Peter Monsen asks a couple of questions about a TT domestic earthing system and the electrode resistance. His question has been answered by the NICEIC (Theme – Fire, Safety & Security):
Question 1: With a TT domestic earthing system, is 8.7 ohms an acceptable reading?
Answer 1: A TT installation having an electrode resistance of 8.4 ohms is more than adequate to comply with BS7671:2008.
Table 41.5, Note 2 mentions an electrode resistance as being as low as practicable, and that a resistance of above 200 ohms can be unstable. Therefore, your reading is ideal for the installation.
Question 2: Many thanks - I've received your email, and I checked Table 41.5 and the maximum Zs for a 300mA RCD is 167ohms. If the resistance on the earth spike is less than 167 ohms, is it OK to use a 300mA RCD to protect both the supply cable and the distribution board?
I would, if possible, prefer to fit a 300mA RCD over a 100mA RCD, as the loads are inductive and may cause unwanted tripping.
Answer 2: Your proposal is acceptable, and will make the installation's earthing arrangements comply with BS7671:2008.
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