Inverex Nitrox F24 — PV Insulation Failure (Earth Fault)

⚠ DANGER — this fault involves lethal voltage. Do not attempt a DIY fix. Call a qualified technician.

What it means: The inverter has measured the electrical resistance between the solar array and earth and found it too low. Inverex's hybrid fault table gives F24 as "DC insulation impedance resistance is too low" with the cause "PV isolation failure"; the Nitrox three-phase string manual's troubleshooting table puts the same code more bluntly as "PV+ or PV- to the Ground resistance too low". In plain terms, current now has a path from the solar array to earth that should not exist. That path can be through wet insulation, a damaged cable, a failed panel — or a person.

Why does Inverex show F24?

The Pakistan factor

Earthing is the weak point in a large share of Pakistani residential installs, and this is the code that exposes it. Many houses have a nominal earth rod that has long since dried out or was never properly bonded, so the manual's own check — whether the inverter's PE cable actually reaches ground — fails here far more often than it should. Rain is the trigger. Water works into unbranded MC4 connectors and unsealed roof junction boxes, and F24 duly appears on the first wet morning of monsoon and clears by noon. Rodent damage to DC cable run in open trays is the other common source. The reason this one is a stop-and- call rather than a troubleshoot is the combination: with poor or absent earthing, an insulation fault that would trip harmlessly on a properly earthed system can instead energise the array frame and the roof structure around it.

How to fix it

  1. Treat this as a live electrical hazard from the outset. Do not touch the array, the DC cabling, the connectors, the mounting frame or the inverter's DC side. An insulation fault means metalwork you would assume is dead may not be.
  2. If anyone has felt a tingle or a shock from the array frame, a metal roof, a gutter or the inverter body, keep people and animals away from the array, switch the system off at the AC and DC isolators only if you can reach them safely while standing dry, and call an installer immediately. Do not wait for a convenient appointment.
  3. From ground level, note what changed — whether it started after rain, after a storm, after work on the roof, or after rodent activity. Photograph the fault screen. That costs you nothing and genuinely shortens the technician's visit.
  4. Do not go onto the roof to look for it, do not open DC connectors, do not unplug PV strings and do not open the inverter. The array generates whenever there is light and cannot be switched off the way a mains circuit can, so this is qualified-installer work.
  5. Do not simply clear the fault and carry on because production comes back once the panels dry. A fault that appears and disappears with the weather is still a real insulation failure — it is telling you exactly where water is getting in.
  6. Call a qualified solar installer to insulation-test the strings and inspect the earthing, including whether the inverter's PE cable is genuinely connected to ground, which is the manual's own second check. If the fault is traced to the inverter rather than the array, it escalates to an Inverex service centre.

References