The unseen hazards in electrically driven vehicles

We are all aware of the impending gloom of queueing at the public charging points as vehicles get their 30 minute “top-up” due to the limited mileage of electric vehicles, and the concern over how those home owners who have no garage or drive will safely charge their vehicle parked on the roadside without trailing electrical leads over the pavement. No, the more serious issues, life safety issues, are not yet being considered, those issues relating to the inherent risk of fire and the possibility of toxic gases to which members of the public as well as firefighters may be subjected to.

The ignition of a battery fire within a car park caught on CCTV

There is still little known of the full hazards associated with electrically powered vehicles despite the tests that have been undertaken, and there are many questions still to be publicly addressed. The responses received from MHCLG to date have been platitudes, such as, ‘the government is determined to ensure that all residents are and feel safe’ and ‘we welcome constructive engagement with industry as we work together’. There appears to be no genuine practical attempt to address issues such as:

  • Is thermal runaway a real danger?
  • What are the real hazards associated with thermal runaway?
  • Are there harmful gases discharged during a fire?
  • What changes are needed to the current Building Regulations approved documents to accommodate electrically driven vehicles?
  • What hazards are members of the public exposed to in the event of an electrically driven vehicle fire within a confined space such as a car park?
  • Should electrically driven vehicles be restricted to “safe areas” in enclosed and underground car parks?

Such a lack of information will cause concern and alarm when incidents such as a vehicle fire occur and which cannot be suppressed in the way that a “conventional” car fire can be tackled.

With legislation in place stating that all new cars from 2030 onward must be electrically driven, information is needed now to ensure that regulation and guidance is in place to cope with the actual risks. Based on research into what peoples’ real-life experiences and the results of tests that have been conducted, I believe that there is still a lot of work to do.

What do we know of research undertaken?

The short answer to this question is still, very little. We are aware of a significant amount of research work being undertaken to enhance safety and increase battery life, but what of the current hazards?

BRE research in 2010 into fire spread in car parks specifically excluded electrically driven vehicles. Consequently, no knowledge could be gleaned of the potential fire risks associated with electrically driven vehicles.

In Norway, SP Fire Research, in cooperation with Skein Fire Department, the University College of Southeast Norway and Greenland Energy, have undertaken full scale tests on two electrically powered vehicles.

The results of the tests showed that in a simulated heavy collision, a large amount of smoke was first released from the battery followed seven minutes later by a fire that “burned freely”.

In August 2020, the Swiss Federal Department of the Environment, Transport, Energy and Communications (DETEC) published a highly informative and comprehensive report entitled Risk minimisation of electric vehicle fires in underground traffic infrastructures which produced evidence that burning electric vehicle batteries change the chemical hazard situation in underground traffic infrastructures.

The report states that ‘several experiments showed that severe mechanical and thermal damage to the lithium-ion batteries of an electric vehicle instantly lead to uncontrollable fires with high energy expenditure, strong smoke generation and new pollutant emissions’.

The study concluded that ‘the thermal fire hazards of electric vehicles are comparable to those of conventional vehicles. However, given an immediate vicinity to the fire and unfavourable ventilation conditions, electric vehicle fires lead to new, potentially more severe chemical hazards. The pollutant analyses indicated critical concentrations of the heavy metals in the form of aerosols. The emitted metal dust exceeded maximum concentration values above which an exposed person will suffer severe or permanent damage after 30 minutes (immediately dangerous to life or health). These pollutants are toxic both to humans and the environment’.

The report states that ‘as long as the potential effects of electric vehicle fires on the operation of these traffic infrastructures remain unknown, they present a risk for operators that is difficult to predict’.  

Hazards Within Confined Spaces

The greatest risk to the public is within confined spaces such as enclosed or underground car parks. The Lithium-ion batteries used within electrically driven vehicles when they catch fire do not perform in the same way as vehicles driven by an internal combustion engine.

When the vehicle batteries catch fire, the ignition can be violent and the risk of fire spread significant. Due to the type of fire and the location of the battery often being fully enclosed beneath the vehicle, access to extinguish the fire can be extremely difficult. Add to this the hazard of thermal runaway and reignition, the nature of the fire is quite different to that normally encountered by the Fire Service.

Due to the lack of research, we still know very little and there is no regulatory guidance, as a consequence, designers of car park ventilation and smoke control systems, and those who are responsible for managing those car parks and who will be responsible for risk management, are “working blind” to design or manage a system that takes account of the risks associated with all vehicles.

Thermal Runaway and Re-ignition

New Zealand firefighters have reported that, once a fire has been extinguished, there can be a risk of electric vehicle batteries re-igniting up to five days later. In the USA, following a traffic accident in which a car caught fire, the vehicle reignited on two occasions when being transported to the storage yard.  

If damaged, short circuiting can occur, creating a chain reaction referred to as a thermal runaway and burning at temperatures of up to 1,000°C. Reports have even suggested that sealed cells could explode if safety vents are overwhelmed or non-functional (such as may occur in an accident).

In the incident in the USA referred to above, police attempted to put out the fire of a crashed vehicle using a fire extinguisher carried in their vehicle, but were unable to extinguish the fire. They were unaware that the Lithium-ion batteries inside electric vehicles, once ignited, could not be put out with chemicals from a conventional extinguisher. Whilst firefighters dousing the flames with water seemed to work, the car twice re-ignited whilst being towed away.

Concerns about the risk of re-ignition have also been expressed by in the UK by police authorities responsible for roads policing and attending vehicle collisions. They have responsibility for safely storing vehicles that have been involved in an accident for evidence in possible court proceedings.

The risks associated with Lithium-ion batteries, with thermal runaway, the extreme temperatures that can be reached in the event of fire and the risk of explosion has been taken seriously in Sweden where research has been undertaken by the RISE Research Institute. Their report acknowledges that the ‘most severe’ challenge lies in extinguishing fire inside battery packs and thereby preventing propagation of thermal runaway between battery cells.

The test report states in its conclusion that it should be borne in mind ‘that without [on board] cooling of the battery and without flames, there is a risk that large quantities of flammable gas are released. If these gases accumulate in an enclosed space, e.g. the battery compartment, there is a risk of explosion’. There was no suggestion as to how much water was needed, nor how or where such a storage facility could be housed within a vehicle.

The Tesla online emergency response guide states that ‘Battery fires can take up to 24 hours to extinguish. Consider allowing the battery to burn while protecting exposures’.

What are the Emissions?

Car park ventilation schemes are designed to control harmful pollution caused by routine traffic movement and to clear smoke in the event of a fire. In compliance with current building regulations (Approved Document F), for day-to-day pollution control and ventilation, an air change rate within the car park of six air changes per hour is adopted and for clearing smoke in the event of fire (Approved Document B) ten air changes per hour.

In larger enclosed and underground car parks, an “engineered” solution to the day-to-day ventilation system can be adopted. The basis of this engineered approach is to assess the level of pollutants emitted. This assessment is currently based on the predicted level of traffic movement and the subsequent level of CO and/or NOx gases discharged into the atmosphere within the car park.

It is generally assumed that the emissions from an electrically driven vehicle will be virtually zero, or significantly lower than those emitted by a petrol or diesel powered car. However, one must assume that the vehicle must emit some gases whilst moving or even parked after being driven for any significant distance. The assumption that emissions will be insignificant and harmless may be correct, but it would be wise to confirm that this assumption is true and publish the data.

Quantifying the Fire Hazard

The most concerning issue related to an electrically driven vehicle is the potential fire hazard. Engineered solutions designed to control the flow of smoke in the event of fire rather than simply clear it from a car park are based on design fire loads. BS7346: Part 7: 2013 publishes a table listing suggested design fire loads, 4MW for a sprinklered car park and 8MW for unsprinklered.

From information collated to date, the characteristics and intensity of an electrically driven vehicle fire could be quite different to this published in the BS7346 guidance. The potential severity of the ignition in a battery fire is significant, along with the potentially increased rapidity in fire growth rate and fire spread. Current guidance of ten air changes per hour or designing a system based on a 4MW or 8MW design fire load may be totally inadequate for this new breed of car.

The Swiss research referred to above refers to ‘electric vehicle batteries change the chemical hazard situation’ and ‘the pollutant analyses indicated critical concentrations of the heavy metals cobalt, nickel and manganese as well as lithium in the form of aerosols. The emitted metal dust exceeded maximum concentration values above which an exposed person will suffer severe or permanent damage after 30 minutes (immediately dangerous to life or health)’.

Probably one of the greatest hazards in the medium- to long-term will be from worn or damaged batteries. With the cost of replacing and recycling batteries, both costs borne by the vehicle owner, there will be a real risk of some drivers, unable to afford the cost of replacement, running cars with damaged batteries which will consequently create an enhanced risk of fire.

There are also potentially different challenges faced by firefighters when tackling a fire in an electrically driven vehicle compared to internal combustion engine car, not least the danger of an electrical shock due to the extremely high voltage used in this type of vehicle, which could be as high as 600V DC.


It is obvious that more information is urgently needed in so many areas to enable informed decisions to be made on what risks these vehicles pose to public safety, especially in confined spaces such as enclosed or underground car parks.

Once accurate, evidence-based information is available, risk assessments can be undertaken and the appropriate steps taken to minimise any identified hazards. This would also be the first step in providing worthwhile regulatory guidance for the fire related industry.

More information is needed for architects, consulting engineers, fire engineers, Fire Service and regulatory authorities, including guidance on universal signage for the location of charging points and training on how to tackle a battery fire on a multiple of vehicles.

Clearly, more UK research is needed and more clear information must be put into the public domain. It is government policy that all new vehicles are to be electrically driven by 2030, this already leaves very little time for the work that needs doing.

About the Author:

Richard Brooks is a systems design consultant at Advanced Smoke Group Limited of Oadby in Leicester and has been involved in the smoke control industry for almost 40 years. He was chairman of the British Standard Institute drafting group of BS7346: Part 7: 2013, ‘Code of practice on functional recommendations and calculation methods for smoke and heat control systems for covered car parks’, and was, for a time, BSi’s UK delegate to the European committee drafting the standard for car park ventilation. He was also chairman of the Smoke Control Association group drafting the design guide to Design of Smoke Ventilation Systems for Loading Bays and Coach Parks.

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