Board member Berthold Bonekamp told a press conference in Essen that RWE had asked an independent expert to study the causes of the blackouts to comply with requests by energy regulator BNetzA for a full report on the incidents.

"We are certain that we have not acted negligently," he said.

Pressure is mounting on RWE to explain whether it had taken sufficient and timely precautions to ensure its pylons and cable network, where usage charges are among the highest in Europe, are able to cope with extreme weather.

Germany's top power producer blames the extreme weather for the rare outages.

Dutch and Belgian pylons had also collapsed when they were hit by sudden snowfalls and gusts of winds, it said.

A weekend report by the Spiegel magazine said the company had been slow to revamp faulty pylons made from 40-year-old steel.

But RWE said stress on power lines in the affected Muensterland region was 15 times higher than normal, and new pylons built in the 1990s with more modern steel had also been torn down.

Long before the outages, it had earmarked 550 million euros to gradually replace old pylons in a programme due to be completed only by 2015, as swapping takes a long time when disruptions are to be avoided.

Christa Thoben, economics minister of North-Rhine Westphalia state, where Muensterland is located, said earlier on Tuesday RWE had known for years that there were deficiencies.

"As the supervisory authority, I am responsible for the security of energy supply ... I want to know the internal time schedules at RWE," she said on German radio.