SP Report on Bus Fire Safety

Between 1999 and 2004 an average of 49 bus and coach fires were reported each year in Norway, and 122 per year in Sweden. However, it can be assumed that the actual number of fires is considerably higher than this, as a large number of fires are not reported. This means that about 1.0-1.4 % of all buses and coaches in service are involved in fires each year. In percentage terms about 5-10 times as many buses and coaches catch fire as do heavy goods vehicles. Fortunately, despite the surprisingly high number of fires, injuries to persons have been very limited. However, the risk of a catastrophe is high if, for example, a fire should occur in a situation where escape is difficult. Examples of such cases are: Poland in 2005, when 13 persons died and Sweden in Fjardhundra accident in 1998 and that in Arboga in 2006 where luckily no fatalities occurred. As recently as November 2008 a bus fire in Germany killed 20 persons.

In the first experimental part of the project a number of materials typically used in the interior of buses were reviewed in terms of their fire performance. The tested materials were three seats, eleven wall and ceiling materials and two floor systems coming from modern buses and coaches with a mass more than five tons and with more than 22 passenger seats. All materials were tested according to ISO 3795/FMVSS 302, a simple horizontal flame spread fire test presently required for buses and coaches in Europe, and in several state-of-the-art fire test methods used for other applications such as trains, ships and buildings. The tests aimed to evaluate flame spread behaviour, heat and smoke release rates, ignition resistance and generation of toxic gases. The test results were compared to existing criteria for other applications and the present level of fire safety was discussed. The main conclusion was that the present horizontal fire test does not provide a sufficiently high level of fire safety in the passenger compartment of buses. Further, other fields with fire performance requirements are typically much more stringent. For example, products fulfilling the horizontal test for buses would not even meet the lowest performance level for buildings. A short review of the scientific literature shows that several publications have come to the same conclusion.

Since it was clear that the ISO 3795/FMVSS 302 test method is inadequate for discriminating between different levels of fire performance, an initiative has been taken at GRSG (Groupe de rapporteurs securite geneale) at UNECE (United Nations Economic Commision for Europe) to address this issue. The aim is to amend Regulation No. 118 where it is specified that ISO 3795/FMVSS 302 is the test to which bus interior materials should comply. The proposed amendment contains other and better methods that are employed by IMO (International Maritime Organization) and in the new EN-standard for fire safety of trains.

One part of the project was to identify designs and routines that create high risk for fire, and means to mitigate these risks. Fires are typically a result of heat, vibrations, defective maintenance, and compromises in the design. Most fires start in the engine compartment and therefore actions to reduce fire risk were focused on this part of the vehicle. Examples of suggested actions were:

Insulate all hot surfaces in the engine compartment (thermal insulation).

Equip the engine compartment with better detection systems in combination with
fire extinguishing systems.

If a fire occurs in, for example, the engine compartment or in a wheel it is important to contain it and delay spread of smoke and fire to the passenger area. The ability of a construction to resist spread of smoke and fire is referred to as its fire resistance. A high fire resistance in the critical partitions of the bus translates directly into more time for safe evacuation of passengers. Moreover, a high fire resistance will reduce the overall expected damage and therefore also reduce costs for repair and due to downtime. Typically fire resistance is tested according to the standard EN 1363-1 which is an advanced full scale test that is relatively costly. An alternative new small scale test method for fire resistance of bus partitions is suggested. This will enable cost effective testing in the early phase of development. In brief the suggested method consists of a furnace with dimensions W x H x D = 120 cm x 100 cm x 80 cm that is heated with three LPG-burners. The properties studied during the test are thermal insulation and integrity of the partition. A number of materials were tested with this new method. The results show that the method is effective in quantifying insulation capacity and integrity.

In addition to the method for testing partitions a new method for testing fire extinguishment equipment in the engine compartment was developed. The purpose was to define a test that can evaluate the fire fighting performance of different extinguishment systems in a well defined and objective way. The proposed test consists of a reduced scale engine compartment, scale 1:3, configured in a similar manner as that of a city bus with a rear engine. Since the scale of the test compartment is 1:3 only one third of the nozzles should be used in the test compartment, as compared to the number used in the full scale end application. Three different fuels are used at different positions in the test engine compartment: fibrous, liquid, and gas, and three different operating conditions are simulated: full load, idling, and engine off. The operating conditions are set by setting a pre-defined temperature on the engine mock-up and on the exhaust system mock-up, and by using different forced ventilation inside the test engine compartment. The proposed test method can be useful for manufacturers of fire fighting equipment, especially in the developmental phase. It can also be useful for bus owners and manufacturers by allowing an objective comparison of the performance of any given system and an objective definition of the required capacity of the system.

One part of the project was to investigate to what extent CFD (Computational Fluid Dynamics) models can be used as a complement to full scale fire tests. It was shown that CFD modelling can be particularly useful when the movement of smoke and heat inside the bus is the object of the study. Simulation results should be interpreted carefully, however, by a trained professional and cannot presently be used as an alternative to experiments, but rather as a complement. The reason for this is that there are many unknown parameters affecting the simulations. Experiments are therefore needed in order to validate and help interpret the simulation results.

In the last part of the project a conventional coach for 49 passengers was used for three different real scale tests. The tests were:
Fire in the engine compartment

Fire in a tyre

Full scale fire of the entire coach

In the first test a fire was started in the engine compartment. The bus was equipped with a system for fire detection which was tested at the same time. The main conclusion was that stopping the engine is an effective way of reducing or extinguishing the fire since the oxygen level drops rapidly when ventilation of the engine compartment stops. It was also found that the detection system enabled an early detection which in practice would drastically reduce any damage to the engine compartment and eliminate or reduce spread of fire and smoke to other parts of the bus when coupled to rapid fire intervention.

The test with a burning tyre did not result in a cracked window. This was an interesting result since fire spread from a tyre into the passenger area via a broken window is often believed to be the root cause of fire inside the bus. One should note, however, that the tested vehicle was a coach where the distance from the tyre to the window is larger than for a typical city bus. This means that for the tested coach the flames did not reach the window to the same extent as would be the case for many other bus types. Smoke and fire did spread to the passenger area, however, from the wheel house via the floor and side wall.

In the last test a fire was allowed to develop in the entire bus. The heat release rate reached 12 MW before the fire was extinguished in order to protect the laboratory equipment. The scenario was a fire starting in the engine compartment in the rear end of the coach. The purpose with this test was to study:

Fire development from the engine compartment into the passenger compartment.

Smoke spread and visibility in the passenger compartment.

Concentrations of toxic gases in the passenger compartment.

Heat release rate from a developed fire in the coach.

The test showed that the time for evacuation of the passengers was 4 – 5 minutes at a maximum. After this time the concentration of toxic gases reached dangerous levels. The visibility in the passenger compartment decreased rapidly. After 5 – 6 minutes the visibility was just a few meters.

In summary, the number of bus fires is disproportionately high as compared to other types of vehicles. In this project a number of strategies for improving the fire safety of buses have been defined. A new test method for partitions of buses has been developed and is promoted internationally. This test will considerably simplify testing as compared to existing methods. A method for testing fire fighting equipment in engine compartment has also been developed.

A full scale test showed that the time available for evacuation in the case of fire is very short. Therefore, improvement of the fire behaviour of buses is of the highest importance to ensure passenger safety in the future.

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