Inverex Nitrox F42 — Grid Under-Voltage (AC Line Low Voltage)

What it means: The mains voltage at the inverter's grid input fell below the permitted window, so the inverter disconnected from the grid to protect itself and your appliances. Inverex's hybrid fault table lists F42 as "AC line low voltage" under the heading "Grid voltage fault", and its first instruction is to check that the AC voltage is inside the range given in the specification. The problem is in the incoming supply or the grid-side wiring, not inside the inverter. On the three-phase Nitrox string inverters the same number is narrower — there it means the W–U line pair specifically is reading low.

Why does Inverex show F42?

• Weak or sagging feeder voltage, which is what the manual's first check is looking for
• Loose, corroded or under-tightened AC input connections, which the manual asks you to check second
• Undersized or aluminium grid-side cable dropping voltage under load
• Very heavy neighbourhood demand pulling an end-of-line feeder down
• On a three-phase Nitrox, a weak or lost phase on the W–U pair

The Pakistan factor

Under-voltage, not over-voltage, is the characteristic Pakistani grid fault on loaded urban feeders and long rural runs, and it bites hardest between roughly 6 and 11 pm and right through summer. Swings between 180 V and 250 V on one feeder in a single evening are routine. There is a trap worth knowing: undersized or aluminium grid-side cable and a hand-crimped lug will drop enough volts under load to produce F42 at the inverter while the meter a few metres away still reads acceptably. That is an installation fault, not a DISCO fault, and complaining to the utility will not fix it — it needs an electrician to re-terminate and re-size the run.

How to fix it (safe to try yourself)

1. Read the grid voltage on the inverter display and note the time of day. Log it for a few days — a feeder problem clusters at particular hours, a wiring problem does not.
2. Cross-check with a plug-in voltage meter at a wall socket, and compare what you see against the grid voltage range in your model's datasheet. If the supply sits below that range, it is out of spec and no inverter setting will fix it.
3. Widen the grid-voltage window in the inverter settings ONLY within the range your manual permits, and only if the supply is marginal rather than badly out of spec. Never disable the protection — it is what stops a collapsing supply from browning out your motors, compressors and LED drivers.
4. Report persistent under-voltage to your DISCO in writing. Low voltage burns out fridge and pump motors as surely as it trips the inverter, so it is worth pursuing.
5. If the display shows healthy voltage but F42 keeps latching, or the voltage at the inverter is much lower than at the meter, stop here. That points at the grid-side cabling or a loose terminal, which is live work — do not open the inverter's AC compartment or the distribution board yourself. Call a qualified electrician or your installer, and escalate to an Inverex service centre if the unit itself is suspected.