BRUSSELS - Swiss officials failed to respond quickly and adequately after a power line shorted out in a storm, creating a chain reaction that blacked out nearly all of Italy in under half an hour, an investigation by European grid operators said recently.

The Union for the Co-ordination of Transmission of Electricity, which groups almost three dozen European operators, also noted the continent's aging grid was not designed to cope with today's "high level" of cross-border energy sales.

Transmission system operators thus are running "close to the limits" of system security, the report said.

"We are using the system now for situations for which it was not built," investigation chairman Frank Vandenberghe of the Belgian operator Elia told The Associated Press.

But that did not make blackouts inevitable, he said, insisting the system "should be able to withstand" such events if handled properly - which he said the Swiss failed to do.

The Swiss company ETRANS, which co-ordinates the sharing of electricity with neighbouring countries, called the report technically accurate but "one-sided."

"We are of the opinion that we acted correctly and quickly enough," ETRANS chairman Thomas von Weissenfluh told a news conference in Bern, the Swiss capital.

The Sept. 28 blackout in Italy left 55 million people - about 95 per cent of the country - without power for up to 18 hours.

The report said it was triggered when a tree hit the Mettlen-Lavorgo line in Switzerland at 3:01 a.m. during a storm. Because of high loads on remaining lines, an automatic response intended to protect the line blocked attempts to bring it back in service.

The ETRANS centre in Laufenberg, Switzerland, called the Italian grid operator GRTN's control centre in Rome at 3:11 a.m. to ask it to decrease imports by 300 megawatts to relieve the overload in Switzerland. The Italians complied within 10 minutes, the report said.

But the report criticized the Swiss for "lacking a sense of urgency" in making only one call and criticized the Swiss measures as "inadequate" and "insufficient."

At 3:25 a.m., when the nearby Sils-Soazza line also shorted out when hit by a tree, all other interconnectors were tripped automatically and Italy was isolated from the European network about 12 seconds later.

"About two minutes and 30 seconds after the disconnection of the country, the blackout was an unavoidable fact," the report said.

Vandenberghe said the Swiss could have prevented the blackout if they had responded better after the first line was knocked out because the second didn't go down for nearly 30 minutes.

In a statement, ETRANS said the European study brushed over "grave weaknesses" in Italy's handling of its network and "focuses one-sidedly on the events in Switzerland," largely because the Swiss, unlike the Italian grid operator, made available "all the necessary information."

"It is incomprehensible that the line disturbances on the Swiss-Italian border could have caused the collapse of the entire Italian electrical system," the Swiss statement said.

Vandenberghe declined to make any comparisons to last August's massive outage in the United States and Canada, saying the cause there has yet to be determined conclusively.

The investigating committee included representatives from five countries involved - Austria, France, Italy, Slovenia and Switzerland - as well as outside experts from Belgium, Germany, Spain and the Netherlands.