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The power-unit monitoring system was not working properly when the control tower at Tokyo’s Haneda airport suffered an electrical failure Tuesday because circuit breakers on the switchboard were being replaced, the transport ministry said.

Airport officials monitoring the power surveillance system were unaware of the malfunction to the uninterruptible power supply units until the outage hit because they had been told beforehand that the system would not function properly while circuit breakers were being replaced.

“We apologize for causing inconvenience to passengers,” Teiji Iwasaki, chief of the Land, Infrastructure and Transport Ministry’s Civil Aviation Bureau, said at a news conference Wednesday, adding that the ministry did not have adequate measures to detect problems in the UPS units during the breaker-replacement work.

“We need to study whether the monitoring system needs to be equipped with additional measures when engineering work is being done and whether the work should be conducted when the airport is open,” he said.

Regular power to the units was cut during the period somehow and the airport went on emergency-battery power.

UPS units ensure power is not disrupted in the event of a blackout. The units run on emergency batteries until the emergency power generator starts running.

Haneda’s control tower has two UPS units, but both had their power cut at the same time — something that rarely happens, according to ministry officials.

But airport officials misinterpreted the warning signal on the power-unit monitoring system telling them that the emergency power had kicked in. They believed the surveillance system was not working properly, according to the ministry.

The emergency batteries ran out about 45 minutes later and the blackout ensued.

The power failure shut down the tower’s operating system — radar, computers and radios — forcing the airport to stop operations for about an hour.

The transport ministry is still investigating the cause of the blackout, but since the time that the breaker replacement was completed matches the time the power went out, it suspects the two incidents were linked.

The airport reopened at 12:33 p.m., when electricity was restored through a stopgap measure of diverting the transmission line to circumvent the UPS units.

The airport was operating on normal power Wednesday.

A total of 49 flights were canceled, 19 flights rerouted and 429 delayed for more than 30 minutes, affecting nearly 100,000 people, according to the ministry.

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