Ok hard hat on. Ive wired a garage with a water pipe. Ive taken a 10 mm back from the pipe to the MET in the house (TNS). The board in the garage is RCD protected. So far all the same as the IEE Supplying a Detached Out building booklet recommendations. Can I run another 10 mm from the pipe to the earth bar in the garage consumer unit? This would help reduce the Zs readings of the garage consumer unit and in my opinion make the installation safer.