Richard asks this question concerning the effective earthing of an existing commercial installation situated on rocky ground. It has been answered by the ECA (Cables, Wiring and Wiring Accessories):
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Question: I’m working on an existing commercial installation that is situated on a rocky ground. There is the main building with the main panel, which supplies two other buildings, each with their sub-panels.
Each building panel is earthed via earth electrodes. The loop impedance is very high (164 ohm) at the location and in an effort to lower the resistance, I’ve linked all the earth electrodes together. This has brought down the loop impedance significantly (42 ohm) but it is still too high.
The supply authority earth is to close to the main panel and I’m thinking of installing an earth mat as the main earthing electrode (to replace the main earth rod) about 20 feet away from the main pane/ supply authority earth. I will thus use a main earthing terminal bar to link all other building earth.
The issue is that the entire area is solid rock and the place was backfilled with about four feet of soil, and I don't think the earth mat will drop it to a resistance required - below 2 ohm.
What can I do to significantly improve the resistance and soil resistivity-earth mat and special backfill, etc?
Answer: The most suitable way is to connect an electrode earthing terminal within the building earthing system.
I’m pretty sure you’ll reach a good rate of resistance; or you’ll just have to put more earth electrodes in an accessible place and connect them to the others using strip connection.
Note that if you have a TT system, all your circuits must be protected by RCD. If, however, it is a TN system, then you should protect all the outlet sockets up to 20A with an RCD.
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