Thanks. The impression I get is that this is not what most people do (i.e.I thought they tended to measure the whole installation, at least for starters) - am I wrong in this belief? I would have thought that it's usually going to be worth first testing the entire installation (at least, the entire cu/db) - since, if that gives a high reading, then one doesn't have to spend time testing individual circuits.I find that when I am testing the whole installation, it would normally be a new installation measuring form the tails. When doing work on older installation this is not always possible or practical. So I generally test each circuit individual. (I mostly work in the domestic sector)
I suppose this all comes down to what one means by 'no fault'. IR values even remotely that low (indeed, any which are not >>100MΩ are clearly 'not right' - but I presume when you say 'no fault', you mean that the low IRs are due to damp/dirt/whatever, rather than to deterioration of any components of the installation.But If measuring from the tails again the guideline would be 2Mohms.
You could have say 6 circuits on a board with the overall resistance reading being 1.85Mohms. But once all circuits are tested individually there would be no fault found and all the readings complied.
Kind Regards, John.