Insulation measurement gives too low values for longer cables?

Concerns: PROFITEST 204 and PROFITEST 0100:

Longer cables have a larger capacitance. The insulation resistance measurement is performed with DC voltage. Consequently, the capacitance of the DUT must be charged. The test equipment continues to measure until charging is complete.

The test instrument recognizes that the charging process is still in progress by the fact that the voltage at the DUT rises. If the voltage rise has fallen below a certain limit, the test instrument assumes that charging is complete and that the insulation resistance now measured has been correctly determined.

In the case of very large capacitances, charging slows down so much that the test instrument incorrectly detects the end of charging even though the voltage is still rising very slightly. However, as long as the voltage is still rising, the charging current is also flowing. This charge current is then converted to the insulation resistance, which is of course much too small.

• Simply hold down the start button of these devices until the voltage display no longer changes. Then you will get the correct insulation resistance.