TT and TNCS (PME) at same address!

[Piratepete]

I'm working at a farm with a 3 phase PME supply to a cupboard in the house containing the house CU and two 3 phase isolators feeding SWA cables to other parts of the farm.
There's an earth terminal block connected to the PME connection and the SWA armour and earth cores; but not to the earth terminal on the CU. This may be connected to an earth rod (I've got to confirm this).
Ze at the CU is 5.1 ohms (fine for TT). Ze for the other supplies is 0.22 ohms.

Is there any reason why I shouldn't connect the CU earth to the PME terminal?

Cheers.
Pete

 

[NMc]

Very unusual. A supply company may bring a PME supply into a property but only make a TT supply available for take-off. Maybe somebody before you has connected as PME not know what that the supply was PME


Check with the DNO to see what supply has been made available. If it is TN-C-S the you will have to bring the cu installation up to TN-C-S standards before using the available earth facility

 

[Deleted member 9648]

Tncs supplies are not generally used for farm supplies,as there are too many complications with bonding. It would be expected that the house CU would be Tncs,and the farm supplies separated...it is possible that the PME has just been used to protect the SWA's and separated at the farm buildings.....Further investigation is required.

 

[NMc]

Good point wirepuller. the farm itself is a special installation in part 7

 

[Deleted member 26818]

The residential parts of the Farm, are allowed to be PME.
It is just the parts of the farm where there may be livestock that restrictions apply.

 

[Piratepete]

Hi Guys
As usual your answers have raised more questions!
1. More investigation of SWAs required
2. So I can PME the CU? It currently has a 30mA RCD Main Switch. Would I be expected to upgrade it to a 17th Ed Dual RCD unit?
Cheers
Pete

 

[mike the sparky]

I work on a large house which had tns, tncs and tt all going in a different places. We moved the incoming supply with the local electricty board to incorporate a back up generator and it was then set as a tncs system.
As I see it, it depends on how old the cable is and what work is involved. 3 core pme is cheaper and easier to install; 4 core tns is still available usually on steel buildings where high start-up currents cause problems with RCD's tripping.
TT should always be used on farm buildings and tncs to the farm house if available.