According to research, fire is the cause of truck accidents in nearly eight per cent of cases, today PowerTorque looks at the common causes of fires in trucks and trailers. This places fire behind causes like inappropriate speed, fatigue, driver error and accidents with no discernible cause, but fire is more prevalent a cause than mechanical failure, theft and contributory negligence.
NTI’s Major Accident Investigation Report in 2017 breaks down the stats to tell us over 65 per cent of the accident causing fire incidents surveyed were engine and cabin fires, 14 per cent were caused by a truck wheel bearing, while just over 10 per cent were caused by a trailer bearing fire. Around six per cent were brake fires and a further four per cent were trailer freezer fires.
Common Causes
In a study of truck fires from 2016, Peter Hart of Hartwood Consulting identified a wide range of issues operator should bear in mind in regard to reducing the incidence of fires in a fleet.
Truck fires can have many and varied causes and the most common causes of heavy-vehicle fires are:
- Arcs on the starter (or battery cables), the alternator cable or the positive feed wire into the cabin.
- Flammable material resting against the turbo charger or the exhaust;
- Fuel line rubs or failures that result in leaks / sprays of fuel onto the exhaust.
- Lubrication oil line failures near to the exhaust.
- Turbo charger failures that cause excessive temperatures in the exhaust;
- Leakage of hot gases from the exhaust.
- Electrical arcs at terminals or connectors resulting from hot terminals that cause insulation to melt and catch fire.
- Addition of heavy add-on loads onto a circuit not intended for it. If the fuse rating is increased, the wiring may not be adequately protected.
- Collection of carbon dust/organic residue in alternators leading to short circuit.
- Tyres catching fire because they are flat or poorly inflated or rubbing on hard surfaces.
- Wheel bearing failures resulting in bearing grease catching fire. Sometimes overheated brakes cause the bearing grease to catch fire.
- Road debris that catches under vehicles and is combustible.
Minimise the Risk
There are a number of ways a truck operator can minimise these risks:
- Specify that vehicles have circuit breaker protection on the alternator, trailer feed and cabin power supplies from the batteries.
- Specify that the starter motor cable is double insulated, conduited and taped closed.
- Use plastic conduit to protect electrical cables that has fire-retardant properties.
- Insist on rubber-block clamps to hold main electrical cables in place. Avoid clamps that have a metal spine and a thin rubber insert. If the rubber insert come out, a sharp metal edge will cut into the loom.
- Keep combustible parts, such as noise shields and fibreglass engine tunnels, well away from the turbo and exhaust.
- Check and adjust the wheel bearings regularly.
- Don’t add heavy electrical loads onto existing electrical circuits.
- Use compressed air to blow residue out of alternators and starter motors.
- Check for rubs on the fuel lines and on the turbo charger oil line. Ensure that that fuel hoses are clamped at the ends and properly fixed.
- Keep fittings in hoses that carry oil or fuel well away from the exhaust. Anticipate whether a failed fitting could spray the fluid onto the exhaust pipe.
- Check that the tyres are pumped up and cannot rub on hard surfaces.
- Ensure that heat shields that might be on the exhaust (for diesel engines) are tight and in-place. Exhaust clearance to combustible materials should exceed 150mm.
- It is also advisable to carry a fire extinguisher and to open circuit the batteries using an isolation switch when the vehicle is parked.
Pre-conditions for Fire
Fire takes hold under the following three conditions:
- Some (combustible) material is heated to its ignition temperature;
- There is an adequate supply of oxygen (air);
- There is a propagation path for the fire.
The rate at which a fire spreads also depends on these factors. Well-oxygenated fires in materials that have low ignition temperatures spread quickly while fires in, for example, wheel bearing housings are usually slow to propagate.
The materials that burn are usually:
- Wiring insulation (normally has an ignition temperature about 150oC).
- Plastic and polymer materials used as noise shields, insulation, flooring, trim, etc.
- Fibreglass used in the cabin.
- Hydrocarbon fluids such as diesel fuel, lubricating oil, bearing grease,….
- Rubber tyres.
- Plastic tubing, straps, and air hose rubber coatings.
- Hydrogen build-up in batteries leading to explosion.
Experience shows that metals are never the initiating materials. Solder always melts and aluminium often melts as a result of fire, but this is consequential. Copper, brass and steel are oxidised by exposure to fire but only melt at electrical arc points. Electrical short circuits may produce local hot spots with temperatures sufficient to melt these metals. Therefore, electrical short-circuits can usually be identified from solidification globules and arc craters on either electrical or adjacent earthed metal.
(PowerTorque would like to acknowledge Peter Hart from Hartwood Consulting for his work in this field and supplying this data.)
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