well the idea was that the mosfet is turned on by the comparator, and it shunts the circuit to ground (so full 20A goes through a low Ron mosfet), and at the same time, a relay to break the circuit is triggered.
The fuse is connected to the mosfet thermally, incase the relay does not open the circuit, the mosfet will get hot and open circuit.
The idea is that the mosfet only works for as long as the relay needs to open, so it should not need a heat sink I think.
It seems more reliable to use a pass relay and a "grounding" mosfet then a pass mosfet. Oh yea, the comparator output would latch, and the mosfet/relay would be driven by slightly more beefy transistors then in a regular comparator. The relay is normally closed.
This is just a mod to attach to the side of a old school CV 30lb power supply to make it a bit more safe/lab worthy.