This question is about domestic installations, Zs values and earthing under BS7671:2008. It is answered by the NICEIC (Theme - Regulations & Legislation):
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Question: I recently re-wired an old terraced house. When I came to test and inspect the new installation, I found that the Ze at the supply authority cutout was 4.08 ohms. This was tested in the correct fashion and a good contact was made on the incoming cables armouring.The system is a TN-S and, bearing in mind the maximum allowed for this type is 0.8 ohms, I decided that I had two options:
1) To contact the local supply authority to see what it could do about it (I know it is not obliged to provide an earth).
2) To install an earth electrode and, therefore, turn the system into a TT.
Before doing either, I thought I would ask for some advice from the NICEIC Technical Helpline. This I did and the people I spoke with agreed with everything I'd said, but they also said that I didn't need to do anything as long as I had installed a new 17th Edition consumer unit (which I already had) - so that the whole installation was RCD protected, and as long as the Zs readings were within the max values permitted for circuits protected by RCDs as given in BS7671: 2008.
This made sense but got me thinking that as long as you've got an RCD protected installation you can effectively scrap your type of earthing system and max Zs values given for 60898 breakers, BS88 fuses etc under the new regulations. Surely this can't be right?
Answer: Your assumption is correct for 30mA RCD protected circuits providing that a TT system is installed and the earth electrode reading is below the 1667 ohms required to ensure that the touch voltage generated in the event of an earth fault is not exceeded (though above 200 ohms may be considered unreliable).
However there are still installations or circuits that will not require RCDs to be fitted. We must also consider that the O/C device is there to protect against overload and short circuit faults.
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