Gidday Jon,
Welcome back, mate.
An interesting question you ask, it is also one that can't be answered with a "broad-brush" answer.
Insulation resistance testing was taught to me at night school.
There are a few "fish-hooks" involved in meggering gear as well.
AVO Megger came out with a really good interactive CD ROM, about 6 years ago, that explained the in's and out's of proper use of the Megger, if you Google that, it may point you in the right direction
In a general sort of a way, insulation testing with equipment like motors and transformers, is really only ever done between any of the line terminals on the equipment and (Ground/Earth/The Metallic body of the equipment).
Transformers are however slightly different, in that you test between the core of the transformer and the winding ends.
You also test between the primary and secondary sides of the transformer to make sure there is no "blow-through".
At the end of the day, the readings you get with each test are meaningless, if you don't know what they should be in the first place.
Only experience can really tell you how high or low a reading should be.
Take for instance an electrical cable, it is say 100' long, it will have an insulation resistance of say 300 Megohms, if you then double it's length to 200', the insulation resistance will halve to 150 Megohms.
This is because of the capacitance between the cores in the cable.
{Please note that these figures are not what you would get from an actual cable, they are only an example}
One other thing about Insulation Resistance testing is that it is defined as:
A test that will stress the insulation of wiring and components, to test thier electrical integrity.
This test is required to be done, using a DC voltage of twice the working voltage of the circuit/equipment.
{Foot-note:
I'm going to move this thread to the Electrical Theory and Applications Forum, I'd like to see what Scott35 has to say about this subject}
