"Lurch" wrote in message
...
On Tue, 6 Mar 2007 15:57:05 -0000, "Ian_m" mused:

I can undertstand after fitting, testing between live/neutral
and earth as a check against accidental insulation damage, but before
installation ?

You check before installation so you have a reference measurement.
I have now have both a before and after reference measurement, both 20M.as
the test equipment I have has 0-20M and greater than 20M readings @ 500V.