Testing an Earth Rod


A

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 ?
 
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A

Andy Wade

ac1951 said:
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)
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.
 
A

ac1951

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.
Ok thanks for the clarification
 
R

robgraham

Ok thanks for the clarification
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
 
J

John Rumm

ac1951 said:
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 ?
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:

http://wiki.diyfaq.org.uk/index.php?title=TT_Earthing#Without_specialist_test_gear
 
S

Steve

John said:
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:

http://wiki.diyfaq.org.uk/index.php?title=TT_Earthing#Without_specialist_test_gear
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
 
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J

John Rumm

Steve said:
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 :)
Fair comment. Constructive suggestions always welcome!
 

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