I have been using the PrimeTest 250 for a few weeks now testing IT equipment, and it is apparent that the manual is not the easiest thing in the world to understand.
One important omission appears to be about cord tests - nowhere can I see that it say I should not have the PrimeTest 250 plugged into the mains when performing a cord test. It always fails when I have done this (usually it is plugged in because I have been doing a Class 1 test on an appliance and just forget to unplug it for the next cord), the same cord will pass fine if the PrimeTest 250 is unplugged. I think it just assumes that I am testing a Class 1 appliance and want to perform a differential leakage test if it is plugged into the mains, so it performs the earth bond test using the earth bond lead rather than the IEC socket (it fails on the earth bond test as an open circuit).
I can understand that the device is not a mind-reader, but this omission from the manual just seems to be unhelpful.
On the subject of plugged or unplugged, is there any advantage or is it better to perform differential over substitute leakage where IT equipment is concerned? The only difference I notice is that substitute appears to give a higher reading but still passes well below the 3.5mA threshold. I have always performed the differential test (tester is plugged into the mains) so far as that way I can turn the appliance on during the test, as was shown in my training on a Europa style tester.