Fox ESS grid code fault — voltage & frequency diagnosis

Open Fox Cloud and check the alarm history. The grid alarms are Grid Lost Fault (code 1, listed in Fox Cloud as No Utility), when the grid supply has been lost; Grid Volt Fault (code 26, listed as Grid Voltage Fault), when grid voltage is outside the allowed range; Grid Freq Fault (code 27, listed as Grid Frequency Fault), when grid frequency is outside the allowed range; and 10min Volt Fault (code 28, listed as 10 Minute Average Grid Voltage Fault), when grid voltage has been out of range over the last 10 minutes. Fox ESS's manuals give the names, and on an H3-Plus or P3-Plus each alarm also has a four-digit code. Note the alarm and the time it occurred. A midday-only pattern is worth telling us, because Fox ESS says an inverter raises its own voltage to export and, on a weak grid, that can trip it.

In normal running the inverter needs a live grid connection, and if the grid supply is lost it shows a Grid Lost Fault. Owners report Grid Lost, Grid Volt and Grid Freq alarms appearing together in a power cut. Fox ESS says the system reconnects by itself once the grid is back to normal. A system with EPS wired in by your installer and a battery fitted can still supply its EPS circuits in a power cut. If the rest of the house has power, the supply to the inverter may be lost because the breaker feeding it has tripped. Look at the consumer unit, without opening anything, for the circuit labelled Solar or PV. If it has tripped, ask your installer or an electrician to find out why and put it back on. Choosing the right breaker or RCD is your installer's job.

The UK grid standard is 230 V with a tolerance of +10% / -6%, giving a permitted range of 216 V to 253 V. Check the real-time grid voltage on the inverter display (on the H1 series, System, then Grid). The inverter disconnects if the grid voltage goes outside its own protection limits, which are set for the UK grid code at commissioning and are wider than that range. High grid voltage can happen at the end of long supply cables and in streets with many solar systems exporting at once. If readings regularly exceed 253 V, the supply may be too high, although the inverter's own export and the property's cabling can add to the voltage it reads. Ask an electrician to confirm the voltage at your incoming supply, then contact your DNO to report it.

The inverter carries a country (grid code) setting. It defaults to the local setting at first start-up and the installer checks it at commissioning, so it should be set for the UK. If it is set to the wrong country, the voltage and frequency protection limits will be wrong and the inverter may disconnect unnecessarily. On the H1 series the grid-protection settings are under Settings, then On-Grid, and changing them needs the installer password, so ask your installer or us: we can change grid settings if requested. G98 or G99 is not a menu choice. G98 applies to a system up to 16 A per phase of total registered capacity (3.68 kW on a single-phase supply, 11.04 kW on three-phase) and G99 to a larger one, which needs an application to the DNO before installing. Anything bigger is always G99, even with export limited: a 5 kW inverter limited to 3.68 kW is still G99, never G98.

Loose or corroded AC connections between the inverter and the consumer unit can cause intermittent grid loss. The inverter sees a momentary voltage drop or spike as the connection makes and breaks, which can raise a Grid Volt Fault or a Grid Lost Fault. Check that the AC isolator switch is fully on, and look for any visible sign of heat damage or discolouration on cables or connectors, without touching them. If you suspect a wiring fault, a qualified electrician must check it: do not open the inverter or consumer unit yourself. Smoke, fire or a burning smell? Do not go near the equipment. Get everyone out, call 999 and tell them there is a solar battery.

Fox ESS says the system reconnects by itself once the grid is back to normal. If the supply is on, the voltage looks normal and the settings are correct but the alarm has not cleared, shut the whole system down and restart it. Our order for a Fox ESS system is the battery's power button first, then the AC isolator, then the inverter's DC isolator, and the battery's DC MCB last if it has one, so the MCB is never switched under load. To turn it back on, go in reverse: battery DC MCB, DC isolator, AC isolator, then the battery's power button. Read the full shutdown guide. If the alarm comes back, note the readings and the times, and contact STS for a remote diagnostic, or contact Fox ESS or your installer.