Insulation monitoring devices continuously monitor the insulation resistance of IT systems (unearthed systems) and issue an alarm if the value falls below a response value.

To obtain a measurement, the device has to be connected between the IT system and the protective earth conductor (PE). A measuring current is superimposed onto the system which is detected and evaluated by a measuring circuit. Insulation monitoring devices detect deteriorations of the insulation level at an early stage and in a reliable way.

Insulation fault locators are used in combination with insulation monitoring devices for localisation of insulation faults in unearthed power supplies (IT systems). They detect the test current signals generated by the insulation monitoring device using measuring current transformers and evaluate them correspondingly.

Insulation fault locators allow locating the fault source within a short period of time and during ongoing operation.

Residual current monitors are used to monitor earthed systems (TN and TT systems) for fault currents or residual currents. The sum of all conductors except the protective earth conductor is measured by means of measuring current transformers.

In general, residual current monitors (abbreviation RCM) are used to send a message to the user even before reaching the shutdown threshold of the RCD. Residual current monitors detect deteriorations of the insulation level at an early stage and in a reliable way.