Ground-fault Monitoring in Ungrounded Systems

How ground-fault detection through insulation monitoring works

Ground fault monitoring solutions must be approached differently in ungrounded systems vs. solidly grounded or resistance-grounded systems. In ungrounded systems, the best practice is to actively monitor the “strength” of the insulation system to ground, even while the system is in operation.

In the past, passive methods such as the “three lightbulb” method had been used, but these techniques cannot detect symmetrical failures such as where insulation has degraded relatively evenly across all active conductors or a connector has been flooded causing relatively equal degradation in the isolation to ground. The modern solution is to use a dedicated insulation monitor device (IMD) that can provide early detection of ground faults to allow critical systems to remain online while faults are identified and resolved. By continuously monitoring the system's insulation resistance, IMDs can provide early indication of both immediate and trending ground faults before leakage current may be of concern.

The insulation 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, resulting in 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 the detection of both symmetrical and asymmetrical ground faults.

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 pure-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. Larger systems tend to have higher leakage capacitance which can cause the measurement to take more time due to the longer charge times associated with increased capacitance.

AMP measurement method

The Bender patented AMP (Adaptive Measuring Pulse) measurement method is a different technique that allows IMDs to be used in a broader array of applications. AMP adapts automatically to the monitored system and its leakage capacitance. Software-based evaluation filters inherent leakage currents and accurately calculates the insulation resistance. This means that broadband interferences as they occur, for example, during converter operation, do not adversely affect the precise determination of the insulation resistance.

The AMP Plus measurement method takes interference suppression to the next level. IMDs supporting this measurement method can be used universally in AC, DC, and AC/DC systems, such as systems with varying voltages or frequencies, high system leakage capacitances, or DC voltage components. This makes Bender AMP-enabled devices ideal for use in today's industrial and commercial power distribution systems, which are typically subject to this type of interference.