Dear Tom and Ray:

You recently answered a question about a car fire started by an automotive cigarette lighter that did not "pop out.", You suggested the fire could possibly have been prevented by a fuse blow. This is not the case. Fuses do not directly prevent fires. Fuses blow only as a result of passing excessive current, a condition that occurs only in the case of an overload or short. As long as excessive current isn't drawn, the fuse won't blow. The problem is, a cigarette lighter is capable of getting quite hot WITHOUT drawing current above the rating of its fuse, since getting hot is an essential part of its function. What usually stops the cigarette lighter from getting hot enough to start a fire is its own mechanical thermostatic action, which causes it to pop out and disconnect itself from power. If this mechanical thermostatic action is interfered with, the cigarette lighter will get hotter and hotter, but will not actually draw excessive current, and will therefore not blow its fuse. The cigarette lighter could, in theory, get hot enough to burn out its element and stop working, but this would happen long after the dashboard is already in flames. The situation is comparable to that of a kitchen electric range. While fuses and circuit breakers will prevent fire which might result from overload or short within the range itself, they do not prevent the cook from burning food. Everyone knows that it is possible to set food on fire with an electric range without ever blowing the fuse. In my opinion, cigarette lighters are inherantly unsafe. There is absolutely no protection against fire if the mechanical thermostatic action fails for whatever reason. Since I do not smoke, my cigarette lighters usually ride around in the glove compartment where they cannot hurt anything. Since my father does smoke, his cigarette lighter rides around in my glove compartment, too, where it can't hurt him.
Michael

TOM: You're right about the fuse, Michael. If the "mechanical thermostatic action" (that's scientific notation for "popping out when it's hot") doesn't work for some reason, the cigarette lighter WOULD just keep getting hotter and hotter. And in the letter you referred to, that's what apparently happened after a mechanic had tried to "adjust" the lighter.

RAY: And we certainly do know that food can be burned without blowing a fuse. How else could my brother make what he calls his "cajun style" toast every morning!