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Reading Topic #184
There is a part on all the L&PS motor cars that I am unable to identify. I noticed this box on the roof of every car - even ones purchased second-hand from other roads. Can anyone identify this and maybe provide a little info on its function?ThanksWayne
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It's either a lightning arrestor or fuse box. They both looked similar and served the same purpose.
Dave
Dave
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I wouldn't know if either fuse box or lightning arrestor is correct, but they do not serve the same purpose.A fuse, like a circuit breaker, is an overcurrent device. It is installed in the circuit so that the flow of current in the circuit goes through the device. The magnitude of the current is some how measured, then, on a moving train, it would have been thermally or magnetically. A fuse is just a thermal device, circuit breakers would have had both. At that point there were also electrico-mechanical instruments, but the motion of the train would have interferred with proper operation. There are now also electronic devices. So anyway, a fuse is used to protect a particular circuit against excess current flow. The excess current flow might be caused by a fault (short circuit) or an extended overload.A lightning arrestor is a surge supressor that responds to overvoltages. At that time, a lightning arrestor would have been a spark-gap device, where two electrodes are held a fixed distance from each other. One of the electrodes would have been connected to the overhead side of the power circuit and the other electrode connected to ground, the rail side of the power circuit. The distance between the electrodes is set so that air, even air with a high humidity level, remains an insulator at the operating voltage, plus some margin. At higher voltages, the air between the electrodes would not be a sufficient insulator and a spark would jump the gap. Once a spark jumped the gap, the air forms a plasma, with a much lower resistance than the air had, creating a low impedance path to ground for the overvoltage. Should a lightning strike, or other source, such as a switching transient, cause an overvoltage condition, the lighting arrestor will conduct, protecting the circuits downstream.Overvoltages can certainly cause overcurrents, and so a fuse may respond to the associated overcurrent, but fuses and lightning arrestors respond to different electrical phenomena, doing different things, and are connected into a circuit differently.David Beach.Palouse & Lewiston
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