Question about earth rods and desired impedance

It will always be variable due to the e amount of water in the clay I'd suspect. I have to say earth resistance is all a bit of a black art to me, I see one go down below my house but I have never tried to measure it, and I doubt many ever do. Brian

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.

So leave it in. Options include: Screw another section onto it, hammer in connect to both rods

NT

If you are trying to create your own PME supply then the best of luck.

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.

Belt, braces and a piece of string ;-)

If you are worried about losing the overhead neutral, you could go for an all RCBO CU.

When selecting your RCBOs, choose ones with functional earth wires. These are thin and white.

Should a loss of neutral occur, the RCBOs will all trip.

S.

I had another rod so screwed it on and drove down another 4 feet (parallel rods need to be a long way apart so that wasn't an option). Initially it made little difference, down to about 50 ohms, but after a few days the resistance between the MET and the new rod had dropped to less than an ohm (measured using PSU and ammeter and also by DMM). I don't understand why it changed, but the new figure has been stable for a week or so - it's possible that I had a bad contact before, but I'm confident that I didn't.

When you first hammered the rod into the clay there was probably some sideways movement making the hole a larger diameter than the rod. The rod was only making good contact at the far end. Since than the clay has flowed back to fill the void between the rod and the original side of the hole.