Error message Riso
(en) The Riso error message indicates that the inverter has discovered a ground fault in the PV array or that the PV module’s insulation resistance (R-Iso) is too low. Possible causes for this are damaged PV modules, DC cables or PV connectors.
Normally, 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.
In addition to the Riso error message (red LED), there is also the Riso status message (with flashing green LED) and the Riso-Sense error (with blink code 5).
The Riso status during the connection phase indicates that the transformerless inverter is presently carrying out the prescribed insulation test before each grid connection. This is a normal procedure.
However, if this status message is displayed over an extended period of time, or the Riso-Sense error is reported, this indicates a failed insulation measurement.
Possible causes of the failed insulation measurement are:
- Defects in the measuring unit (e.g. after lightning strikes)
- Insulation resistances that fluctuate heavily or are too low
- Moisture in the inverter
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).
Information:
- Further information on the Riso threshold for systems with SMC 9/10/11000TL(RP)-type inverters and thin-film PV modules can be found in the technical information “Riso-UDE10452.” This can be downloaded from the Archive section at https://www.sma.de/en/service/downloads.html.
- Also 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).
Corrective measures for PV 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.
- Where possible, read out the inverter values via communication (e.g. Sunny Explorer or Sunny WebBox) and note down the actual Riso value.
Information: The Riso value fluctuates depending on the weather conditions (humidity / moisture on the modules). - Check whether there is evidence of a lightning strike and whether other devices in the PV system or in the household are affected.
- Contact an installer immediately. The following 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, USB-Serviceinterface or Sunny WebBox). Depending on the communication product used, visualization in the Sunny Portal may be possible.
- Check if there is moisture in the device. Moisture can cause creeping currents, which interfere with the electronic measuring equipment and cause the error message. Check whether any moisture has got into the inverter during installation, whether the inverter’s enclosure lid is firmly screwed on, the enclosure openings are correctly configured and the unneeded DC inputs and enclosure openings are closed with filler plugs.
- 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.
Important information: The Riso value varies depending on the weather conditions (humidity / moisture on the modules). For this reason, take the measurements in the relevant weather conditions, if possible while the inverter is displaying the error.
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
- Exact error message (Riso, F-Riso, Riso status or Riso-Sense) and LED display
- How long has the error been present for now? Is it sporadic or continuous?
- Is there evidence of a lightning strike? Are other devices in the PV system or in the household affected? If yes, please indicate the system address and date of failure.
- Was there moisture in the device?
- Riso values that have been read out via communication, weather conditions during the measurement
- Measured DC voltage between +/- and PE of all strings, weather conditions during the measurement
- Measured insulation resistance of all strings, weather conditions during the measurement
The Riso spot value measured by the inverter and the Riso minimum value set in the inverter 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
Riso minimum value under Settings/Grid Monitoring/Minimum insulation resistance
Sunny WebBox (485)/ USB service interface after the inverter has been selected in the system tree:
Riso spot value under Spot Values/Riso
Riso minimum value under Parameters/Riso or Iso.LeakRisMin
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.