On 09/07/2019 11:56, John Rumm wrote:
On 08/07/2019 22:27, wrote:

I'm new to earth rods so would like a little advice.
First, some background. We bought the house a couple of years ago and
at that time it was supplied via a 2-wire overhead line from a
transformer some (unknown) distance away. The 2 wires terminated on a
bracket at the back of the garage and then came through the wooden
window frame to the main fuse. As soon as we moved-in I got the local
electricity board to put the final leg underground and convert the
supply to TN-C-S - it looks as if they have added a ground connection
at the final pole but the existing connection to a local earth rod was
left in place.
I'm replacing the old CUs as part of a major refurb and decided to
keep a local ground rod, but to use a different position. I've just
put a 4ft rod in the ground and made some measurements (using 30V DC
and checking with both polarities). The soil is clay and the rod
went-in remarkably easily using the SDS hammer.
New rod to MET: 60 ohms
Old rod to MET: 150 ohms
New rod to old rod (about 4m apart): 230 ohms

60 ohms seems rather high - what impedance should I be aiming for and
is there any point (with clay soil) in driving the rod down another 4'?

60 does sound a bit higher than I would expect (I have found getting
under 15 is usually easy with a single length rod in clay). It might be
worth trying a different location. Having said that, if you have TN-C-S
anyway, you have to question what you actually want to achieve.

I'm doing it because I would only have one earth point if I lost the
overhead neutral cable, so it seemed reasonable to have a second that is
under my control. I accept that's not the strongest argument and I
probably wouldn't have thought about it if there wasn't an existing rod
(which, it turns-out, is virtually worthless at 150 ohms).

Is there a recommended maximum impedance that I should be aiming for?

Trying somewhere else is easy to say but getting the rod out might be
not be so easy. I'll try a strap and a long piece of 2x4.