300 Amp Fuse Blowing with 4KW of Load from an Inverter - Safety Mechanism Prepare for the Unexpected

I bought a couple of 300 Amp fuses from an eBay seller and decided to build an enclosure on the cheap. Be careful as wood is FLAMMABLE.
Using 4KW of load at 240 volts the fuse did blow... After a while of uncertainty.
The temperature of the fuse did peak at 90 degrees Celsius at one point with 200 Amps flowing through it at 12v but this seemed to be down to the retaining bolts being poor conductors.

Prepare for the Unexpected

Link to the fuses http://www.ebay.co.uk/itm/MEGA-FUSE-100Amp-500Amp-TOP-QUALITY-CAR-MARINE-VAN-AUTO-/262254066650?var=&hash=item3d0f8f7bda

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