SCR is a circuit component that conduct when it is been triggered. typical circuit is as shown below:
When there's overvoltage across Rload, zener Z1 will conduct and trigger SCR X1 into conduction - this cause huge current that blow the fuse.
Without this protection, the current level during over voltage might be too little for fuse to burn open
Now compare this protection mechanism with explosive (reactive) armoured tank shown below (picture from http://en.wikipedia.org/wiki/Reactive_armour).
Those “bricks” are actually explosive material. When the tank being hit by bazooka, the plate will explode, thus dispersing the piercing force of the bazooka, and thus preventing injury for personnel in side.
Both SCR protection (with fuse) and reactive armour protection work in similar way – namely by conducting large amount of current to burn fuse versus exploding of “brick” to disperse piercing force of bazooka– to protect the cargo, by preventing the destructive force from 'hurting’ the cargo.