Isolation and Ground Short Monitors

Isolation and Ground Short Monitors are designed for specific voltages.  Choose the appropriate device based on the system characteristics listed in the table below for more information.

Isolation Monitor Selection

System VoltageSupply VoltageMethod of Measurement
Device Type
0 to 230VACSystem VoltageActive, with DC measurement voltageNWI1100
System VoltageNWI110x-V
100-260VAC  100-350VDCNWI5810
0 to 690VACSeparate AC or DCNWI5120
0 to 5000VACSeparate AC or DCNWI5121 + HTC5000
0 to 300VSeparate AC or DCActive, with square wave measurement voltageNWI4110
0 to 690VSeparate AC or DCNWI4120
0 to 220VDCSystem VoltagePassive, without measurement voltageNWI2100
NWI2105

Measuring Procedures Comments

There are two different ways of detecting an isolation fault or a ground short, depending on the power of the monitored network.  These are known as active and passive measurement methods.

The superimposed positive DC signal or the pulsed DC signal indicate an active measuring system; while the passive measuring systems measures the residual voltage between the network connection and ground.  The standard DIN EN61557-8 states that active measuring systems can be used as isolation monitors, however passive measuring systems may only be used to detect ground faults.

Isolation monitors must follow DIN EN61557-8, which states that they need to be able to detect both symmetric and unysmmetric isolation deterioration.  The measuring system with the superimposed DC signal should only be implemented in AC systems.  The use in AC systems with connected rectifiers is possible, however not recommended due to the inaccuracy and high sensitivity of isolation faults.  A result is early triggering of the alarm.  The use of the pulsed DC signal is possible in every system type.  Due to the high measurement efforts, the longer evaluation time, and the higher associated costs, these should not be used in pure AC systems.  However, devices with a simpler method of measurement, pure DC superimposed signals, should be used.

The passive measurement method works without a superimposed signal.  It is used in only DC systems (usually only control voltage systems) and is referred as a ground fault monitor.  A symmetrical isolation fault detection is not possible along with the display of the current isolation value.