Earth Rod Positioning

[PeatBog]

Hi,

This is the scenario:

1) Scout hall with TN-C-S supply.

2) Outbuilding needs electricity supply. This is an equiment store with what I expect to be an "earthy" steel frame and "earthy" concrete floor (although I have not yet measured to see if frame is extraneous).

Because of the TN-C-S supply i'm not keen to export the earth. So I'm looking at TT'ing it at the outbuilding end.

The only practical position for the earth rod is very close to the perimeter of the site and close to a post of the 50m long 10ft high metal security fencing with steel posts embeded deep into ground.

What are the issues with the rod being so close to the boundary and the fence post?

Thanks for anyones views.

Pete.

 

[Evans Electric]

None , but make sure you're not in line with any services leaving the site.    Keep away from where the uprights are concreted in .

Thats what I hate about rods , never know what they might hit .

 

[Bruspark]

Yep, always position over the gas pipe everyone knew was there  . . .

 

[Manator]

There could be a problem if the rod is close to a metal fence. Remember step voltages may differ. During fault conditions the earth potential from one point to another may decide to pass through whatever is in contact with both in order to equalise.

 

[Sidewinder]

Test the outbuilding before putting the supply in to see if it is OK to export the TN-C-S earth, after all unless it's PME, there can be no issue.

Remember PME does NOT EQUAL TN-C-S.

 

[PeatBog]

I know the theorectical difference between TN-C-S and TN-C-S with PME but without a PME label on the head can I prove whether it is or not by test?

(It is a 20 year old building in a TN-S area)

 

[Andy™]

Remember PME does NOT EQUAL TN-C-S.

isnt it the other way?

 

[PeatBog]

Thanks for the replay Manator. I sorry but us townies do very lillte TT work!

What I'm concerned about is getting a voltage on the fence from a fault current which could be sustained if the RCD fails?

My thinking:

I understand that Earth rods can be hazardest to four legged animal as the one set of legs could be ajacent to the rod and one set 1m away, upon fault. This tells me that 1m away from the rod the earth could still be at >50V from true earth.

If my metal fence post is within 1m of the rod then it could have at least 50V on it (for a sutained time if the RCD fails).

As my choice of earthing is a risk based assessment between the available options then a fault and a failed RCD would seem more likely (if less dangerous) than a lost neutral with an underground supply from the sub-station.

Also, can I check my understanding:

With a failed neutral:

TNC-S without PME  then neutral and earth within installation will rise towards 240V depending on the load resistances between line and neutral?

TNC-S with PME the voltage on the neutral and earth will be dragged down toward 0V by the points of PME earthing? So therefore safer?

Is that right?

 

[Steve3948]

Why can you not export and make sure the outbuilding fabrication is all bonded to the electrical earth, there should then be no difference in potential.

 

[PeatBog]

The plan is to re-use the existing 10mm2 Twin & Earth sub-main that runs the length of the main building with SWA to new building (total length of 35m ish). To extend the equipotential zone to the out-building would require laying a 10mm2 G/Y (or possibly 16mm2) along the length of the main building, which I would be very difficult! 

 

[steptoe]

Localised PME,?