NTSB finds moisture in battery system as probable cause of 2022 BEB fire in Hamden, Conn.

April 1, 2024
The board explained the lack of a safety plan by CTtransit for mitigating risks associated with high voltage lithium-ion battery fires during an emergency response contributed to the injuries of facility personnel.

The National Transportation Safety Board (NTSB) has determined moisture in the high voltage lithium-ion battery system as the probable cause in a summer 2022 incident where an electric bus caught fire in Hamden, Conn.   

On July 23, 2022, at about 3:39 a.m., the BEB, owned by the Connecticut Department of Transportation and operated by CTtransit, began emitting smoke while parked inside a CTtransit maintenance facility in Hamden, New Haven County, Conn. The BEB had been placed out of service two days earlier due to an error in the bus charging system.  

NTSB noted responding fire department personnel did not observe any visible flames and the bus was pushed to an outdoor, isolated parking area. In the process, two CTtransit maintenance workers suffered smoke inhalation and were treated at an area hospital.  

According to NTSB, later that same morning, the bus was again emitting smoke and fire was observed coming from the rear of the vehicle. Fire personnel returned to the site and the incident commander decided to let the bus burn in the controlled environment. The fire remained active for several hours and fully consumed the vehicle.  

Following the departure of fire personnel, the bus continued to smolder while remaining isolated in the parking lot. Two days after the bus had caught fire for the first time, NTSB says smoke and an orange glow were observed emanating from the right rear wheel well of the burned bus. Fire department personnel responded for a third time and applied water to the smoking battery compartment. No additional injuries were reported.  

What NTSB found 

NTSB says moisture in the high voltage lithium-ion battery system led to battery damage, which resulted in the fire. Contributing to the injuries to facility personnel was the lack of a safety plan by CTtransit for mitigating risks associated with high voltage lithium-ion battery fires during emergency response.  

Lessons learned 

NTSB notes high voltage lithium-ion battery fires require a specific firefighting approach that relies on application of water to cool the high-voltage batteries. Stranded energy in the batteries presents a risk of battery reignition.  

NTSB previously issued safety recommendations to manufacturers of electric vehicles equipped with high-voltage lithium-ion batteries to provide vehicle-specific information in their emergency response guides about safely extinguishing fires, mitigating reignition events and transporting and storing damaged vehicles. All three manufacturers of the buses mentioned in the report have the recommended information in their training materials and/or emergency response guides. 

After the fire events in Hamden, the Federal Transit Administration published a handbook for deploying BEBs that includes updated guidance for managing risks associated with high-voltage BEB fires consistent with actions CTtransit implemented after the initial bus fire, including outdoor bus storage and providing isolation areas should a BEB fire occur.  

In addition, best practices for managing risks associated with BEBs are an area of active research by the Fire Protection Research Foundation, under a project awarded by the Transportation Research Board. As transit agencies continue to electrify their bus fleets, NTSB will continue to monitor issues related to BEB fires and ongoing research in the area to understand the safety impacts of these alternative-fueled vehicles.