You must ensure the earth loop impedence is low enough so that the RCD
will
trip before the earth potential reaches 50V. This requires an earth loop
impedence below a certain value (around 1600 ohms). However, I believe the
recommendation is to have one below 200 ohms, which is well within the 50V
requirement when using a 30mA RCD. (It will drop 6V by the time it reaches
guaranteed trip current). It allows for changes in ground resistance and
is
very easily achievable, so accepting less is asking for trouble.

So as the earth for the garage is using the SWA of the cable from the house
this is surley much better than the earth rod in the mud which I imagine
must alter its resistance wildly between the very wet times and the very dry
times of the year if a regular RCD test proves ok surley this is good????

Rich






so I could test from a live wire in the fuse box to the earth rod for
example and get a reading??

Yes. The testing will attempt to draw a predetermined current (usually
20A).
It then measures the voltage drop. If the earth loop impedence is high,
the
voltage drop will be greater. If you are working in low current circuits,
such as lighting, it might blow the MCB, though. Lower current testers are
available for this reason, but you should use the full current to test
when
possible.

Christian.