Why does the Insulation Resistance test PASS with nothing plugged into the PAT tester?

This is a question we often get asked during the PAT testing courses. On any PAT tester, just press the Class II button. The tester will carry out the Insulation Resistance test and display a PASS - even with nothing plugged into the tester.

Why does this happen? Is the tester faulty? The answer is NO.