Ground-fault monitoring: The concept

Operating principle of a ground-fault monitoring device

The ground-fault monitoring device is connected between the live supply conductors and ground and superimposes a measuring voltage Um. In the event of a ground fault, the ground fault RF closes the measuring circuit between the system and ground, generating a measuring current Im that is proportional to the ground 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 ground-fault monitoring devices. ground-fault 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 ground-fault monitoring devices must support a prescribed measuring principle which enables them to monitor both symmetrical and asymmetrical deteriorations in insulation.

Single-pole (asymmetrical) ground fault
Symmetrical ground fault