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.
Notes
- A symmetrical deterioration in insulation can be said to occur when the insulation resistance of all conductors in the system to be monitored declines to (approximately) the same extent. An asymmetrical insulation deterioration can be said to occur when the insulation resistance, e.g. of a conductor, declines to a significantly greater extent than that of the other conductor(s) in the system.
- Devices known as ground-fault monitors, which use the voltage unbalance (voltage shift) occurring in the event of a ground fault as a sole measuring criterion, are not ground-fault monitoring devices under the terms of this standard.
- Under certain system conditions, a combination of various measuring techniques, including unbalance monitoring, can be used for monitoring.
Notes:
Symmetrical ground faults are common in DC voltage systems and control circuits. If the resistance of both ground faults is approximately equal, ground-fault monitoring devices operating on the basis of the principle of superimposed voltage measurement will not be able to detect these ground faults. It is for this reason that IEC 61557-8 requires the use of continuous ground-fault monitoring devices.
