Causes
Power went off at about 03:20 hrs. local time on 28 September 2003. Initial reports from Italy's electricity supplier, ENEL, stated that the power line which supplied electricity to Italy from Switzerland was damaged by storms, causing it to trip and also the two 400kV power lines between France and Italy to trip due to sudden increased demand from those two power lines. The cascading effect disrupted power supply to Italy from France and Switzerland. ENEL lost control of the grid in the next 4 seconds, with the lines tripped one by one amid the cascading effect. Swiss electricity company ATEL later concurred that a power line between Switzerland and Italy went out for a few hours.
Read more about this topic: 2003 Italy Blackout