Grid operators of the five countries involved in the Sept. 28, 2003, blackout in Italy (Austria, France, Italy, Slovenia and Switzerland) released a joint Interim Report investigating the reasons why the sequence of events evolved from a single line trip in Switzerland into the impossibility for the Italian system to operate separately from the European network.

The committee was set up within the Union for the Coordination of Transmission of Electricity (UCTE).

The report states that the sequence of events was triggered by a trip of the Swiss 380-kV line Mettlen-Lavorgo (also called Lukmanier line) at 3:01 p.m., caused by a tree flashover. Several attempts to automatically reclose the line were unsuccessful. A manual attempt at 3:08 p.m. failed as well.

The report lists the main reasons for the blackout as unsuccessful reclosing of the Lukmanier line because of a too high-phase angle difference; lacking a sense of urgency regarding the San Bernardino line overload and call for inadequate countermeasures in Italy; angle instability and voltage collapse in Italy; and right-of-way maintenance practices.