NTSB turns on the link device from Dali headquarters

The NTSB has released an update on its investigation into the disastrous boxship alliance Dali in March with the Francis Scott Key Bridge, which destroyed a span, killed six workers and blocked a federal shipping canal in Baltimore for months.

The container ship lost electrical power twice as it approached the bridge during the voyage during which the accident occurred. Each time, mission-critical circuit breakers that connect the ship’s generators to the rest of the electrical distribution system tripped (opened), turning off all the lights – even though the generators continued to operate.

The loss of electrical power temporarily disabled the ship’s rudder and forced the main engine to shut down, causing Dali lose drive and course control as it approached the main part of the bridge. Although backup steering control was restored after the emergency generator activated, it was too little too late to prevent alliance with the southwest pier of the bridge. Dalí the momentum crushed the pillar on impact, collapsing the entire bridge truss within seconds. Repairing the damage will likely take years.

According to the NTSB, the first set of breakers (HR1 and LR1) tripped when Dali was only three ship’s lengths from the bridge, causing the first power failure. With the help of the OEM, crew and other experts, the NTSB thoroughly examined these breakers at the component level to determine what could have gone wrong.

During testing, investigators noticed an “open circuit in the control circuit of the HR1 undervoltage release.” An undervoltage release is a device that trips a circuit breaker when the voltage drops below a set threshold.

The team removed the terminal strip – the female connector socket for the plug-in components – from the circuit breaker’s undervoltage release control circuit (illustration on the left, courtesy of WAGO). Two sections of control wiring associated with this terminal block were also removed from the switchgear and these components were taken back to the NTSB laboratory for further testing.

The agency did not provide further information on its findings and stressed that the update should not be considered a conclusion about the root cause of the accident. The broader investigation into the alliance is still ongoing; although the NTSB does not have a formal timeline for completion, given the length of previous investigations, the process often takes about a year.