In unearthed (ungrounded) power systems, also known as IT systems, the insulation resistance must be continuously monitored to ensure operational safety and reliability. This is done using insulation monitoring devices (IMDs), which trigger an alarm when the insulation resistance drops below a defined threshold (response value).

An IMD is connected between the active conductors of the IT system and the protective earth (PE). It injects a low-level measuring current into the system and evaluates the response via a built-in measurement circuit. This allows the device to reliably detect insulation deterioration at an early stage, preventing potential hazards or system failures.

When selecting an IMD, the following factors must be considered:

• Application area (e.g., medical, industrial, or renewable energy)
• Voltage level of the monitored system
• Type of voltage (AC, DC, or mixed AC/DC, as in inverter-based systems)
• Maximum permissible system leakage capacitance

Proper selection and installation of an IMD help maintain safe, efficient operation while meeting the requirements of modern electrical standards.