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Diagnostic guide · Fox ESS
Fox ESS ground fault — earth and insulation diagnosis
Fox ESS lists two different alarms here. Ground Fault means the ground connection has failed. Iso Fault means low insulation resistance between the PV side and earth. Do not keep clearing either one: read the exact name in Fox Cloud and call an engineer.
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We check Fox ESS ground and insulation faults
Fox's list says an Iso Fault can go back to normal after a while, but that does not mean it is fixed. Do not keep clearing it. Tell us the exact alarm name and the times from Fox Cloud, and with your permission we read the alarm history with you before anyone goes on the roof.
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Genius when it comes to solar and off grid installations we spent days trying to work out what was causing our problems and Ron resolved it in less than an hour! Cant reccomend enough
Ben Carroll · Aug 2026 Google
The Solar Tech Support company was the only one to respond to a request for someone to supply and fit a new inverter for our solar panels after our old one finally expired after 26 years. Ron kept us well informed throughout, came and ascertained that the issue was down to the inverter having failed then sourced and fitted a new one. He got the system up and running again, set up the wi-fi performance feedback and took away the old inverter for disposal. By acting promptly and fitting the job around existing commitments we were able to get our solar power unit back in operation with minimal delay in the short while before we were due to go away for a week. The work was carried out very professionally and we were asked to confirm that all was in order the following day. I was impressed that, having recently taken on additional work for GivEnergy customers we were able to keep our solar panel down-time to a minimum. I would highly recommend for prompt attention and first-rate service.
James Dignan · Aug 2026 Trustpilot
Genius when it comes to solar and off grid installations we spent days trying to work out what was causing our problems and Ron resolved it in less than an hour! Cant reccomend enough
Ben Carroll · Aug 2026 Trustpilot
Particularly useful in helping to fix my GivEnergy All in One system with a disabling fault.
Patrick Collins · Sep 2026 Google
Solar Tech Support answered my initial enquiry swiftly. They provided remote support to establish what the issue was, and through their experience and network, managed to get my clients system back online. Once the system was online, STS managed to diagnose the main issue to get the ac coupled inverter and battery back online, a battery repair was required. A site visit was organise and within a few days, the system was tested and back up and running. This has saved my client, and probably myself huge costs and inconvenience now the system is back online and working correctly. Would highly recommend.
Dean Wilkinson · Jul 2026 Google
Start here
Ground Fault or Iso Fault? They are not the same alarm
Before you start: open Fox Cloud, go to your device and check the alarm history. Write down the exact alarm name and the date and time it first appeared, then check what the weather was doing on those dates, which is our advice rather than Fox's. Fox's alarm list gives each alarm below its own entry and meaning, so read the exact name before doing anything else. Fox's manuals use the same names for Ground Fault, Iso Fault and Res Cur Fault across the H1-G2, H3, KH and EVO, and we cover all Fox ESS models.
Ground Fault
Ground connection failed
Fox's meaning is that the ground connection has failed. Fox's action points to the voltage between neutral and earth and the AC wiring. That check, and any disconnecting and reconnecting of PV, grid and battery, is an electrician's job. If your RCDs trip when backup power starts, EPS wiring and earthing is an installer's job, and the EPS not working page covers it.
Iso Fault
Insulation failed on the PV side
Fox's list says the isolation has failed: low insulation resistance between the PV side and earth. Fox's action is to check whether the wire insulation is damaged, see whether the alarm goes back to normal after a while, or ask for help. The insulation check is an engineer's job. Do not ignore this alarm, because it points to a possible safety issue.
Res Cur Fault · Res Cur HW Fault
Residual current high, or its detector failed
Res Cur Fault means the residual current is high. Fox gives it the same action as Iso Fault but lists residual current monitoring as a separate protection, and not all leakage is an insulation failure: rain and humidity can change normal leakage from the panels. Res Cur HW Fault is different again: the residual current detection device itself has failed, so call an engineer.
Bms Insul Fault
Battery insulation fault
This one is on the battery side, not the PV side and not the ground connection. Fox's meaning is a battery insulation fault, and its action is to contact the battery supplier. Do not open the battery. Tell your installer or an engineer the exact alarm name.
Diagnosis
Step-by-step ground fault diagnosis
Work through these in order. Step 1 tells you which alarm you have. Steps 2, 4 and 5 apply to every alarm here, step 3 is for Iso Fault and Res Cur Fault, and step 6 says who to book.
1
Read the exact alarm name in Fox Cloud
Open Fox Cloud, go to your device and check the alarm history for timestamped alarms, or look at the inverter's own screen. Write down the exact alarm name and when it first appeared, because the name decides everything that follows. Fox Cloud's wording can differ from the name in Fox's manual, so copy it exactly as it shows. Ground Fault means the ground connection has failed. Iso Fault means low insulation resistance between the PV side and earth. Res Cur Fault means the residual current is high. Bms Insul Fault is a battery insulation fault.
2
Do not repeatedly clear the alarm
Do not keep clearing the alarm or restarting the inverter. For an Iso Fault or Res Cur Fault, restarting is not a fix: Fox's list gives those two no restart remedy, only a check of the wire insulation, a look at whether the alarm goes back to normal after a while, or a call for help. If the alarm stays or keeps coming back, switch the system off and leave it off until an engineer has looked at 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.
3
Compare the alarm timestamps against the weather
This step is our advice, not a Fox check. Note whether the alarm appears in wet weather or on damp mornings, and whether it clears when things dry out. A pattern helps the engineer, but it does not prove where the fault is. If the alarm clears in dry weather and returns when it rains, it has not been fixed.
4
Do not use the PV switch as a test
The inverter's DC isolator, sometimes called the PV switch, is only for switching the system off in the order in step 2. Turning it off to see whether the alarm clears is not a test, and Fox gives owners no such test. Where it sits depends on the model: on an H1-G2 hybrid it is on the underside of the inverter, other models may use an external switch, and AC-coupled AC1 and AC3 models have no PV input of their own. Do not open your inverter or touch DC cabling to find it: ask your installer or an engineer.
5
Look from ground level, and touch nothing on the roof
From the ground, without touching anything, you may see obvious damage: melted plastic, scorch marks or water dripping from a connector. Note what you see and photograph it if you can do so from the ground. Do not open the inverter, disconnect cables or touch MC4 connectors: DC voltage from the panels is present whenever there is daylight and cannot be switched off. A look from the ground cannot confirm where the fault is, and cable runs and roof voids are for an engineer to inspect. 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.
6
Book an engineer or electrician
The checks in Fox's alarm list are a job for a qualified person, not an owner. Iso Fault or Res Cur Fault: a qualified solar engineer checks the wire insulation, finds any damaged part and repairs or replaces it. Ground Fault: an electrician checks the voltage between neutral and earth and the AC wiring, and does any disconnecting and reconnecting of PV, grid and battery. Res Cur HW Fault: the detection device itself has failed, so call an engineer. Bms Insul Fault: Fox's action is to contact the battery supplier, so tell your installer or an engineer which alarm you have.
Insulation resistance: what the documents say
- H1, H1-G2 and KH
- Fox prints no insulation figure for these inverters.
- Other Fox ranges
- Fox prints figures for some ranges, and they differ, so a figure from one range does not carry across.
- Wet weather
- Rain and humidity can change normal leakage from the panels to earth. The one Fox manual we found that links low insulation to cloudy or rainy weather is the H3-Plus and P3-Plus manual.
- If left
- Contacts exposed to water are likely to corrode, and over time that can lead to heat, an arc or a fire.
- G98
- G98 is an AC grid-connection standard and sets no DC insulation limit.
These rows are about PV-side insulation. Ground Fault and Bms Insul Fault are separate alarms.
Reference
Possible causes on the PV side
Fox ESS does not publish a list of causes for this alarm. These are the causes we look for, in general words, and finding any of them is an engineer's job. They sit in the PV wiring and panels, not the battery: a Bms Insul Fault is a separate, battery-side alarm.
MC4 water ingress
Moisture inside MC4 connector housings
Connectors can let water in as they age or if they were badly fitted, and water inside the housing can make a path to earth. A corroded connector also adds electrical resistance and heats up under load, which is a fire risk as well as an insulation fault. Replacing a connector is an engineer's job.
DC cable damage
Cracked or split cable insulation
UV can cause fine cracks in a cable sheath over time, and cables can also be damaged during installation or later roof work. Damaged insulation lets moisture reach the conductor. An engineer inspects the cable run, including exposed sections and entry points through walls or roof tiles.
Junction box failure
Cracked or separated panel junction box
Heat cycling and UV can make the adhesive fail on the box at the back of a panel, so the box separates from the panel and lets water in. An engineer works out whether it can be repaired or the panel needs replacing.
Rodent damage
Chewed cable insulation in loft or roof void
Squirrels, rats and mice can chew DC cable insulation in lofts or under panels. The exposed conductor can then touch metalwork or damp surfaces and cause an insulation fault. Roof voids are for an engineer to check, not an owner.
Panel backsheet crack
Moisture reaching the cell layer
Older or lower-quality panels can crack in the rear backsheet, which lets moisture reach the cells. A crack can be hard to see, and the fault may show only when the panel is damp, such as on a dewy morning, so a dry test can miss it.
Fox ESS ground and insulation faults: what you need to know
Iso Fault is about insulation
Iso Fault means the inverter has detected low insulation resistance between the PV side and earth. Fox lists insulation monitoring as a built-in protection, and its manual says an earth fault alarm shows on the inverter's screen with the red LED on. Fox gives this alarm no restart remedy, so clearing it does not deal with the cause.
Ground Fault is a different problem
Fox lists Ground Fault on its own, with its own action: the voltage between neutral and earth, then the AC wiring. Fox's installer guide adds polarity, poor earthing and the supply itself, and looks at the inverter's own measuring circuit only after those checks. That work is an electrician's. If the alarm persists, the installation may have an earth fault that needs an electrician to diagnose.
Where insulation damage comes from
Fox does not list causes for this alarm, so this is our view in general words. Cables, connectors and junction boxes age, and they can also be damaged during installation or later roof work. Water reaching a damaged point can make a path to earth. When it dries out the alarm can clear, which does not mean the damage has gone.
Why not to leave it
Contacts exposed to water are likely to corrode, and over time that can lead to heat, an arc or a fire. Get it looked at while the alarm is still coming and going. With your permission we read your Fox Cloud alarm history remotely to establish when it started, before an engineer attends. We are independent from Fox ESS and from your original installer.
Related
Related Fox ESS and cross-brand guides
EPS backup not working
Fox ESS gave no backup in a power cut? Check whether EPS was wired and switched on, the reserve settings and the load, and which jobs need an electrician.
See EPS backup not workingGrid code fault
Fox ESS showing Grid Volt (26), Grid Freq (27), 10min Volt (28) or Grid Lost (1)? What each alarm means, what you can check and when to tell your DNO.
See grid code faultBMS communication fault
Fox ESS battery offline or BMS Lost (code 14)? What the battery alarms mean, what you can check in the Fox Cloud app, and when to call your installer.
See BMS communication faultFox Cloud offline
Fox ESS offline in FoxCloud 2.0 or the app? See if it's the app, the dongle or the system, follow Fox's own checks, re-pair the dongle and deal with 40004.
See Fox Cloud offlineBattery not charging
Fox ESS battery not charging from solar or the grid, or not discharging? Check the work mode, Mode Scheduler and reserve settings in the Fox Cloud app first.
See battery not chargingFox ESS documents
Download Fox ESS datasheets and manuals for the H1 (G2), H3, EVO and the EP, EQ and ECS batteries, plus quick installation guides for the H3 and H3 Smart.
See Fox ESS documentsInverter red light (all brands)
Red light on your inverter? Usually a protection trip: grid or isolation fault, not a dead unit.
See inverter red light (all brands)How to read inverter fault codes
Learn how to find and read solar inverter fault codes across all major UK brands including GivEnergy, Growatt, Sunsynk, SolarEdge, Solis, Huawei, Fox ESS.
See how to read inverter fault codesSystem not producing
Solar panels and inverter installed but generating zero or near-zero output? This guide covers every common reason a solar system stops producing.
See system not producingQuestions about Fox ESS ground and insulation faults
There are two separate alarms behind that phrase. Ground Fault means the ground connection has failed. Iso Fault means the inverter has detected low insulation resistance between the PV side and earth. Neither should be cleared again and again: note the exact name and call an engineer.
Not on the inverter: Ground Fault is an earth alarm, and Fox's list points to the ground connection and the AC wiring, which an electrician checks. Iso Fault is insulation on the PV side, and Res Cur Fault means the residual current is high; Fox gives those two the same action. Res Cur HW Fault means the residual current detection device itself has failed, and Bms Insul Fault is a separate, battery-side insulation fault. See the full Fox ESS fault code index for the other alarms.
Fox's alarm list says an Iso Fault can go back to normal after a while, so clearing by itself does not mean it is fixed. No Fox document for the H1 or H1-G2 ties the alarm to the weather, though Fox's manual says rain and humidity can change normal leakage from the panels to earth. In our view, water reaching a damaged cable, connector or junction box is one possible reason an alarm follows the weather. Contacts exposed to water are likely to corrode, and over time that can lead to heat, an arc or a fire, so have it checked.
No. You can read the alarm history in Fox Cloud, note when the alarm appears, and look at what you can see from the ground without touching anything. Do not open the inverter, disconnect cables or touch MC4 connectors, because DC voltage from the panels is present whenever there is daylight and cannot be switched off. Finding the fault is a job for a qualified solar engineer, and an earth fault on the AC installation needs an electrician to diagnose.
Fox prints no insulation figure for the H1, H1-G2 or KH, so we do not quote a pass mark here. Fox prints figures for some other ranges, and they differ, so a figure from one range does not carry across. G98 is an AC grid-connection standard and sets no DC insulation limit. Checking insulation is a job for a qualified solar engineer, never an owner.
It depends on the alarm. The remote diagnostic is free: with your permission we read your Fox Cloud alarm history, through your own login or after an account transfer (£95), and tell you which alarm you have and what it points to. A ground or insulation fault is usually physical, so it needs an on-site visit from £295, quoted in writing before we start. Where it is something we can correct remotely it is £125, and you only pay if we fix it.
Reading the alarm name and its history is usually a same-day job, in one remote session, once we can see your Fox Cloud account. You tell us what the system is doing and what the weather has been. We then send a written summary of what the alarm means and what needs checking. If the fault is physical, that summary tells the engineer or electrician which alarm they are dealing with before anyone goes on the roof.
Book
Get help if your Fox ESS inverter is still showing a ground or Iso Fault
Tell us the exact alarm name from Fox Cloud, the date and time it first appeared, and what the weather was doing. With your permission we read your Fox Cloud alarm history remotely, tell you which alarm you have and what it points to, and set out what the engineer or electrician needs to check. Independent from Fox ESS and your installer.
- Not affiliated with or endorsed by Fox ESS
- No fix, no fee covers remote fixes. On-site work is quoted in writing before we start.
- On-site visits available
Service notice
We’re busier than ever. Thank you for your patience.
Due to unprecedented demand we’re currently only carrying out on-site work in Yorkshire, and replies are taking up to 7 days. Remote diagnostics continue for the whole UK as normal.
Every message is read and answered, so please don’t send it twice; that only slows the queue.
Ronald Director, Solar Tech Support
On-site: Yorkshire only, for now
Remote: UK-wide, as usual
Remote fix: £125 if we fix it, no charge if we don’t
Replies: within 7 days
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