Sunsynk Error F08: GDFI Relay Failure Root Cause and UK Resolution Guide

Key Takeaways

• Sunsynk error F08 denotes GFDI_Relay_Failure, indicating an internal or external ground fault detection interrupter relay anomaly.
• In the United Kingdom, the most frequent cause on newly commissioned or updated inverters is a software configuration mismatch where Grid Type is mistakenly set to split-phase rather than Single Phase.
• Never service or rewire inverter terminals without isolating all energy sources, ensuring full circuit isolation before inspecting terminals.
• Clearing the fault requires switching Grid Type to Single Phase, followed by a controlled cold reboot powered strictly from battery storage with AC and DC solar circuits disconnected.
• Persistent relay adhesion or internal contact failure after proper configuration demands immediate escalation to an MCS installer or Sunsynk engineering support.

What is Sunsynk error F08?

Hybrid solar inverters must continually verify the integrity of their earthing references and protective isolation relays before exporting power. On Sunsynk single-phase and three-phase hybrid inverters, the alert code F08 represents GFDI_Relay_Failure (Ground Fault Detection Interrupter Relay Failure).

When this fault occurs, the inverter control logic detects that the internal ground fault relay has either failed its pre-startup self-test or that external neutral and protective earth relationships contradict programmed system topology. As an immediate safeguard, the inverter halts power conversion, opens its grid connection relays, and displays F08 across the front LCD touchscreen.

For British homeowners and solar electricians, diagnosing an F08 error requires differentiating between software configuration mistakes and genuine physical earthing faults. For detailed guidance on domestic wiring regulations and protective bonding compliance in Great Britain, see our UK solar electrical installation guide.

Primary cause: Grid Type configuration mismatch

While the name GFDI_Relay_Failure suggests a damaged mechanical relay, Sunsynk technical field data confirms that the vast majority of F08 occurrences in the UK result from a firmware setup parameter.

When a Sunsynk inverter is initialized or undergoes a firmware refresh, the grid standard selection screen may default to an overseas profile. In official manufacturer troubleshooting documentation, Sunsynk specifies: "first check that the Grid Type in Grid Mode settings is set to Single Phase and not 120/240V Split Phase."

In overseas jurisdictions such as North America, split-phase networks rely on dual live legs with a center-tapped neutral. In such configurations: "F08 GFDI_Relay_Failure 1. When the inverter is in a Split phase (120/240Vac) or three-phase system (120/208Vac), the backup load port N line needs to connect ground;"

Because UK domestic supplies operate on standard single-phase alternating current rather than a 120/240Vac split-phase arrangement, leaving the inverter configured for split-phase causes the internal firmware to look for an earthing bond on the backup neutral conductor that does not exist. Consequently, the inverter flags an F08 relay failure and locks out.

Step-by-step resolution: correcting the grid profile

To rectify this configuration error and reset the inverter, follow this official four-step sequence:

1. Access Grid Mode Settings

1. Tap the gear icon on the top right of the Sunsynk home touchscreen.
2. Select the Grid Setup icon to enter grid operational parameters.
3. Locate the Grid Type field.
4. If the display shows 120/240V Split Phase or an overseas multi-phase standard, select Single Phase.
5. Tap OK to commit the setting to non-volatile memory.

2. Execute Battery-Only Cold Reboot

Simply changing the setting while the inverter remains connected to live grid power and DC arrays may not clear the locked relay state. As dictated by Sunsynk engineering: "Setting to Single Phase should clear the F08 Fault and the Inverter can be rebooted from Battery Power only with AC and DC PV isolated."

To execute this clean reboot:

1. Turn off the external AC rotary isolator or consumer unit miniature circuit breaker connecting the inverter to the mains supply.
2. Turn off the DC solar array rotary switch on the underside of the inverter casing.
3. Power down the inverter using the front power button, leaving only the low-voltage battery storage system active.
4. Allow the inverter display to turn off completely.
5. Power the inverter back on using battery power alone.

3. Restore AC Grid and Solar PV Supplies

Observe the touchscreen initialization sequence. Sunsynk instructions confirm: "If the unit goes into 'Normal' state and the F08 has cleared the AC and DC PV can now be restored."

1. Once the home screen indicates Normal operating status without the F08 warning banner, switch on the external AC mains isolator.
2. Switch on the DC solar string isolators.
3. Wait for the inverter to complete its internal safety countdown and synchronize with the mains supply.

4. Verify Active Power Flow Telemetry

Finally, review system operation on the central display. Sunsynk guidelines mandate: "check settings and check Power Flow for correct operation and functionality"

Ensure that solar generation, battery charging, and domestic consumption dials report expected values without intermittent relay chatter or secondary warnings.

Neutral-earth bonding and island mode considerations

For systems fitted with emergency power supply (EPS) backup circuits, earthing rules require careful design to ensure safety during power cuts without triggering false ground faults during grid-tied operation.

Class Conversion and Earthing References

In normal grid-tied operation, the installation relies on the distribution network operator earthing arrangement. However, off-grid backup requires dedicated protection: "To change the classification of a Sunsynk Inverter from a Class II to a Class I an Earth-Neutral Bond must be created."

Signal Island Mode Configuration

To ensure an earthing reference is established exclusively when the mains grid fails, Sunsynk provides an automatic relay control signal: "To activate the Earth-Neutral Bond when GRID power is lost Signal Island Mode MUST be selected in the battery settings."

Selecting Signal Island Mode commands an external or internal contactor to bridge neutral and earth only when mains voltage drops to zero.

Relay Hardware Configurations

Installers must review the specific hardware version in use: "Some inverter models have an Earth-Neutral Bond built in whereas other may have hot contacts, dry contacts or a combination of that can be used to create this bond." "Different makes/manufacturers of contact relays may have different Pin Out configurations. Please refer to the manual of your specific relay before making any connections."

Avoiding Upstream RCD Incompatibility

A critical installation mistake is creating a permanent hardwired bridge between neutral and earth on hybrid systems: "If an Earth Bond is required between neutral and earth and your system is a hybrid system, then you can have a permanent earth wire since this can cause faults with an RCD before the inverter."

A permanent neutral-earth bond allows circulating return currents to bypass the neutral sensor of an upstream residual current device, leading to continuous nuisance tripping. Grid compliance requirements are further detailed in our micro-generator grid connection guide.

Safety precautions during earthing inspections

Working within hybrid inverter terminal bays carries high risk. Installers must confirm that both AC distribution and high-capacity battery DC circuits are completely de-energized before accessing internal terminals. Auxiliary earthing conductors must remain bonded to maintain touch-voltage protection.

Installers operating on commercial sites must also verify network compliance under our commercial grid connection guide.

When to escalate to Sunsynk technical support

If the Grid Type has been verified as Single Phase, a battery-only reboot has been completed, and external wiring has been confirmed to be free of earth faults, yet error F08 persists immediately upon startup: "2. If the fault still exists, please get in touch with us for help."

A persistent F08 under correct single-phase configuration indicates that the internal GFDI relay contacts have welded or that the relay drive circuit on the main power board has failed, necessitating warranty repair or inverter replacement.

Ensure the inverter serial number, installation date, and full display event log are documented before opening a service case. Promptly resolving inverter lockouts preserves your daily solar self-consumption and protects ongoing export revenues under your energy contract, as detailed in our Smart Export Guarantee guide.