Portakabin supply and TNCS

[Oscar21]

I have to connect up 3 site cabins which are to be used on a permanent basis, one is a portakabin type and the other two are container type cabins (one a toilet block). A new 3 phase underground TNCS supply has been put in (permanent not temp supply).

Now I know you are not meant to export the earth and I intend to TT the three cabins but cant decide whether it would be a good or bad idea to drop and earth rod at the supply head as well as the cabins, I would say a good idea as it shortens the fault path under normal conditions. And the PME supply should have various stakes along its length anyway, but I am open to critisism as I dont do this type of install very often.

This also got me thinking, what would be the problem with exporting the earth and putting rods in as well, getting the benefit of a good earth and also having the backup of the TT supply in the event of a dropped neutral. Apart from a faulty RCD.

 

[Sidewinder]

What you mean true PME?...

 

[Oscar21]

What you mean true PME?...

Dont get you, its a PME supply (3 core ali cable wrapped in the bare earth/neutral, with sheathing of course) and the link across the N-E. Didn't se what type of cable it was jointed to though.

Or are you saying the DNO dont bother to stake their PME supplies.

 

[Sidewinder]

DNO may or may not stake, however they provide TN-C-S, NOT PME, in ESQCR this is supposed to be PME, but they have until the end of next year as I understand it to make it PME.

Think of what PME stands for...

Then think again about my other post!

 

[Oscar21]

So why stick red "PME" stickers on cutouts then? anyway the joint is not far from the sub station so probably no need to put an interim rod in between. I would be doing my bit towards a true PME by rodding at the service head though, wouldn't I?

When we did a block of 10 apartments the other year the DNO brought a 3 phase supply in and they rodded the CNE at the joint on the main road and also put a rod in just outside the building wired to the MET.

 

[steptoe]

1st point, you CANNOT mix earthing systems,

you either TT it or PME it,

you as designer need to decide what you wish to do.

---------- Post Auto-Merged at 22:57 ---------- Previous post was made at 22:55 ----------

So why stick red "PME" stickers on cutouts then? anyway the joint is not far from the sub station so probably no need to put an interim rod in between. I would be doing my bit towards a true PME by rodding at the service head though, wouldn't I?When we did a block of 10 apartments the other year the DNO brought a 3 phase supply in and they rodded the CNE at the joint on the main road and also put a rod in just outside the building wired to the MET.

they should NOT have done that,

the MET is NOTHING to do with them, they are NOT allowed to touch it, their PME rod should be connected either inside the HED or preferably straight to the cable just as it enters the property on the outside.

 

[Oscar21]

TT'd the three cabins got 61, 48 and 48 ohms, didn't bother to rod the service head. Job done.

 

[steptoe]

at 48 ohms Ra then I would hazard a guess its deffo NOT PME that the DNO have supplied you with.

 

[Oscar21]

1st point, you CANNOT mix earthing systems,you either TT it or PME it,

you as designer need to decide what you wish to do.

---------- Post Auto-Merged at 22:57 ---------- Previous post was made at 22:55 ----------

they should NOT have done that,

the MET is NOTHING to do with them, they are NOT allowed to touch it, their PME rod should be connected either inside the HED or preferably straight to the cable just as it enters the property on the outside.

When I said MET I meant in the ryfield board.

I always thought PME was a condition of supplying a TNC-S supply, common sense really. But now I know a TNC-S is not a PME but a PME is a TNC-S, well you live and learn.

 

[steptoe]

When I said MET I meant in the ryfield board.I always thought PME was a condition of supplying a TNC-S supply, common sense really. But now I know a TNC-S is not a PME but a PME is a TNC-S, well you live and learn.

whats that?

is that a generic term? sorry, I really have no idea.

its a common misconception about TNCS and PME BTW so not to worry.