Automatic reset fuse (thermal Breaker unit) failure

[shotgun-cam]

On installing the rack lights and driving lamps I used 4- 30 amp breaker fuses ( from Princess Auto) mounted on the outside of a circuit box with connections and power distribution inside.

One of these failed and created enough heat so as to melt the box fusing the fuse to it.

Trouble shooting the circuit showed the fuse had popped while truck was moving (Possibly mechanical not electronic fault) shutting down the circuit. It improperly reset becoming a high value resistance causing the heat. Replacing fuse and main circuit relay restored function. Max circuit draw is under 20 amps.