Export PME?

Colin33 · Sep 17, 2011

Sorry, that boring subject again about which so much has been written. However, I would just like to hear views on the following: 6mm 3 core swa running off 40A MCB from consumer unit in house. Armour and 3rd core connected to PME in house. This feeds an RCBO'd consumer unit in new building in the garden to be used as an office. There's no metal water pipe etc in new building. Is there any point in the addition of a TT earth to the new building? (One actually exists from previously before the house was PME'd.) The way I see it, if there is ever a problem with the suppliers' earth at a nearby pole for example, isn't it good to have an earth stake connected as a back-up?

Engineer54 · Sep 17, 2011

No problem, connect the rod to the EMT in the new CU.

Just don't use an RCD protected MCB in the house CU, there will be no discrimination between that and the RCBO in the new CU...

Hawk81 · Sep 17, 2011

I agreee 100% with above

MarkieSparkie · Sep 18, 2011

Hi xt600, as usual E54 has given sound advice on the subject.

Colin33 · Sep 18, 2011

Engineer54 said:
No problem, connect the rod to the EMT in the new CU.

Just don't use an RCD protected MCB in the house CU, there will be no discrimination between that and the RCBO in the new CU...

It's running off an MCB in the house. All other circuits in the house are RCBO, so no problem with discrimination.

I s'pose my question is: is there any advantage to having the PME system backed up by the earth rod? (I think so)

And if not, is there a disadvantage to having the two earthing systems connected together?

telectrix · Sep 18, 2011

i can't see a problem with having an earth rod connected to a PME system. it's only similar to bonding the PME to a water pipe that's buried underground.

Chr!s · Sep 18, 2011

The only real purpose would for it to tie down the touch voltage upon a broken PEN. The PME link will still be in place so it will always be PME.

Regards chris

Engineer54 · Sep 18, 2011

xt600 said:
It's running off an MCB in the house. All other circuits in the house are RCBO, so no problem with discrimination.

I s'pose my question is: is there any advantage to having the PME system backed up by the earth rod? (I think so)

And if not, is there a disadvantage to having the two earthing systems connected together?

That's fine then, if your supply to the out building/shed is via a non RCD protected MCB.

I can't say that i can think of any disadvantages to having a TT rod tied into a TN-S or a PME system earth. (I'd use a minimum 6mm cable from the rod to the EMT). What you cannot mix, is any of the TN systems earths together.

As Chris stated above, the TT earth rod, can and will tie down the touch voltage in the unlikely event of a broken Pen conductor. It's strange that the previous TT system conductor, wasn't connected to the MET when the installation was converted to PME...

By the way, have you measured the Ra value on that existing rod, if so what is that measured value??

Colin33 · Sep 19, 2011

The system was TT, but changed to PME during a change of cut-out. I called them in because the cut-out was damaged and I wanted it replaced before I renewed the CU. Never got round to measuring Ra, but will do when I re-connect, although it's probably not so important now with PME connected. Because most supplies in my area are off poles, and being Cornwall with it's occasional high winds, I have know of several times when they have lost the Pen, so it's possibly fairly common

Thanks all for your help...