An electrical system is like a piping system, where voltage is analogous to liquid pressure, current is analogous to the flow rate of the liquid, and electrical insulation is like the pipe wall. Insulation prevents electrons from leaking from the conductor – its effectiveness is expressed in terms of insulation resistance. An effective insulation system has a high resistance value, typically greater than several megohms (MΩ). A poor insulation system has a lower insulation resistance.
To detect leaks in a piping system, you need to pressurize it. Since leaks are easier to find at high water pressure, you cannot turn off the water supply to check for leaks. However, you would limit the available water so that when a large leak is found, not too much water is sprayed around. The ideal test is to provide a limited amount of water at a high (but not extremely high) pressure. This is exactly what an electrical insulation tester does.
An insulation tester (megohmmeter) applies a DC voltage to the insulation system and measures the resulting current. This allows it to calculate and display the resistance value of the insulation (the degree to which the insulation confines the current to the wire, or prevents current leakage).
Insulation testers (megohmmeters) typically output test voltages of 50 V, 100 V, 250 V, 500 V, or 1000 V. As in the piping system, the purpose is to provide a pressure (voltage) that is not particularly high. We want to detect existing leaks, but we do not want to overstress the system and create new leaks. Lower voltages are used for low-voltage systems, such as telephone, network, or control lines; higher pressures are used to test the insulation of power systems.
