I intend wiring up a remote garage so it will have a TT earth.What is the best way of protecting the exposed part of the earth rod and the cable clamp from the elements outside?Could I drill a hole through the garage base and hammer the rod into the foundations so the top is inside?I don't have and specialised test equipment, how do I know for sure I have a good earth except by deliberately causing an earth fault to check the RCD? Mark Atherton
Borrow an earth loop impedence tester. Connect earth to the rod and live to your supply (before the RCD, or with the RCD shorted out). Press Test. You need a much better earth than that required to simply trip the RCD. The tester will attempt to draw up to 20A and measure the voltage drop.
Technically, you can get away with higher impedences without breaking the rules, but 200 ohms is a recommended maximum to allow some margin. You should be aiming for much less that this, unless you have some sort of problems with soil or rocks.
Christian.
Use a Furse lightweight 'earth pit'. This is a small-ish plastic enclosure that you set into the ground (flush with the finished ground surface) and which provides a housing for the top of the rod and its cable clamp. The top has a removable lid for access and looks rather like an outdoor stop-c*ck cover. I couldn't find a picture on the Web but any descent electrical wholesaler should be able to come up with the item in question. If you've a branch of Newey & Eyre nearby they sell the same item in their 'Newlec' own-brand range as product code NL786.
That sounds like hard work; if you encounter an obstacle and have to move the rod and start again you'll have the first hole to make good. You'll also puncture the DPM (if any). Outside the building is the conventional place for earth rods. There's also a risk that the soil under the floor may be drier (not rained on) and will have higher resistivity.
See . With a 30 mA RCD the earth electrode resistance must be less than
1600 ohms (500 ohms with a 100 mA RCD) but, as Christian says, the OSG recommends a figure of under 200 ohms to give a large margin of safety in case of drying out or freezing of the soil.HTH
Something like:
That's the sort of thing. The Furse one is round at the bottom, and probably a bit smaller than that though. Thanks for finding the picture.
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