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. An effective insulation resistance system has a high resistance value, usually greater than a few megaohms (MΩ). A poor insulation system has a lower insulation resistance.

In order to find a leak in the piping system, you need to pressurize it. Since leakage is most likely to be found 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 insulation tester (megohmmeter) will apply a DC voltage to the insulation system and measure 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).

Insulation testers (megohmmeters) generally output test voltages of 50 V, 100 V, 250 V, 500 V or 1000 V. As in piping systems, the purpose is to provide a pressure (voltage) that is not particularly high. We want to find existing leaks, but we don't want to overstress the system and create new leaks. Lower voltages are used for low-voltage systems, such as telephones, networks, or control lines; higher pressures are used to test the insulation of power systems.