Fox ESS Grid Freq Fault
Grid Freq Fault, often shown as Error 27, means the utility grid frequency has drifted outside the safe range your inverter is set to allow (in the EU and UK that window is commonly around 49.5 to 50.5 Hz). A grid-side disturbance has pushed the frequency too high or too low, so the Fox ESS disconnects to protect itself, exactly as the grid code requires. This is beyond your control and almost always clears by itself within a minute or two as the grid rebalances.
What usually causes it
- A short grid-side disturbance. A sudden change in supply or demand on the network pushes frequency briefly outside the allowed band.
- Local network instability. Switching events, faults or maintenance upstream can wobble the frequency for a moment.
- A regional protection setting. Your inverter is configured to a national grid code, so the exact limits (the 49.5 to 50.5 Hz figures are a regional example) depend on where you live.
- A weak or unstable supply in some rural or end-of-line locations, where the grid is more prone to drift, less common but worth noting if it keeps happening.
How to handle it safely
- Wait a couple of minutes. This is a protective disconnection, not an inverter fault. The Fox ESS reconnects on its own once the grid frequency settles back inside its limits.
- Check it is not a wider issue. If lights flicker or other devices act up at the same time, the disturbance is on the grid, not in your home.
- Do nothing to the inverter. There is no setting for an owner to change here. The frequency limits are set to match your grid code and must not be altered.
- If it recurs constantly, note the times and report it to your installer or your DNO (Distribution Network Operator). Persistent frequency faults can point to a genuine supply problem they should investigate.
Quick decision flowchart
Related Fox ESS codes
FAQ
Is Grid Freq Fault something I broke or need to fix?
No. It means the utility grid frequency moved outside the safe range, which is on the network side, not in your system. Your Fox ESS simply disconnected to protect itself, as the grid code requires, and reconnects by itself once the frequency settles. There is nothing for you to repair.
It cleared in a minute, should I still report it?
A one-off that clears itself is normal and needs no action. Only if it happens repeatedly, say several times a day or on most days, is it worth noting the times and telling your installer or DNO, since that can signal an unstable supply they should check.
Helpful guides
Sources
- Fox ESS H1/AC1 inverter user manual and FoxCloud alarm list (Grid Freq Fault / Error 27 = grid frequency outside the configured protection range).
- Fox ESS installer guidance on grid-code frequency limits and the regional protection settings applied at commissioning.