Hotter and More Persistent Electric Vehicle Fires Are Changing the Way Transit Facilities Are Designed

The risk of a zero-emission battery electric bus (BEB) catching fire is no greater than it is with a traditional internal combustion engine bus, but when they do ignite, the intensity and persistence of the fire can have devastating consequences, particularly in structured facilities.

“Electric vehicle (EV) fires experience extremely high temperatures and explosive hazards within the battery cells,” said José Del Solar, PE, assistant vice president and supervising tunnel ventilation and fire life safety engineer at WSP. “Gases in the batteries, such as hydrogen fluoride, can explode, and the resulting toxic fumes and smoke can cause breathing irritation … 60-80 percent higher concentration of hydrogen fluoride in an EV vehicle fire than what is present in an internal combustion engine fire.”

Additionally, fire codes that were created with diesel-fueled vehicles in mind are still catching up to the rapid changes BEBs are bringing. Today, agencies and municipalities are looking beyond current code requirements to do what is necessary to protect their fleets, buildings and people in close proximity to a potential EV fire.

“Fire departments strictly adhere to applicable codes when evaluating plan approvals for facilities, but most current codes do not yet specifically address facilities wherein an EV fire could occur,” said William Connell, senior vice president of tunnel systems at WSP. “We are helping municipalities and jurisdictional authorities navigate the code process while at the same time develop a better understanding of how the different risks presented by EV fires might require consideration of a different approach to some key aspects of the fire protection design, which we have concluded will better protect facilities intended to house and service these vehicles.”

WSP uses a performance-based design (PBD) approach when working with transit agencies to plan BEB facilities and garages. The approach makes use of computational fluid dynamics (CFD) modeling to determine the consequences of a fully engulfed EV and establishes best practices for those scenarios.

“PBD is important to fully understand the consequences of an EV fire in new or existing enclosed infrastructure housing EVs,” said Hasan Raza, a fire-life safety and computational fluid dynamics engineer at WSP. 

The models test and evaluates:

• structural thermal exposure,
• appropriate water application rate for sprinklers,
• need for passive fire protection for structures,
• appropriate vehicle spacing to limit fire spread,
• toxicity levels and irritant gas products, and
• appropriate ventilation.

Raza, Del Solar and Connell have performed considerable research analyzing and defining these fire risks and establishing a methodology to mitigate and prevent these fires from spreading and causing catastrophic consequences. Their expertise has been sought in safety and emergency management for EVs and structures, which included a presentation to the U.S. Climate Alliance in October.

Every project adds to a deeper understanding of fire behavior. One such project involved renovations to an urban bus maintenance facility to accommodate BEBs, where WSP was the engineer of record. The enclosed structure was next to a parking deck, which presented an added risk of damage extending beyond the maintenance facility itself.

“The city fire department wanted assurances that what was being proposed was adequate from a fire protection standpoint, especially since there was limited guidance from existing codes and standards that they could fall back on for this project,” Connell said.

The results of the fire modeling analysis using CFD provided guidance on the type of sprinkler system that should be used along with an appropriate water application rate that could reduce the impact of a fully engulfed BEB fire. Passive fire protection for some structural members was also recommended to help protect the structure and avoid extensive fire damage.

“Since it may be several minutes before the fire department arrives, every element of fire mitigation is essential in such scenarios,” Raza said.

This collaboration set the groundwork for the transit agency’s overall zero emissions fleet program and established the criteria it would use moving forward for all facilities scheduled for upgrading.

Using the appropriate type of fixed firefighting system, along with an appropriate water application rate, is the key to reducing the consequences of a fully engulfed EV or BEB fire.

The research conducted by Raza and the team determined that, for EV and BEB fires, the Extra Hazard Group classification of water density, as identified in the National Fire Protection Association (NFPA) 13 code addressing sprinkler systems, would be more appropriate to effectively cool the hot gases released and reduce the structural thermal exposure resulting from the fire.

Reignition is also a concerning risk. The key is to use enough water capable of controlling the heat release and combustion until the emergency responders can intervene respond and extinguish the fire completely.

“The moment a battery ignites, that bus can be considered a lost asset,” Connell said. “So the focus is not on trying to save it, but to remove it from an enclosed facility to a place where the fire can burn out without spreading. These buses are very expensive and if fires are allowed to spread, extensive loss of assets, and the structure itself may occur.”

The best approach to controlling an EV fire? It isn’t revolutionary, it’s water.

“When fire departments have had to battle EV blazes, they douse the bus with as much water as possible, until it reaches a point where it can be removed from the building safely before it reignites. The key is to make sure there is enough water available to prevent the fire from spreading. That may require an onsite water tank to maintain the system.”

While fire-suppressing foam can help control a blaze, Connell said that water has proven to be more effective at cooling EV fires and keeping them contained.

Click on the buttons in the infographic to learn seven important facts about electric vehicle fires.

Rethinking Maintenance Facility Design

WSP works with public transportation agencies and policy makers to transition to zero emission fleets, including development of support infrastructure. That work includes a state-of-the-art facility that will charge, service, operate, maintain and house a sustainable fleet of BEBs.

“We have designed the fire protection system of the facility in such a way that allows people to safely evacuate in the event of a fully engulfed BEB fire, protect adjacent parked BEBs, and reduce structural thermal exposure,” Raza said. “We have incorporated a strategy to exhaust the smoke using a combination of both natural and mechanical ventilation.”

“What we’re doing for that facility is cutting edge,” Connell said. “It is leading us to understand how these facilities need to be designed, and what transit agencies and jurisdictional authorities need to consider when transitioning to zero emission vehicles to assure their facilities are safe and protected from the fire risk posed by EV technology.”