An insulation monitoring device monitors the ungrounded system between an active phase conductor and earth. It is intended to give an alert(light and sound) or disconnect the power supply when the impedance between the two conductors drops below a set value, usually 50 kΩ.The main advantage of the device that it allows uninterrupted power supply to important consumers such as medical, military and etc. Even if there is a failure(i.e phase touches ground) there are still no danger to the personnel.
It works by injecting low level ac or dc on the line and detecting.