Armour cable

[John Rumm]

From the measurements I have taken from my installation (heavy clay soil) a 4' rod will give a resoanably consistent 11 - 13 ohms year round.

[Andy Wade]

I'd say that's exceptionally low for a single 4 ft. 50 - 100 ohms is more common, so you've clearly got very good soil.

For my workshop & shed installations I've got two 8 footers in parallel (one of them deliberately near a soakaway) and find a consistent 7.5 to

8 ohms.

[Sparks]

Sorry to butt-in here, but can you tell me how you measure earth rods? What do you use to do the measuring, and what is the other test lead connected to?

Ta :-)

[John Rumm]

Yup, I was quite pleasently surprised. When we moved in the *only* earth connection was to the gas supply (also about 12 ohms), so I was wondering what reading I would get from a rod on its own without any contribution from the gas and water bonds. After some experimentation it seems that close to the house any length of rod that goes more than a few inches into the ground gets you about 12 ohms.

My workshop earth was reading about 22 ohms (probably in dryer soil with lots of tree roots about). So I added a second rod in parallel about 14' away to that to get it down to the same as the house.

What sort of earth connection would they have at a typical substation?

[John Rumm]

I usually use a Megger LT5 digital loop tester. It actually measures the earth fault loop impedance for a whole circuit, so is not quite a true reading of the electrode resistance in isolation, but does give a reasonable indication. Obviously you need to disconnect the main equipotential bonds when you take a reading so as not to include any fortuitous lowering of impedance they may give you.

[Frank Erskine]

On Sun, 23 Jul 2006 18:28:18 +0100, "Sparks" had this to say:

[Andy Wade]

a) with a proper earth electrode testing kit, or

b) with a loop-tester, as John Rumm has already said (and this method is endorsed by the On-Site Guide), or

c) my DIY cheat's method, see

[<me9]

Isn't that how it was prescribed in round about the 15th edition? It sounds familiar.

[Sparks]

Thanks for that :-)

You say... "connect one end of the secondary via a suitable length of wire to the main earth terminal in the house and connect the other end to your earth electrode via an ammeter."

Do I assume this would be done with the rod disconnected from this main earth terminal?

If so, if this is a TT installation, and there is no other path to earth in the house (plastic water pipes etc..) how will you get a reading?

If not, surely this is only testing the cable between the rod and the earth terminal (and your test cable!), not the rod it self?

Sparks...

[Andy Wade]

Yes, obviously - otherwise you'd have a short across the transformer secondary.

Ah, yes, OK. There is an implicit assumption in that article that you're wanting to test an earth rod for an outbuilding, etc., and the house already has a functioning TN earth kindly provided by the DNO. If you don't have the latter then there are probably two options:

(a) sink a third temporary electrode as far away as you can from the other two and use that as the return connection for the test current. You might have to increase the test voltage available to get enough test current flowing (make sure to mask any residual voltage that might be present between the rod under test and the reference electrode);

(b) resort to hairier methods such as using the mains neutral as the test current return (it will be earthed somewhere), or obtain your test current directly from the mains (phase) via a series resistance (such as a couple of 1 kW fire elements in series) instead of using the transformer. Clearly such methods have the potential to be dangerous, so don't attempt them unless you have a clear understanding of what you're doing and appreciate the dangers involved etc., etc.

Well, quite.