Working on a project. Old house in the sticks, TT earth system. Ra 160 ohms, Ze with bonding 5.05 ohms. Water pipe is lead and copper so doing most of the earthing. Outbuilding being converted to bedroom and kitchen. Another outbuilding double garage/workshop. 16mm2 T&E been used for a few years with a 32A commando socket on it. This is going to be jointed with more 16mm2 T&E to run a supply in to the first outbuilding. Total run approx.30m. From the outbuilding new CU another supply to garage. In duct buried in ground 25mm2 3-core SWA. Total run 26m. So total length of supply from house CU to Garage via bedroom CU is 56m, 30m 16mm2 T&E, 26m 3-core 25mm2 SWA. I have spent some time trying to get estimates of the rod Ra's on the 2 new CUs and tried using a metal electric gate post as an earth. It was buried 1.2m down in concrete. I could get 50ohms between the gate posts but couldn't get anything off the house cpc or various rods shoved in around the yard. I was only using a low ohms meter but it was obvious I am going to struggle to get decent rod resistance/impedance. The ground has large clusters of stones and clay under the initial made ground 300-400mm deep. Suggestions for a rod please?
Deeper rods i'd say mate, I had a job a couple of years ago in chalky ground, (don't know how deep the chalk was) and the best I could get with a normal 1.2 m rod was about 160 ohms, so I got hold of a couple of 5/8 inch thick rods with a coupler and then got well under 100 ohms, in fact from memory I think it was about 60 ohms!
You could try 2 x 4ft rods screwed together but no guarantee the reading will be better than 1 rod, you could link rods together spaced about twice the distance of the length of rods, you could also buy a bag of bentonite which is supposed to make a better earth.
If your main rod in the house is giving 160, this is fine. Ideally whack a rod in at other DB locations too, but your main rod is fine, and assuming you are connecting the earths all the way through, what's your issue?
Don't know if you have a 'lower' part of garden to try(ie the part likely to be wettest where all the rainwater runs to). Mr. HandyAndy - Really
Hello Lec. It didn't quite sit right with me to do that. I would be taking a poor earth (160ohms is not particularly good for a rod) through a 6mm2 cpc in the 16mm2 T&E then on to a 25mm2 cpc to the end CU. All this lot relying on one rod just doesn't seem right. I was hoping to have one rod per CU in an attempt to reduce the entire earthing impedance down utilising three rods. Am I being too pragmatic here?
The extra rods are nice, just not required. You will have a time delayed 100 (or 300) mA RCD at the origin.
It is a fully RCBO board and one of the ways is serving the 16mm2 T&E limiting supply to outbuildings to 40A (currently) but could be replaced with a 50A RCBO. So, if you were me would you just add everything on and use the one rod?
As long as everything is on 30ma then I have done many where my earth stake readings are 160 to 200 ohm & have not considered it an issue. I would however carefully check the rod & connection to it.
30mA? Why? With 160 ohm there is nothing wrong with a 300mA! You could go upto 166ohms at 300mA. 500ohms at 100mA. 30mA is asking for trouble IMO. 30mA for additional protection (where required), but not for the submains! I would still whack some more rods in at each location. Go for the larger diameter rods with screw tops, and whack a couple in (screwed on top of each other). Do them at an angle, not vertical. If you are opening a trench for new cables, run a hard drawn bare copper conductor in the trench. It's nice to get a good low Ra, but as long as you're below 200 (or 166 if using a 300mA), you're fine.
In some ways I agree but we don't know if the ground is a bit moist or dry. If its dry then things can only improve, if the ground is a bit moist then the reading could become worse. They seem to indicate that the ground resistance does not alter much 3 ft down in the ground but there are always exceptions
Thanks. How about I use an mcb on the existing fuseboard, say 50A, for the sub-main, then RCBO board in the first out building, then loop off directly to the second outbuilding and stick another RCBO board on that. There is no RCD protection to the sub-main but full 30mA RCD protection on the circuits. Is that allowed?
The buildings are on a slope starting high away from the road down to low near the road. I am told by the owner that the ground higher up is very stony and he doesn't feel confident about getting in a rod. If necessary I would SDS drill first then drop the rod in and even try bentonite (if its not too expensive) clay slurry.
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