trunk & branches)
There never have been, nor at the end of the services.
Thanks. One learns something every day! I had always thought (assumed - hence 'never assume'!) that PME earthing was a lot more 'multiple' than you are indicating. If I understand you correctly, the only difference, earthing-wise, between a PMEd unbranching CNE main (serving many consumer's services) and a TN-S/SNE situation could be just one earth at the very end of the "PMEd" CNE main.
Kind Regards, John
Got it in one!
Just to be awkward.....
If a service supplies an individual customer, what do you call a cable from a main to say, a pair of houses with a loop taken from one cutout to another, or a small block of flats? Is that a shared service? Or a sub-main?
That is a master & loop service.
If more than 6 (I think) with centralised seperate metering at the end, that would be classed as a main so we would also install an earth rod
I would say it is TNS.
Just because it has a label on it saying it is PME does not make it PME. As far as i can see, you just have 2 earthing points, 1 clamped, the other sweated, to the lead sheaf of the incoming servise.
For it to be PME, the 2 earths would of been removed and a short piece of G/Y would of been taken from the Neutral block to the MET.
One explernation, the DNO was called out a high Ze, found that the solder was crumbling away so cleaned up a piece of the lead sheaf and fitted a new clamp, fitted the henley block because the braid would not fit in the MET but the only block he had, already had a PME lable on it and he could not be bothered/forgot to remove it.
As has already been said, the only sure way is to phone the DNO.
Just because it has a label on it saying it is PME does not make it PME. As far as i can see, you just have 2 earthing points, 1 clamped, the other sweated, to the lead sheaf of the incoming servise.
For it to be PME, the 2 earths would of been removed and a short piece of G/Y would of been taken from the Neutral block to the MET.
One explernation, the DNO was called out a high Ze, found that the solder was crumbling away so cleaned up a piece of the lead sheaf and fitted a new clamp, fitted the henley block because the braid would not fit in the MET but the only block he had, already had a PME lable on it and he could not be bothered/forgot to remove it.
As has already been said, the only sure way is to phone the DNO.
Untill a couple of days ago, I would have thought that it can't possibly be TN-C-S (so must be TN-S), since the DNO have not provided an earth derived from a CNE conductor within the premises. However, as a result of all the discussion in this thread, I'm now far from certain ...
...westie, Wontdothatagain, or whoever ... if there were a CNE main which fed a service to a consumer through a SNE cable, would/could you call that TN-C-S?
Kind Regards, John
Deleted, just re-read your post again, not sure, westie would answer that better.
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