Cascading failure in holiday lights
Close-up of a standard holiday light bulb. The shunt wrapped around the posts will do nothing, unless the filament fails. Upon filament failure, the coating on the shunt burns off, creating a conducting pathway to keep the series circuit complete.
Image, captured from a high-speed video, showing the purple-white glow of the shunt resistors and the yellowish glow of the filament. Note the filament is centered within the bulb, while the shunt is near the bottom. This was filmed by cutting a string of lights down to only 12 bulbs and connecting them to 120 VAC, while filming at 240 fps. The slow motion video can be viewed at TPT Online at http://dx.doi.org/10.1119/1.4792024.1 and shows a fantastic dance of filament/shunt resistors cycling on and off. doi: 10.1119/1.4792024.1.
Frames from high-speed video of the failure. A failed filament (frame 1) causes the top left bulb to lose current (it is cooling off). The coating on the shunt resistor of the bottom bulb is burned off, thus reestablishing the electrical current (frame 2). This filament then fails and we see the coating on the shunt resistor within that bulb firing up before the filament has even cooled down (frame 3).
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