(Reuters) — Japan's All Nippon Airways had three instances of electric distribution panel trouble in its Boeing 787 Dreamliner before it grounded the aircraft in January and had to replace the panel twice, a spokesman said.

In the most serious case, which took place in April 2012, ANA found burns in the protection circuit and the breaker of an electric distribution panel during on-ground inspections after pilots received a generator-related bug message, ANA spokesman Etsuya Uchiyama said on Thursday.

The company believes a foreign material entered the panel and caused the short circuit. The material is likely to have scattered and damaged the breaker, and ANA replaced the damaged panel, he said.

"We believe this is not serious. Fire damage in electric distribution panels are also found in other types of aircraft, and it has no impact on safe operation of aircraft," he said.

ANA, which is Japan's largest airline and the biggest operator of 787s worldwide, reported the case to Japan's Civil Aviation Bureau, Mr. Uchiyama said.

The airline also replaced an electric distribution panel in June 2012 and exchanged parts in a panel in March 2012, he said. These two cases were not reported to the Civil Aviation Bureau.

On Wednesday, the Japan Federation of Aviation Workers' Unions said ANA experienced short circuits in its 787 electric distribution panel at least five times between December 2011 and October 2012, citing ANA mechanics.

Asked about the Federation's information, ANA's Mr. Uchiyama said the panel problems occurred three times, not five.

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The federation, which groups unions representing aviation workers including mechanics, flight attendants and pilots, did not mention Japan Airlines, the second-largest 787 operator.

JAL could not be reached immediately for comment.

Boeing did not immediately respond to a request for comment.

ANA operated 17 Dreamliners before the aircraft was grounded in mid-January after lithium-ion batteries burned on two jets, one in the United States and one in Japan.

The 787 uses a large electrical system with multiple generators and power distribution panels to perform many functions that on other jets are powered by compressed air from the plane's engines.

In 2010, a power distribution panel caught fire during a 787 test flight. In December 2012, a United Airlines 787 made an emergency landing due to electrical problems stemming from a distribution panel. Boeing later said it traced that problem to a faulty circuit board made in Mexico.