A loose wire caused a power failure on the 300-metre cargo ship Dali, leading to its collision with Baltimore’s Francis Scott Key Bridge that killed six construction workers and destroyed the span.

The US National Transportation Safety Board investigation found a single loose wire in the electrical system caused a breaker to unexpectedly open, launching a sequence of events that led to two vessel blackouts and a loss of propulsion and steering.

Wire-label banding prevented the wire from being fully inserted, causing an inadequate connection, the NTSB said in its final conclusions on the cause of the accident in March 2024.

NTSB chair Jennifer Homendy compared the painstaking search for the wire to finding a single loose rivet on the Eiffel Tower.

The NTSB and the ship’s manufacturer HD Hyundai Heavy had to test thousands of wires to find the problem, she said.

“It’s like finding a needle in the haystack,” Homendy said.

The board also said contributing to the collapse was the lack of countermeasures to reduce the bridge’s vulnerability from impacts by ocean-going vessels. They could have been implemented if the Maryland Transportation Authority had carried out a vulnerability assessment.

The board staff said they recommended operators complete periodic inspections of high-voltage switchboards and proposed changes that would allow ships to more quickly recover from a loss of power.

HD Hyundai said in a statement it delivered a safe, seaworthy vessel in 2015 with automated systems and critical redundancies to swiftly respond to blackouts.

“Unfortunately, the owner and operator circumvented these safeguards,” it said.

The company added that when it delivered the ship “there was no indication that any wire was loose” and said if any wire were to come loose “over the course of a decade, through vibrations or otherwise, the owner and operator should have detected that in a routine inspection and through normal maintenance”.

-AAP