Ground Fault Detection in Ungrounded AC/DC Systems
An active IMD is the ground detector of choice for ungrounded systems. The active IMD can detect ground faults regardless of
quantity and severity, as well as the ability to have early warnings and trending.
The active IMD is considered to be an online megger. It connects via pilot wires between the system and ground.
A constant measuring signal is sent from the IMD into the power wires. It will spread out evenly into the secondary
side of the transformer and the attached loads. If this signal finds a breakthrough path to ground, it will take
this path of least resistance and return to the monitor. The IMD's internal circuitry will process the
signal and trip a set of indicators when the resistance of the fault reaches a certain trip level. By the nature
of the ungrounded system, leakage current may or may not be present at this time, however the ground fault
will still be seen through the insulation resistance. Because of this, IMDs measure in Ohms (resistance)
and not in Amps (current). A ground fault will be indicated as "insulation breakdown."
A typical system would be considered to have good insulation with a resistance value of multiple
kilo ohms or mega ohms, and would be considered to have low insulation at the low kilo ohm to
less than one kilo ohm range. However, the value of a system's overall resistance may vary
depending on the number of loads, type of insulation used, age of the installation,
environmental conditions, etc.
Figure 1: Ungrounded three-phase AC system with BENDER IMD
The active IMD is the preferred choice for ungrounded DC systems as well. As in floating DC systems, a DC
IMD will be connected via pilot wires between the system and ground. A constant measurement signal will be sent
from the IMD into the power wires. From there, it spreads out evenly into the secondary side of the supply
(e.g. a battery) and the attached loads. Again, the signal will take the path of least resistance and
return to the monitor if it finds a breakthrough to ground.
For DC systems, as well as AC systems that have varying voltages or power conversion equipment
including variable frequency drives, a special measurement signal is applied. BENDER's AMP Plus measurement principle,
found in devices such as the IRDH275, have the ability to be used universally in AC, DC, and AC/DC systems,
as well as overcoming adverse system conditions, such as high leakage capacitance.
Figure 2: Ungrounded 120/250 VDC system with BENDER IMD