Why do e-buses catch fire?

Updated: Apr 27

Electric buses are rightly gaining popularity on our city streets, supporting better air quality in the places our children live, work & play. Like all vehicle fuel types, they do catch fire; our research has found at least 18 verifiable e-bus high voltage battery fires globally since 2010, in a stock of over 110,000 vehicles. This incident data may provide some insight into how we can reduce fire risk in depots & at charging hubs.


Key points:

  • Fires occur in buses of all fuel types, & e-buses - those powered by a high voltage lithium ion traction battery - are no exception

  • By examining these incidents as a group we can observe common threads that may help design safer depots & charging hubs

  • Our research is primarily focused on passenger EVs (EV car fires) & this post is only intended as a high-level look at e-bus fire incidents, captured as our research progressed (not an in-depth study). We have not compared the number of e-bus fires to other fuel types.

Firefighters standing on the top of an electric bus with an extinguisher & smoke coming from the battery pack
An improperly crimped wire connecting to the battery caused a rise in temperature over several days, leading to thermal runaway & fire. 27 September 2016, Frederick County, Maryland, USA

An overview of electric bus fires since 2010


Similarly to passenger vehicle companies, the world's major bus manufacturers & operators are decarbonising their fleets through the development of electrified & alternative low-emission transport.


E-buses are particularly vital for our schools; children are repeatedly & often exposed to diesel particulate from idling buses as they arrive & leave. Additionally, the rollout of e-fleets will improve air quality in our cities.

However, as with passenger EVs, electric buses have experienced a number of fire events, some of them catastrophic enough to destroy multiple vehicles. Thankfully, at the time of writing, only minor injuries & no fatalities have been recorded.


One of the early observations of these incidents is the rate of fire spread when thermal runaway occurs at depot charging hubs, potentially due to the fact that electric bus batteries are typically roof-mounted.



How many electric buses are in operation globally?


The March 2022 Research & Markets Global Electric Bus Market by Propulsion report states 'the global electric bus market size is projected to grow from 112,041 units in 2022 to reach 671,285 units by 2027, at a CAGR of 43.1%'


Thirty nine e-bus manufacturers are mentioned in this report, including Volvo, Scania, Iveco, BYD & Yutong, with North America being the fastest growing market, & the Asia Pacific region as the largest overall market.


An interesting example of this is the city of Shenzhen, China. The Shenzhen Bus Group have transitioned to a fleet of 16,000 electric buses to service a population of 12 million citizens, enabling an estimated 440,000 tonne saving of carbon emitted & halved the annual refuelling bill. How their depots are designed & emergency responders trained to handle e-bus incidents is unknown.

In Australia, it's difficult to find a total number, however a quick online search shows all states & territories are funding, rolling out or investigating electric buses.


It's sensible for Aussie emergency responders, particularly those in capital cities, to assume a bus is electric until determined otherwise.


What are the challenges to emergency responders with e-buses?


The main challenge from an emergency response perspective is the location of batteries on an electric bus.


Low floor requirements to enable easy access for passengers, particularly those living with disability, means e-bus batteries are typically roof-mounted, not within the chassis like passenger EVs. Additionally, roof-mounted batteries are easier to retrofit to existing diesel buses & may require less cooling due to airflow during travel.


While directing water underneath a passenger EV in thermal runaway can be difficult, getting enough water onto a roof mounted e-bus HV battery to cool it effectively can be similarly tricky.


Diagram of HV battery locations by range of e-bus manufacturers
Electric bus HV battery locations from the Handling Lithium-Ion Batteries in Electric Vehicles: Preventing and Recovering from Hazardous Events report*

How does an electric bus charge?


These roof-mounted batteries mean e-bus charging is often delivered using an overhead pantograph, however opportunistic or overnight charging is more commonly conducted at public or at-depot AC or DC charging hubs.


As with passenger EVs, it's important for emergency responders to be aware of bus charging hubs in their local area & the additional risks & considerations this presents should the e-bus go into thermal runaway.


Two images of e-buses, one charging with a pantograph, the other at a charging unit
Credits: Pantograph charging via OppCharge & DC charging hub via Solaris Bus & Coach

Does the same emergency response apply as with passenger EVs?


Mostly, yes, primarily following similar identify, immobilise & isolate steps.


For example, the BYD K9M Series Electric Bus emergency response guide outlines the power down, HV disconnect & how to activate the in-built fire suppression system via a driver-access emergency button.


While this internal system seems a useful suppression tool, there is often little to no information about external attack for firefighters in e-bus emergency response guides.


E-bus manufacturers should follow ISO 17840


When writing an emergency response guide, the ISO 17840 for electric vehicles should be followed. This standard creates a uniform set of guidelines for all emergency responders, assisting them to easily recognise identifying & isolating features.


This handy guide by ISO 17840 creator Kurt Vollmacher, written specifically for electric buses or coaches, can be used to streamline the Rescue Card & ERG process.

How to write a Rescue Sheet & ERG for Electric Bus & Coach
.pdf
Download PDF • 2.55MB

What do electric bus fires look like?


Of the 18 incidents we've been able to verify, we've included a selection here that provide supporting images or footage.


2015, Shenzhen, China - E-bus went into thermal runaway while at a charging hub, but was reportedly not charging at the time. Incident occurred inside an enclosed depot parking area (closed on three sides). Images: D1EV.com


2021, Dusseldorf, Germany - 40 buses - both electric & ICE - were destroyed in depot; e-buses were connected to charging, which cut off as fire overheated the units. Point of origin still unknown, however believed to have started in diesel bus carpark.


2021, Hanover, Germany - Five e-buses, two hybrid electric buses & one diesel bus destroyed by fire in bus depot. Unknown if charging at the time. Images: dpa/Francois Klein


2021, Stuttgart, Germany - 25 Mercedes eCitaro electric buses, charging hub & depot destroyed when a 'technical fault triggered when bus was charging'. Image: Feuerwehr Stuttgart


2022, Hyderabad, India - An electrical fault during charging is cited as the potential cause of thermal runaway in an Olectra Greentech Limited e-bus. Image: Unknown.


2022, Paris, France - Driver saw a 'wheel explode' & evacuated passengers before the e-bus became fully involved. Full footage on Twitter, credit JP News Photographer.


Can e-buses explode?

When we talk about explosion, we're referring to the deflagration of gases venting from lithium ion battery cells; what's referred to as vapour cloud explosion, which we covered in a previous blog post.


In at least one e-bus incident - shown below - there has been a vapour cloud explosion


2021, Guangxi, China - four open sided buses were destroyed when one went into thermal runaway, leading to a vapour cloud explosion (see footage). Unknown if charging at the time. Cause unknown. Footage: South China Morning Post


What are the opportunities to learn from e-bus fires?


While this has been a brief look at electric bus fires & what they mean for emergency response, this initial data provides some insight into some of the ways & situations in which e-bus batteries go into thermal runaway & catch fire.


For example, from these figures, we can calculate approximately 27% of e-bus fires occur while charging; this provides a great opportunity to research & test fire mitigation strategies & tools at electric bus charging hubs.


Additionally, a third of e-bus battery fires occur while the vehicle is at the depot, often leading to multiple vehicles & at least one building being destroyed. How can building design perhaps mitigate this risk in future?


What do we still need to learn about electric bus fire safety?


We'll be working with a global network of experts to better understand risks around electric bus fires throughout 2022, & will share these findings as they become available.


If you'd like to keep up to date, we welcome you to join the mailing list where all information is shared first. You can also join us on Facebook, Instagram, LinkedIn & YouTube.


As always, thanks for supporting our website & research.



*Handling Lithium-Ion Batteries in Electric Vehicles: Preventing and Recovering from Hazardous Events



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