Technical Information
Grounded Operation
SMA Solar Technology AG
4/12
3 Grounded Operation
The essential difference to the standard devices in the Sunny Central product range is that one of the
PV generator's poles is centrally grounded in the Sunny Central by a fuse. This system concept was designed
in the USA and has been used in the inverters for the American market since it was first commercialized.
There are no normative stipulations regarding this concept. Determination of the maximum leakage current,
which causes the GFDI to disconnect the system, occurs in accordance with UL1741.
3.1 Behavior in case of Ground Fault
If a fault arises causing a ground fault to occur at the PV generator pole that is not grounded, the leakage
current flows through the GFDI and back to the grounded pole, thus triggering the GFDI. The triggering of the
GFDI interrupts the leakage current and the PV generator is not grounded by the ground fault (see figure 1,
system example with a negative grounded pole). This interruption of the leakage current prevents damage to
the system. The GFDI does not ensure personal protection. The "protection provided through the automatic
disconnection of the power supply" is not technically possible at this time. This is due to the complex structure
of the PV generator.
The period of time that passes before the fuse is triggered depends on the quantity of the leakage current, which
itself depends on the DC voltage and the transition resistance of the ground fault. Please refer to the triggering
characteristic curve of the GFDI for the triggering times (available upon request). If the leakage current is below
the triggering threshold of the GFDI, the fuse is not triggered. This state can, for example, be set for high
transition resistances or low radiation.
The triggering of the GFDI is signaled by an indicator light, a relay output, and the display on the device. The
signal occurs as a common fault, i.e. it can involve different fault causes. Please refer to the Sunny Central user
manual for the overview of the possible faults.
When the GFDI is triggered, the monitoring causes the inverter to deactivate. After the disconnection, the
insulation fault must be removed immediately.
Operation of the inverter with a triggered GFDI could damage the PV modules due to the absence of
grounding. If, while in this state, a ground fault also occurs at the grounded pole, this creates a short-circuit
(see figure 2) to which the inverter could feed current. This can damage the power module. Therefore, the
inverter cannot be operated when the GFDI is triggered.
Leakage current below the GFDI's triggering threshold can remain for an extended period of time and do
not trigger the fuse.
