Testing an Earth Rod

[ac1951]

How do check the effectiveness / restistance of an earth rod ? I read an article on a Wiki web site ... basically it said to use a low voltage transformer and connect one end to the earth rod and the other end to your main earth connection in the CU with an Ampmeter in the cct. Then using simple Ohms law you can calculate the resistance and this value will prodominantly be the restistance of the rod to earth.

Fine underatand all that... BUT.. before starting the article said that you need to disconnect all other bondings (Water and Gas). So assuming I have no earth provided by the Electrical supplier (hence the reason for installing your own earth rod) I fail to see how any current is going to flow in this cct since one end of the supply simple connects to all the protective earths on the house cables and the other connects to Ground (via the Rod)

Am I missing something ?

[Andy Wade]

Quite so. The context in which I originally posted that idea is testing the earth electrode for a separately-TT-earthed outbuilding installation. As you rightly imply it does assume the availability of a supplier's (TN) earth connection to provide the return path. In your case you'll need another temporary earth rod to provide the return - bash a bit of copper water pipe (or similar) into the ground as far away from the electrode under test as you can manage, and use that to complete the circuit. With this technique you must use the separate sensing electrode (a 2nd temporary rod) to establish a voltage reference. Put this one roughly half-way between the other two, and take measurements with it in two or three different places to check that the resistance areas aren't overlapping too much.

[ac1951]

Ok thanks for the clarification

[robgraham]

Thanks guys - I was looking at this the other day and aiming at putting a second rod at the new workshop I'm building at the back of my garage as the run back to the house rod is a bit long. It'll do nicely as the reference to check the house earth, and vice versa.

Rob

[John Rumm]

I was sure I had posted a reply to this the other day... but my sent folder says no!

Yup, the wiki does need a bit of clarification there as Andy pointed out.

I have tweaked a bit, and pasted Andy's response in there as well.

I have a feeling a bit more detail on overlapping resistance areas might help as well. Anyone fancy adding some words?

Revised version:

[Steve]

Apart from one or two obvious slight typos, the only thing that I would change is the use of impedance instead of resistance in the " Without specialist test gear" section. As you previously stated in the article, quite correctly, that reactance would be negligible, it seems unnecessary to introduce impedance and then go on to talk about resistance for the rest of the article.

Just my 2p's worth... but you did ask :-)

Steve

[John Rumm]

Fair comment. Constructive suggestions always welcome!