PTJ Reveals Cause of Fatal Bus Fire

By Don Winner - Inspectors from Panama's Judicial Technical Police (PTJ) held a press conference this afternoon to announce the cause of the bus fire that killed 18 Panamanians on Monday afternoon last week. Apparently the air conditioning system on the bus had a tendency to overheat. There was a safety system built into the system, specifically designed to prevent such a fire. When the air conditioning system compressor overheats it sends a signal to the fuse box which effectively shuts down the bus. In other words, if property wired a fire like this can not happen. Apparently the bus was "fixed" and a mechanic wired the compressor directly, bypassing the safety device. The PTJ inspectors called it a "slow fire" and were able to identify the exact cable that overheated and started the fire. Once it became red-hot, the insulation started burning and there was a relatively small fire inside of the engine compartment cover. This fire spread to other flammable materials inside of the engine compartment, including a flammable industrial-grade refrigerant gas and broken fuel hoses (more).

RPC TV: Channel 4 and RPC TV had the scoop on this one. They were the only station that covered this story on the 6:00 pm evening news. I saw the report and followed up with some research and an hour on the phone with my dad, the retired fire chief. That's the sourcing.

Short Answer Up Front: A mechanic rigged a bypass to a safety device which caused overheating, a short-circuit, and a fire. The small fire grew very large quickly because the air conditioning system was charged with a type of gas called HC-12a that is only supposed to be used in industrial applications. And, at some point the pressurized fuel lines failed, adding (literally) fuel to the fire. The whole thing flashed over when the driver lifted the engine cover and provided oxygen to the fire. And, the fire flashed right next to the only available exit. If there was a working fire extinguisher on the bus, which is doubtful, the driver simply bailed and never tried to fight the fire. Eighteen people died as a result, dozens were injured, and two remain hospitalized with 3rd degree burns.

Fuel Vapors Throughout: This fire almost certainly started on the Northern Corridor. Reports from survivors indicate that the bus had to stop several times and that the air conditioning system was not working and that the "only thing coming out was heat." At that point the compressors had already failed and eventually the overheated wires started burning. Eventually the fuel lines were compromised and diesel fuel was being sprayed onto the hot engine, turning it to vapor. Diesel has a higher flash-point (>62 °C) compared to gasoline (>-45 °C.) The fuel vapor went through the passenger cabin and when the driver lifted the engine cover he provided the oxygen that was lacking and the fire "flashed over" and spread quickly through the bus.

A Common Error: Bypassing a fuse or safety device is a common mistake. Fuses are put into an electrical system specifically to fail (on purpose) before something can get hot enough to cause a fire due to a short circuit. Fire examiners can look at a wire from a fire and tell whether it was the cause of the fire (heat generated from the inside-out) or if it was burned as a result of the fire (heat coming from the outside-in). The copper is effected differently. Also, you can find copper puddling at the point along the wire where the current was high enough to short out, melt the wire, and start the fire. At that point (in time) the engine is still running or there was still electrical power going through the system. A wire or a bundle of wires that was burned in a fire are usually intact with the insulation burned or destroyed, but they usually don't get hot enough to actually melt the copper. The fire eats the plastic and moves on.

Weak AC System: The air conditioning system on these buses was known to be insufficient. A compressor on an air conditioning system is run by a pulley wheel which is driven by the engine. The job of the "inside" of the compressor is to put the refrigerant gas under pressure. This chamber has to be physically separated from the mechanical application of force from the pulley by a clutch system (to keep the gas from leaking out.) The compressors on these buses were wired with a safety system that would send an electrical signal which would disengage the clutch if the compressor started overheating. Because the combination of the engine and air conditioner in the hot and humid conditions were under powered they would often overheat and the safety system would kick in, disengage the clutch, and effectively shut down the air conditioning system. That safety was bypassed with an improvised direct-wire that short circuited the safety control at the fuse box. The improvised rig caused the fire.

Using the Wrong Gas: There are several types of gas that can be used in air conditioning systems. A gas called HC-12a was being used in this bus. HC-12a® and OZ-12® brand hydrocarbon refrigerant blends are flammable refrigerants. Their primary components are hydrocarbons, which are flammable substances such as propane and butane. HC-12a® and OZ-12® are registered trademarks of OZ Technology, Inc. HC-12a® has been marketed since 1994. OZ-12® was a similar blend marketed until the introduction of HC-12a®. It has been illegal in the United States since July 13, 1995 to replace CFC-12 with HC-12a®. OZ Technology is prohibited from marketing this blend as a substitute for any ozone-depleting substance. In addition, any use of this blend as a substitute for CFC-12 or any other ozone-depleting chemical, in industrial process refrigeration or any other refrigeration or A/C end use, is prohibited under the Clean Air Act. Note that the Clean Air Act does not regulate the use of any of these hydrocarbon refrigerants when they are used as replacements for non-ozone-depleting chemicals such as HFC-134a. However, many states prohibit using flammable refrigerants in motor vehicles, regardless of which original refrigerant was used in the vehicle. It is illegal to use hydrocarbon refrigerants like HC-12a® and DURACOOL 12a® as substitutes for CFC-12 in automobile or truck air conditioning under any circumstances.

Industrial Use Only: EPA has not yet received data that adequately address the safety issues of hydrocarbon refrigerants in applications other than industrial process refrigeration. Flammability risks depend on the type of refrigeration or air-conditioning system. Industrial process refrigeration, for instance, does not include air conditioning, which pipes refrigerated air directly into occupied areas. Industrial process refrigeration generally refers to complex customized appliances used in the chemical, pharmaceutical, petrochemical and manufacturing industries. Direct risk to human health is reduced in industrial process refrigeration; for example, access to areas near the system is typically restricted. In addition, other regulations exist to protect the safety of industrial workers.

Basically, A Bic Lighter: So far there are two mechanical errors. First the short circuit to cause a small fire, the compressors are driven to overheating and failure, and the gas they are pumping around the plumbing is basically the same stuff as butane. This stuff simply is not supposed to be used in vehicles for fire safety reasons, and (by the way) it also eats holes in the ozone layer. So a small and smouldering fire was able to grow and spread.

Under Pressure: Fuel has to be delivered to a diesel engine under pressure through a fuel injector that sprays it into a mist to create the correct mix of fuel and air to allow for combustion. At some point a fuel line was compromised, causing fuel to be sprayed under pressure onto the hot and running engine. It could not have continued running in that condition for long because soon the engine would have failed for lack of fuel.

Going to Get the Mechanic: At the time of the press conference the PTJ said that the driver was cooperating with the investigation and that he had identified the mechanic who had made the "repairs" on the bus. Investigators were on their way to pick the guy up. If there's a bright side, at least now inspectors know exactly what to look for, and I wonder how many of the buses that are still driving around the city have their systems charged with HC-12a, running on air conditioning systems with bypassed safety systems. Talk about your nightmare scenario...

Copyright 2006, Panama-Guide.com by Don Winner. Go ahead and use anything you want, just please give credit to the author and original source.