Taken from DTI guide
NOTE REGARDING TEST METHOD
Two test methods are possible:
a) Test between Array Negative and Earth followed by a test between Array Positive and Earth
b) Test between Earth and short-circuited Array Positive & Negative
Where the structure/frame is bonded to earth, the earth connection may be to any suitable
earth connection or to the array frame (where the array frame is utilised, ensure a good
contact and that there is continuity over the whole metallic frame).
For systems where the array frame is not bonded to earth (eg where there is a class II
installation) a commissioning engineer may choose to do two tests: i) between Array cables
and Earth and an additional test ii) between Array cables and Frame.
For Arrays that have no accessible conductive parts (eg PV roof
tiles) the test should be between
Array cables and Building Earth
Still none the wiser as to why i'm getting a reading on my meter on more than one occasion? I've got a new install to do tomorrow so I will see if its the same reading on a job I have wired, I was carrying out IR DC testing wrong when I was installing before Dec 12th so can't comment.