The insulation resistance tester applies a DC voltage to the insulation system and measures the resulting current. In this way, it is possible to calculate and display the resistance value of the insulation (the degree to which the insulation confines the current in the wire, or the degree to which it prevents current leakage).

In order to find a leak in the piping system, you need to pressurize it. Since leakage is easy to find when the water pressure is highest, you cannot turn off the tap water to check for leakage. However, you will limit the available tap water so that you will not spray too much water around if you find a big leak. The ideal test is to provide a limited amount of water under high (but not particularly high) pressure. This is exactly what the electrical insulation tester does.

    An electrical system is like a pipe system. Voltage is like liquid pressure, current is like liquid flow rate, and electrical insulation is like pipe wall. Insulation prevents electrons from leaking from the conductor-the magnitude of its effect is expressed by insulation resistance. Effective insulation resistance systems have high resistance values, usually greater than a few megaohms (M). A poor insulation system has a lower insulation resistance.

Before powering on the system, use the insulation test to verify that it is sound and can improve the performance of the system; the insulation test can find manufacturing process problems and equipment defects, and these problems are generally not found before the equipment fails. In the European Union, this test is mandatory, even for the smallest civil systems.

By testing the insulation resistance of different components in the system (transformers, switchgear, wires, motors), technicians can isolate and repair the failed components.