Iso Fault on a Fox ESS H1 G2 single-phase hybrid inverter: The isolation is failed
Short answer
On a Fox ESS inverter, Iso Fault means: The isolation is failed. The inverter stops until the fault clears or is reset.
Reviewed 24 September 2026 · 1 source
What it means
The manual says: The isolation is failed.
Part involved: PV array insulation resistance.
When it shows: Insulation resistance of PV array to earth below threshold at start-up (IEC 62109-2 clause 13.9 earth-fault alarm; red LED).
Likely causes:
- Insulation of PV wiring damaged or damp; earth fault on the DC side.
Is it safe?
Switch the inverter off at its isolator and call an engineer.
| Does the inverter stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
The inverter stops until the fault clears or is reset.
Resetting itself: yes.
Manual reset: Manual: wait for a while to check if back to normal; else seek help.
What you can check
- Read the full code and message on the inverter screen or in the FoxCloud app and note the date/time for your installer.
- Check the inverter's AC isolator next to the inverter and the solar/battery breaker in the consumer unit are ON (do not reset a breaker that trips again - call your installer).
- If it only appears on wet mornings and clears as the day dries, note the pattern for your installer.
- Do not touch the panels, DC cables or DC isolator.
What the engineer will check
- Insulation resistance test of each string (IR tester); inspect connectors (MC4), roof cable runs and junction boxes for water ingress/rodent damage; check PE.
Who to ask for: MCS-certified solar PV installer (DC/string work) or the original installer.
Certification: Work on DC strings and inverter enclosures: competent PV installer (MCS-certified installer for warranty/SEG continuity); electrical work notifiable under Part P (England & Wales) or equivalent.
What you might notice
Sources
- H1(G2)/AC1(G2) Series User Manual ↗ · V1.0.8 · page 47 · Reviewed 24 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 24 September 2026