I have recently started a job, a simply case of changing over a board I thought. I dont do domestic work (HVAC) normally, and need advice.
The supply comes from the outhouse, and is a TT system nowever the there is no good loop. I plan to place an electrode outside house close to d.b as I can achieve reading of under 70 ohm, leave exsisting prob in and join them at d.b, board is fully rcd protected. and meets time regs

so do i also need an rcd in the outhouse ? with maybe a higher mA rating . or is leaving the old cart fuse and switch ok

The submain to the house will need RCD protection, and you will need to look at a time delayed RCD to achieve some kind of discrimination.

If the outhouse has any other power eg lights or sockets you should also consider RCD requirements for those circuits.

finally if you are in England or Wales don't forget the PART P requirements of the building regulations