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Diagnostics case :

Event number 74, 7401 Varistor defective

Varistor defective

Event numbers 74 and 7401 indicate that at least one of the inverter’s thermally monitored varistors is defective.

Varistors protect the electronics against atmospherically coupled energy peaks, such as those that can occur in the conductor loop of the PV array in the event of lightning strikes in the neighborhood.

Varistors are wear parts. Their functional efficiency diminishes with repeated strain as a result of overvoltage.

During normal operation, the varistors’ resistance is very large. After strain the resistance drops and it is necessary to replace the varistor(s).

Corrective measures for system operators:

• Contact an installer or an SMA solar power professional. 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:

Check that the varistors are operating, as described in the service manual or the device type’s installation manual. They are available for download at https://www.sma.de/en/service/downloads.html.

If there is no conductive connection to the areas described, the varistor is defective. SMA Solar Technology AG recommends replacing all varistors immediately; replacement varistors and insertion tools can be ordered from SMA Service.

If there is a conductive connection, please contact SMA Service with the following information:

• Device type and serial number of the inverter

Event number

• Installation date

• Have the varistors been checked?

• Is the inverter feeding power in?

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.