Earth Rods spin

[George Miles (dicegeorge]

I hammered in two earth rods, but when I screwed in the new earth wire I noticed that they spin - will this degrade the connection, is there anything I can do, or is this normal?

[g]

[John Rumm]

It will depend on the soil type and condition. Even in heavy clay you can often shift them a small amount I find. The important question being what impedance reading did you get from them?

[GB]

Do you measure the impedance between the rods?

[John Rumm]

Not usually - unless you want to see if they are in an overlapping impedance zone.

(also depends on if you mean two separate rods, or two rods joined to make one longer one, which would be the more typical configuration)

You would normally follow one of the procedures outlined here:

[Smolley]

You will need an AC supply for impedance readings.

[GB]

Okay, I think I've got it!

Obviously, if you have an earth from the electricity supply, you'll test against that, although I am not quite sure why you would need an earthing rod as well.

If you don't have an earth from the electricity supply, you'll test using three rods.

[Tim+]

Quite common if you have solar panels and batteries. You need to provide your own earth if you want to use your system “off grid” as a supply failure could potentially mean the loss of your earth connection as well.

Tim

[GB]

Thanks. I definitely hadn't worked that out!

[John Rumm]

A common example might be where you have a PME (TN-C-S) earth, and you want to run power to a greenhouse or similar. If you were to export the PME earth as well, you would also need to export the equipotential zone as well, and maintaining an equipotential zone where you have free access to the ground etc, is pretty much impossible.

So in those cases you would create a TT earth for your out building, and it would not be connected to the suppliers earth.

Yup, or just measure the loop impedance if you have a tester for that instead. That will technically give you Zs rather than Ze, but the difference will be negligible.

[John Rumm]

Same goes if you want to run a generator via a transfer switch. You can't rely on the supplier's earth during a power cut, so adding your own TT earth to that of the PME supply will ensure you still have an earth when running disconnected from the grid. (and your earth just becomes another one of the "many" of the PME earth in normal circumstances)

[John Rumm]

You could do it with a couple of car batteries - but take several readings and swap polarity each time.

[George Miles (dicegeorge]

thanks for replies.

I had the supply and main fuse moved because of the extension: these two rods are screwed into each other as the first one just goes through the new concrete foundation.

I havent measured impedance yet - or connected them.

The existing earth rods are inside the new extension and under a new floor eventually - but not this year.

Its a TT system, with the incoming earth coming in along the Neutral wire from I dont know where.