Sorry to bring this up again but I think that something has to be mentioned here.
A Megohm meter does not work by magic and it is a simple bit of test gear to use.
Like a few test instruments, it works with Ohms Law in mind, having to find relevant results is a no-brainer.
You should be able to work out in your head what the relative condition of the installation is by the test results.
There are fewer simpler test instruments, apart from a Bell Set.
