Abstract: A system for locking caps, to be used especially in those tanks containing inflammable liquids, said cap (10) comprising a body (11) which is seated over a tank nozzle (2), an articulating lock arm (14) bearing a respective lock displaceable between locking and releasing positions relative to the tank nozzle upon the corresponding angular movement of the lock arm, an arm activating assembly provided in the cap body (11) and whereto a drive shaft (51) is mounted in a freely rotating manner and to which torque is selectively applied, an impelling piston (15) coupled to lock arm (14) and being axially displaced between corresponding locking and releasing positions of the lock arm (14); an electromagnetic clutch causing the selective axial displacement of the impelling piston (15) between operational limit positions upon the rotation of the drive shaft (51) and comprising a coupling means displaceable between engaging and disengaging position; coil means (60) causing the displacement of the coupling means to the