What Does This Error Mean?

Fronius State 301 means the grid voltage on the AC output fell below the minimum allowed limit. The inverter shuts down as required by grid safety regulations and will restart automatically once normal voltage is restored. If State 301 appears frequently, your grid connection has voltage instability that needs a licensed electrician to investigate.

Affected Models

Fronius Primo
Fronius Symo
Fronius Galvo
Fronius IG Plus
All Fronius string inverters

Common Causes

Local grid undervoltage — utility supply voltage dropped below the minimum threshold (typically 207V on a 230V grid)
Long cable run from the main panel causing significant voltage drop under load
Loose or corroded AC connection at the inverter's AC output terminal
Grid fault or maintenance by the utility in your area
Inverter AC voltage range settings not calibrated for local grid conditions

How to Fix It

Wait for the inverter to restart automatically.

Fronius inverters shut down with State 301 and attempt to restart every few minutes once the grid voltage recovers. A single or occasional State 301 event on a hot afternoon or during a storm is usually a momentary grid fault — the inverter will resume production without any action needed. Check the Fronius display or Solarweb portal the next morning to confirm production has resumed.
Check the inverter AC output connections.

A loose or corroded wire at the inverter's AC output terminals causes a significant voltage drop, which the inverter reads as low grid voltage. Have a licensed electrician check the AC terminal connections inside the inverter and at the main switchboard. Pay attention to the neutral and ground connections — a loose neutral is a common cause of apparent undervoltage.
Check the AC cable from the inverter to the switchboard.

An undersized AC cable (too thin for the inverter's output current rating) creates a voltage drop that can push the measured voltage below the State 301 threshold. Verify the cable cross-section matches the Fronius installation guidelines for the cable run length. This is an installer task — contact your solar installer if you suspect undersized wiring.
Review State 301 frequency in Solarweb.

Log in to solarweb.com and open your system's Event Log. Filter for State 301 events and note the times they occur. If State 301 appears repeatedly in the early evening when demand peaks, the local grid is experiencing regular undervoltage — report this to your electricity network provider.
Adjust the voltage trip threshold if permitted.

In some regions, an installer can adjust the minimum AC voltage threshold in the Fronius service menu to better match local grid conditions. This requires a licensed installer with Fronius service access — do not attempt to change voltage limits yourself. Contact your solar installer to review whether the inverter's AC settings are appropriate for your grid.

When to Call a Professional

Frequent State 301 events indicate a grid voltage problem. Contact your electricity network operator to report low voltage, and have a licensed electrician check the AC wiring from the inverter to the main panel.