Ground-fault monitoring: The concept
Operating principle: ground-fault detection through insulation monitoring
The ground-fault monitoring device is connected to the live supply conductors and to ground. Much like an ohmmeter, it applies a voltage between these points to measure resistance. In the event of a ground fault, the fault closes the measuring circuit between the monitored system and ground, allowing a resistance measurement that may indicate a ground fault. This technique can be used on AC, DC and mixed AC/DC systems.
If measured insulation resistance declines below a set value, an alarm is issued. Ground-fault monitors serve as early-warning systems, providing operators with the information they need in order to plan appropriate maintenance measures.
Measuring the resistance of system insulation to ground enables detection of both symmetrical and asymmetrical ground faults.
Measurement methods
Applied-DC measurement method
A low-level DC voltage is applied across the system phase conductors and the ground (bond) conductor.
This measurement method is suitable for monitoring AC systems such as those with conventionally controlled motors. It is not suitable for systems with DC components, such as those with motors controlled by adjustable-speed drives.
System leakage capacitances that are present in every electrical system are simply charged to the measuring voltage and ultimately have no effect on resistance-measurement accuracy.