Insulation measurement leads to low values for longer lines.
Subject: PROFITEST 204 and PROFITEST 0100:
Longer lines have a larger capacity. The insulation resistance measurement is carried out with DC voltage. Consequently, the capacity of the object to be measured must be charged. The testers continue to measure until charging is complete.
The charger recognizes that the charging process is still running by increasing the voltage at the DUT. If the voltage increase has fallen below a certain limit, the tester assumes that the charging has been completed and that the insulation resistance measured has been correctly determined.
With very large capacities, the charge slows down so much that the tester will erroneously detect the end of charging even though the voltage is still rising slightly. But as long as the voltage increases, so does the charging current. This charge current is then converted to the insulation resistance and that is of course far too small.
Simply hold down the start button on these devices until the voltage indicator does not change. Then you get the right insulation resistance.