In cold climates, in order to ease starting, automotive vehicles are sometimes equipped with block heaters to keep the engine warm while not in use. Figure 1 is a drawing of the layout of a typical block heater. The heater is usually secured to the block water jacket or coolant hose so that warm water circulates throughout the engine. The AC power cord is typically routed to the front bumper area where a plug is installed to connect to the owner’s extension cord. The vehicle owner plugs the block heater cord into a home extension cord after vehicle use, and before use is supposed to disconnect the extension cord. Occasionally, the owner may forget to disconnect or purposefully not disconnect the cord and merely back the vehicle away, resulting in the block heater plug becoming forcefully disconnected from the extension cord. Over time, this tends to result in electrical insulation breakdown of the block heater power cord or of the extension cord. Chaffing of insulation can occur as a result of repeated contact with the bumper or other structural members by either the block heater power cord or the extension cord. Heating from the ensuing electrical fault can cause a fire.


Figure 1

Figure 2

Figure 2 is a view of a vehicle equipped with a block heater that had been connected to an AC extension cord one evening. A fire was discovered in the vehicle the following morning. Since the vehicle was not being driven, many of the vehicle operational fire causes such as hot surface ignition, fuel injector malfunction and oil leakage could be eliminated as the cause of the fire. Burn pattern recognition is significant in Figure 2, showing severe damage to the front of the vehicle. The aluminum air-conditioner condensing coil is nearly consumed, while the aluminum radiator behind it is not. Severe oxidation of the chrome front bumper and consumption of polymer body parts, suggest an origin at the front bumper area in the vicinity of the block heater power plug. Figure 3 is a view of the block heater power cord and plug showing evidence of an electrical fault. Evidence of an electrical fault was also found on a sharp corner of the bumper suggesting that insulation breakdown from chaffing was the cause of the fire.


Figure 3

Figure 4

Figure 4 is a view of another block heater related loss. Again, burn patterns, especially on the air-conditioning condensing coil, suggest a block heater electrical cord related fire. Figure 5 shows the faulted wire that was on the owner’s extension cord, which was the likely cause of the fire.


Figure 5

As a fire analyst, one is often asked to opine on possibility of subrogation as a result of a loss. Some issues to consider when analyzing block heater power cord fires are as follows: Certainly driving off while the extension cord is connected is a misuse of the product. Subrogation potential is dependent on whether such misuse is foreseeable. As far as the installer or manufacturer is concerned, the routing of such a block heater power cord in the vicinity of a sharp corner is not considered good wiring layout design. If such a condition cannot be eliminated, sufficient warning to the vehicle owner should be conveyed as to the possibility that an electrical malfunction could occur.

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