DC Insulation Resistance of PV Systems
with Fronius Inverters
White Paper
© Fronius International GmbH
Version 1 08/2017
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WHAT IS INSULATION RESISTANCE?
Each single component of the PV system has an insulation resistance to ground. Combined this results in the
insulation resistance of the PV system (R
iso
).
Usually this leads to very small and harmless fault currents.
In the event of a fault this ensures very small and harmless fault currents.
On rare occasions the fault current can reach a dangerous level which may cause damage (e.g. fire) or personal
injury. Therefore the insulation resistance has to be above a certain value to ensure fault currents stay below a
dangerous level.
WHAT DOES THE INVERTER DO?
The inverter has to determine whether or not the whole PV system is sufficiently insulated between live parts and
accessible parts.
In the morning the inverter measures the insulation resistance and will turn on if the resistance level is okay.
If the resistance level is insufficient, the inverter will not connect to the mains and will indicate (and also send) the
appropriate state code.
The threshold level for R
However, these standards each define different threshold values resulting from different approaches.
After the startup, the inverter monitors the insulation conditions with an approved, integrated RCMU unit.
is specified in different, relevant standards.
iso
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RELEVANT STANDARDS
For the inverter:
IEC 62109-2: Safety of power converters for use in photovoltaic power systems –
Part 2: Particular requirements for inverters
This standard requires that the insulation resistance of the PV system shall not be less than R
For example, an inverter with a maximum DC input voltage of 1000V should not connect to the mains if the R
less than 33.3kΩ (= 1000V / 30mA).
iso
= (V
max_PV
/ 30 mA).
iso
For the PV system:
IEC 60364-7-712: Low voltage electrical installations – Part 7-712: Requirements for special
installations or locations – Solar photovoltaic (PV) power supply systems
This standard specifies the insulation resistance for certain PV array sizes (see table below):
It also recommends, where possible, to use higher values than the ones stated, to increase the safety of the PV
system.
is
For PV modules:
IEC 61215-2: Terrestrial photovoltaic (PV) modules - Design qualification and type approval –
Part 2: Test procedures
This standard requires the insulation resistance of a module shall not be less than 40MΩm².
This definition makes the R
The larger the array, the lower the insulation resistance may be and of course will be.
of a real PV system dependent on the area of the PV array.
iso
Past standards (no longer applicable):
In previous inverter standards it was defined that the calculation of R
resulted in a value of 1MΩ for inverters with a V
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max_PV
of 1000Vdc.
may not be less than 1kΩ * V
iso
max_PV
. This
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