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Diagnostics case :

Event number 35, 3501 Insulation failure > Check generator

Insulation failure > Check generator

Event numbers 35 and 3501 indicate that the inverter has discovered a ground fault in the PV array or the insulation resistance (R-Iso) is too low. Possible causes for this are damaged PV modules, DC cables or PV connectors. The Riso value can vary depending on the weather (humidity/moisture on the modules).

The error is not caused by the inverter; it is in the DC installation. Transformerless inverters can be damaged by sporadic ground faults in the module array.

Background information:

PV systems with transformerless inverters are not galvanically isolated from the grid during feed-in operation. In accordance with standard specifications, they must not undershoot a specific insulation limit value before being connected to the grid.

Every PV system has a different potential to ground both before being connected to the grid as well as during feed-in operation. Only sufficient insulation to ground prevents currents from flowing off from the PV system via ground and eliminates any danger in the event of contact as well as additional power losses. The entire ground current, also called leakage current, is made up of the contributions from all system components (PV modules, DC cables, inverters) and can vary depending on the weather (humidity/moisture on the modules). For a given system voltage this leakage current translates into an effective isolation resistance, which is designated by Riso.

In transformerless inverters, the continuous measurement of Riso in operation is not possible due to this missing galvanic isolation . For this reason, transformerless inverters must always measure the Riso value before being connected to the grid and monitor the residual current during operation as even one single short circuit can lead to damage to property and personal injury. Since the level of current is decisive when such damage arises, the minimum Riso value prescribed by the specialist standard depends on the maximum input voltage, e.g., 1000 kΩ with the SB3000HF-30 or 200 kΩ with the STP10000TL-20. As soon as this minimum value is undershot, the inverter must not connect to the grid for safety reasons.


Even the SMA inverters with a transformer measure the insulation resistance. Since other specifications apply in the case of galvanic isolation, they do not forbid connection to the grid; they merely display a warning (in accordance with the parameters laid down in most country data sets).

The Riso spot value measured by the inverter and the minimum Riso value set in the inverter parameters can be read out via communication as follows:

Sunny Explorer / Sunny WebBox Bluetooth / SMA Cluster Controller after the inverter has been selected in the system tree:

Riso spot value under Spot Values / DC side / Insulation Monitoring

Minimum Riso value under Settings / Grid Monitoring / minimum insulation resistance

Sunny WebBox (485) after the inverter has been selected in the system tree:

Riso spot value under Spot values / insulation resistance

Minimum Riso value under Parameter / Iso.LeakRisMin

Corrective measures for system operators:

WARNING! Danger to life due to electric shock

In the event of a ground fault, high voltages can be present. Do not touch the cables, the substructure, the frame or the modules of the PV array.

• Read out the current Riso value via communication (e.g. Sunny Explorer).

Note: The Riso value fluctuates depending on the weather (humidity/moisture on the modules).

• Contact an installer or an SMA solar power professional immediately. The subsequent steps require the specialist knowledge of an electrically qualified person or installer.

Corrective measures for installers:

DANGER! The tasks described below may only be performed by electrically qualified persons:

• Read out the current Riso value via communication (e.g. Sunny Explorer). Depending on the communication product used, visualization in the Sunny Portal may be possible.

• Check PV system by means of voltage measurement +/- to ground, as described in the device type’s service or installation manual.

• If the Riso value read out via communication is conspicuously low, measure the Riso value, as described in the device type’s service or installation manual.

Note: The Riso value varies depending on the weather (humidity/moisture on the modules). For this reason, take the measurements in the relevant weather conditions.

If, however, the message keeps on appearing, even though the PV system has been checked as described, the subsequent steps must be clarified with SMA Service.

In this case, please contact SMA Service with the following information:

• Device type and serial number of the inverter

Event number

• How long has the error been present for now? Is it sporadic or continuous?

• Riso values that have been read out (e.g. via Sunny Explorer)

• Measured DC voltage between +/- and PE of all strings

• Measured insulation resistance of all strings

• Weather conditions during the measurement

All work on the inverter and the cabling of the PV array must be carried out by an electrically qualified person. SMA accepts no liability for the information and solutions given in the Online Service Center. Additional information can be found in our Online Service Center’s terms and conditions of use.