Insulation monitoring: The concept

Operating principle of an insulation monitoring device

The insulation monitoring device is connected between the live supply conductors and earth and superimposes a measuring voltage Um. In the event of an insulation fault, the insulation fault RF closes the measuring circuit between the system and earth, generating a measuring current Im that is proportional to the insulation fault. This measuring current generates a corresponding voltage drop at the measuring resistance Rm, which is evaluated by the electronics.

If this voltage drop exceeds a specific value equivalent to the under shooting of a specific insulation resistance, a signal will be output. IEC 61557-8 specifies detailed requirements to be met by insulation monitoring devices. Insulation monitoring devices serve as early-warning systems, providing operators with the information they need in order to have appropriate maintenance measures taken in good time.

IEC 61557-8 specifies that insulation monitoring devices must support a prescribed measuring principle which enables them to monitor both symmetrical and asymmetrical deteriorations in insulation.

Single-pole (asymmetrical) insulation fault
Symmetrical insulation fault