Could you post some images of your grandfather's solar installation (not the panels) including any largish box which might be called an 'optimiser'?
Some solar installations reduced the incoming mains voltage to 220V using a so-called voltage optimiser. The solar inverter was then connected to the regulated 220V side of the inverter. The idea being that the inverter would 'see' a permanently lower voltage than it was able to produce and need not output much more than 220V.
Some inverters for their own protection disconnect from the incoming mains when directly connected when the mains voltage rises above 255V (figure will be in the inverter spec). If it did disconnect then no PV generation (or battery stored PV energy) could be supplied to the home until the mains voltage dropped below the re-connect voltage.
However, some optimisers, the VPhase being one type, where only continuously rated for about 2.5kW; they would operate for a short period in the range 2.5 - 4ish kW. After the short period or if the load immediately went above 4ish kW, the optimiser would automatically switch to bypass mode which connected the incoming mains at say 240V immediately to the home - a jump of 20V in the voltage supplied to everything in the home. The consequential surge in current through some fuses causes them to rupture.
Or something like this......so some picture would help.
