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County updates investigation of electric bus thermal event; two buses cleared for return to service
Summary
Following a January thermal runaway event on one electric bus, county staff and external engineers determined contactor welding and battery imbalance caused the incident; two newer-generation buses were reprogrammed and will be returned to service, while one destroyed bus remains salvaged under FTA/insurance process.
County transit staff reported the results of a multi-party investigation into a thermal-runaway event affecting an electric transit bus in January. The county said insurance-hired engineers, the bus manufacturer and county fleet technicians all reached the same conclusion about the cause: an electrical contactor welded closed during charging, creating an imbalance across battery banks that led to overheating and thermal runaway.
Transit maintenance staff credited an in-house technician who first identified the fault pattern; external engineers validated the finding. The county said the defect involved a welded contactor that allowed one battery bank to continue charging while others tripped, producing heat that initiated the thermal event.
Officials said the two later-generation buses were reprogrammed by the manufacturer to prevent charging if either side of a contactor stays engaged, and warning codes now appear on the dash to alert drivers. The county demonstrated that if a contactor remains closed, the newer buses will not accept a charge, which staff said reduces the risk of a repeat event. As a result, the county plans to reintroduce two of the electric buses into service after further checks. Older-generation buses without the added safeguards remain offline until a robust retrofit solution is available.
The bus that experienced catastrophic damage will be handled through the insurance and Federal Transit Administration (FTA) process. The county said the insurer placed a salvage value on the destroyed vehicle at $77,500; because the original purchase included federal funds, the county must remit a percentage of the salvage amount (about 89.73% as described in meeting remarks) back to the FTA, leaving a modest residual amount the county may retain for other uses. The county said the manufacturer will retrieve the wreck and debris and that salvage/cleanup logistics are being coordinated with the insurer.
County staff said they are confident in the safety changes on the returned vehicles and that vehicles now report diagnostic data to maintenance and the manufacturer in near real time via vehicle modems.
