OP
moggy1968
depends what book you read but general recommendation is less than 100 ohms for tt system (some recommend 200 ) but in effect you are talking about bringing this down to allow an mcb to trip under earth fault conditions, this is almost impossible to achieve with TT systems unless you are next to the sub station.
any earth fault is dealt with by the rcd, this generally makes the zs readings a bit meaningless if you are trying to compare to max zs for the circuit. what you sometimes find is that the zs is so high that there is less prospective fault current available to trip the mcb than the mcb rating but it still complies!
Looking at the job you had to do I would have been tempted to fit the supply from the rcd switch into a henly which fed a 30ma rcd switch feeding the household circuits and then connected the inverter to a little cu also connected to the henly.
That way you are increasing the safety of the household circuits (albeit at risk of increasing nuisance tripping) and also avoiding using a 30ma for the inverter.
you still need to RCD your PV circuit, also using a single 30ma RCD for all circuits doesn't meet the requirement for discrimination so you would need to fit a 17th ed type board with divided circuits.
I recently had to relocate the board on a TT system (no PV) so had to protect the 5m of tails with a 100ma time delay RCD. The other time I have used this set up was on my folks house. The main board is in the loft, so I fed the garage supply with a 100ma time delay RCD then fitted a 30ma RCD on the garage board so hopefully if the garage tripped you didn't have to go up into the loft to reset the main RCD.
If your board is near the point of entry of course this isn't necessary.