Can't really read the fuse. Looks like. T4A, time delay / slow blow 4a. Does it protect the MOSFET drivers? They didn't expect it to fail given the location.
The fuse is a T4A and exactly as you describe a Time delayed slow blow. I have seen these fuses blow a lot, which is why, while the board is out, it might be a good idea to install a PCB mounted cartridge holder.
This particular fuse is the source of power for the entire inverter "brains". The circuit drops the 48VDC to 12VDC which powers the relays etc. Seeing the damage caused to the PCB under the relay I expected this fuse to have popped. I am amazed by the amount of damage done to the PCB board and the tracks, as you said, it produced a lot of heat to do that! I intend to replace the fuse and solder on some fly leads directly to the relays from the ribbon cable header, this should bypass the damaged tracks completely.