Wednesday , 24 July 2019

Post-collision fires in road vehicles

The loss of human lives and body injuries as a consequence of post-crash fires either by smoke inhalation or due to burn injuries are unfortunately not uncommon. The literature indicates that fire events related to crashes are still a significant problem. The increased combustible load in newer vehicles is an important factor to be taken into account for fire safety, as well as their potential to release toxic fumes while burning. Trends indicate that the survivable collision energy will continue to increase and, at the same time, the probability of post-crash fire will rise with the collision energy. This means that the occupants of a vehicle may probably survive a high energy collision but will sustain severe injuries or death due to a post collision fire.

A study published by Viklund et al shows that 5% of all fatalities in Swedish roads due to collisions in passenger cars, sport utility vehicles, vans and minibuses which took place between 1998 and 2008 occurred in burning vehicles. In USA where, on average, 31 vehicle fires are reported per hour, these are responsible for around 300 deaths and 800 injured persons per year. Furthermore, these vehicular fires are responsible for 12% of deaths, 8% of civilian injuries and 9% of the direct property damage attributable to all the reported fires. For the case of Sweden, the cause of death in one third of the reported incidents was attributable to fire only with no or limited trauma injuries. This means that in these cases the occupants did not die due to the combined effect of trauma and fire injuries, but for the effect of fire only.

The large amount of fatalities following vehicle fires can be understood by the low fire performance of the materials used for vehicle manufacturing. Although there are regulations specifying some minimum requirements with regard to fire performance of materials fitted in vehicles, these regulations are flaccid when compared to the fire safety regulations for the aeronautic, marine and rolling-stock sectors. For example, fire prevention requirements in buses do not cover aspects such as the limitation of peak heat release rate, smoke yield and toxicity as done in regulations for trains, planes and ships. Reducing fires in vehicles is strongly related to the survivability of the occupants against a collision event.

Statistics show that occupants travelling on modern vehicles have a higher probability of surviving a severe impact than of those travelling on vehicles with older technologies. The active and passive safety systems increase the survivability rate but these do not reduce the risks of a fire as a post-collision event.

In order to reduce the number of injuries and human fatalities associated to post-crash fires in road vehicles, it is necessary to study the causes of these lethal fires. Of particular interest are the ignition sources, vehicle types, fire dynamics and toxicology mechanisms which directly contribute to the loss of human lives, for instance, the dynamics and toxicity of fires due to upholstery and materials in passenger compartments or toxic gases due to fires in electric vehicles. New materials and traction systems (e-vehicles) introduce new toxic substances when burning – but intoxication by some of them could be reduced by using available antidotes – which make knowledge of these substances necessary.

This document reports the findings with regard to the problems related to post-crash fires in road vehicles from a multidisciplinary point of view.

The fire performance of materials used in the interior of certain types of road vehicles was regulated by the directive 95/28/EC and was followed by the UNECE Regulation No. 118. UNECE Regulation No. 107 is also applicable.

The directive 95/28/EC stipulates three different flammability tests:

  •  Vertical burning rate of materials.
  •  Horizontal burning rate of materials.
  •  Melting behaviour of materials.

Directive 95/28/EC requires three types of flammability tests for interior materials in the passenger compartment:

  1.  horizontal burning rate of materials,
  2.  melting behaviour of materials and
  3. vertical burning rate of materials. In the first test, five samples in the case of an isotropic material or ten samples in the case of a non-isotropic material, should undergo this test. Each sample is exposed to a low-energy flame for 15s inside a combustion chamber.

Directive 95/28/EC is applied for vehicles carrying more than 22 passengers, not being designed for standing passengers or urban use. This regulation is based on the flammability requirements for buses in USA brought into force as Federal Motor Vehicle Safety Standard (FMVSS) 302 in 1972. Directive 95/28/EC became the UNECE Regulation No.118 in 2005, with updates since then, such as including a new test to determinate the capability of materials to repel fuel or lubricants in 2010.

This regulation has received much criticism mainly because the tests might only be suitable for small ignition sources such as a lighter. For this reason, studies have provided other methods for analysing the flammability of interior materials. The latest revision of Regulation No.118 that of 2012, includes the use of the test method ISO 5658, lateral spread in vertical configuration. The materials that pass this test are considered to also pass the vertical burning rate test and the melting tests. This last version requires the same flammability tests to the materials in the engine compartments.

On the top of the flammability tests, The UNECE Regulation No.107 improved the fire protection of buses in Europe in 2001. This regulation No. 107 and its updates, cover different topics related to bus safety. The engine compartment requires no flammable sound proofing materials or material liable to become impregnated with fuel, lubricant or other combustible material. A partition shall be fitted between the engine compartment and any other source of heat. The use of alarms in the engine compartment is demanded since 2012. Fire extinguishers and first-aid equipment should be placed on appropriate locations. Buses shall be equipped with an alarm system detecting either an excess temperature or smoke in the toilet compartments, driver’s sleeping compartments and all other separate compartments (added in 2012). The driver’s compartment shall be provided by acoustic and visual signal (added in the last updated in 2014) operational whenever the engine start device is operated.

Among the requirements on fire protection, Regulation 107 includes other specifications related to fire protection such as the insulation and protection of electric equipment, the need of the no flammable material near any source of heat, exits and emergency characteristics (e.g. number, position and width), driver’s compartment and interior lighting.

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