found this on Voltim..
Question: Please could you advise me on maximum Zs on a TT system, if a reading is less than 1666 ohms on a circuit - is this is low enough to disconnect a circuit that will satisfy BS7671?
Also, will this reading satisfy perspective fault current and overcurrent protection and, therefore, possibly negating the need for an RCD?
Answer: Section 7.2.6 of the On-Site Guide (BS 7671:2008 (2011) states that the total earth fault loop impedance is verified as being less than 200ohm, and the protective device(s) should be an RCBO or fuse/circuit breaker combined with an RCD.
It is important to note that 1666 ohm is way too high, as it would limit the earth fault current to 138mA which is too low to operate an overcurrent protective device.
From D A Woolgar - other guidance notes detail that a impedance of over 100 ohms should be considered unstable.