Boeing Co.’s 787 Dreamliner suffered a fresh setback after ANA Holdings Inc., the model’s biggest operator, said it discovered wiring defects in three aircraft.

The fault, found in the fire-suppression system of a 787 due to depart Tokyo’s Haneda airport today, would cause the wrong extinguisher to be activated in the event of a blaze in one of the plane’s two engines, spokeswoman Megumi Tezuka said.

Boeing’s flagship jet is already under scrutiny following a fire in London last month that initial findings suggested may have been sparked by wires in an emergency beacon. The 787 had returned to service in June following a three-month global grounding in the wake of fires involving ANA and Japan Airlines Co. jets, later traced to the failure of lithium-ion batteries.

“We will thoroughly examine this issue and take the appropriate steps,” Boeing spokesman Rob Henderson said from Tokyo following the latest incident. “The safety of those flying on Boeing airplanes is our top priority.”

ANA’s Tezuka said parts were replaced in two of the jets in which the Tokyo-based carrier found the defect, adding it must have occurred during the manufacturing process. The third aircraft will also be fixed by the end of the day, she said.

JAL Turnaround

Japan Air today ordered a 787 bound for to Helsinki to return to Narita airport in Tokyo as a precautionary measure after being informed of the new wiring issue by the national Transport Ministry, company spokesman Seiji Takaramoto said.

The carrier subsequently inspected all 10 of its Dreamliners and found no problems, he said.

“These things happen with a new aircraft,” said Robert Stallard, an analyst at RBC Capital in London with an “outperform” rating on Chicago-based Boeing. “When the airlines ground the plane or regulators start becoming involved, then it becomes something to watch out for.”

U.S. regulators ordered Dreamliner operators to check emergency radio transmitters for wire damage after a beacon was linked to the July 12 fire in London. The Federal Aviation Administration is working with Boeing to develop instructions for the inspections, the agency said at the time.

Boeing had delivered 73 Dreamliners to 13 customers through August 7, the company said on its website, with more than 29,000 flights flown. Japan is the biggest customer of the jet.

Like other operators, ANA and Japan Air restarted services with the 787 in June after authorities approved a redesign of the batteries, which included more protection around individual cells to contain any overheating.

Editors: Chris Jasper and Anand Krishnamoorthy and Benedikt Kammel.

To contact the reporters on this story: Kiyotaka Matsuda in Tokyo at kmatsuda@bloomberg.net; Robert Wall in London at rwall6@bloomberg.net. To contact the editor responsible for this story: Anand Krishnamoorthy at anandk@bloomberg.net.