Transport for London (TfL) has removed 90 of its ‘Switch Mobility MetroDecker EV’ electric buses from the streets following a major fire at its Potters Bar garage which saw six of the electric buses engulfed in flames.

The company took the precautionary measure after the fire on Sunday and is currently carrying out an investigation into the cause of the fire.

A bystander captured on video the moment when the fire spread to adjacent electric buses causing explosive thermal runaway. Thermal runaway occurs when the cells in lithium-ion batteries that power electric buses and electric cars are damaged or defective. This can cause a cell to heat up extremely quickly and start a fire. Adjoining cells are compromised and a domino effect created resulting in the fire spreading through the complete battery array.

“Thermal runaway is very difficult to stop,” explains Matt Butcher from Prosol UK. “You can clearly see the moment thermal runaway is achieved in the video. The second bus almost explodes and the flames and toxic smoke increase substantially. It’s a very frightening and dangerous experience.”

Eyewitness Shaun Cunningham is reported to have said he heard an “unbelievable noise that sounded like a jet” and he saw a bus had “exploded into a ball of flames”.

Multiple fire crews from across Herfordshire attended the blaze and worked with police to evacuate the surrounding area. Fortunately no-one was injured in the incident which could be seen over nine miles away in St Albans.