On 09/07/2019 18:47, ARW wrote:
On 09/07/2019 13:37, wrote:
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.


200 ohms is the recommended maximum. Assuming the new CU is all RCD then
you will be fine for 1666 ohms.
Thanks, I hadn't managed to find the recommendation before (and hadn't
woken-up to the fact that 30mA is what sets the max z - doh!) but
searching backwards it seems that there's some debate about whether the
recommended max is 200 or 100 ohms. Given what I'm trying to do I'll
leave the rod in place, whack-in the last few inches and accept the 60
ohms.
If you are trying to create your own PME supply then the best of luck.
Belt, braces and a piece of string ;-)