Going by your figures, you have in a way answered your own question to a point, at the very least the lower rated protective devices WILL trip!! (So the lower you get your Ra value down, the better and it's not always the chore it's made out to be!!) Maybe not in the desired times but they will trip!! Absolutely no chance whatsoever with either 100/200ohms suggested by others. Combine a decent low double figure Ra with decent bonding at the MET and your overall Zs could well improve further to trip out a 32A breaker. Now if i had driven a rod in the ground and achieved an Ra of 10/12 ohm, i'd be, without any thought whatsoever, adding another rod on top Knowing full well that my Ra value will almost halve again, and ''continue to improve''!! And we haven't even talked about system stability, that your rarely achieve with a single 1.2m rod of any size, especially in some of the soil conditions found in parts of the UK.
Most here only look directly at the Ra figure they attain at time of installation, which is fine to a point, but with a deep driven rod, that value will continue to decrease over a given time period as the soil consolidates around the rod and connections. My system for instance, was around 5 to 6 ohms mark when the overall system was completed. I am now at a sub 0 ohm value, 3 years or so later (checked/measured just last week) and my ground/soil conditions are far, far from ideal i can tell you!! ...lol!!
The only reason i can see for the old standard 10 ohm for a TT system being left behind, is the advent of RCD devices, which is all fine and dandy till the things fail for one reason or another and your left with this numpty 100/200 ohm as your earth fault protection. Which comes to the point where we DO agree, that TT systems should have an up-front additional S type protection to cater for such failures of single RCD devices.
No other country that i know of, has a standard anywhere near that of what the UK (IET) suggests, even the Yanks want top see a 20 ohm Ra, and if it's higher another rod is to be installed. (and the Yanks (god bless them) use 3/4'' 10 foot rods, not 3/8'' 4 footers as you have in the UK!!) and many more still use what we used to have, 10 ohms!! I think that is the recognised value in S Africa too, pretty sure Marvo has stated this figure in past Threads. And he, as i remember often has to use several coupled rods in his sandy soil conditions!! He still uses RCD protection, so what's so different in the UK having to comply with realistic Ra values on TT systems??
Now if all these other countries sensible standards don't or can't convince you, then just stick to your die hard support of IET's numbty 200 ohms!! lol!!
