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| Re: Fourth Rails and Safety (832321) | |||
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Re: Fourth Rails and Safety |
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Posted by SelkirkTMO on Sat Sep 12 04:14:27 2009, in response to Re: Fourth Rails and Safety, posted by Bill West on Sat Sep 12 03:46:16 2009. Understood ... but bear in mind I'm also no expert in electric railways, but as I've read the issues, the problem in many DC traction systems where electrolysis was an issue was that the rails provided insufficient current flow back to the powerhouse, and instead, the traction current found its way back to the dynamo by means of soil conductance. Little was known about stray voltage back in the early days which of course is why railroads use thermite bonds these days. :)As to Amtrak, and other AC-powered traction, electrolysis cannot be an issue because the current reverses at a regular rate. Any ions stripped from a metal are returned when the current reverses. Try electroplating metal with AC voltage across the soup. Nothing gets plated. Only under DC supplies does this issue rear its ugly head even though practical mitigation techniques DO exist, and WORK on DC electric railways. It was unknown back then how MUCH of an influence a few ohms of resistance can raise at high voltages and high currents such as streetcar lines and such. Nowadays, that stray current can be reduced to mere milliamps. In the OLD days, the leakage was substantial. And in poorly maintained modern railways, it still can occur from time to time due to negligence. That all said though, what I can't wrap my mind around is that if you have a DC circuit and you measure across the mains, or with a load in parallel on it, you get only ONE voltage reading. Only when you add a ground reference on a bipolar circuit, can you measure a minus 200 on one power supply rail (and yes, railroad rail too) and a positive 400 on the other. Therefore there HAS to be a ground reference which is an *active* part of the circuit, and in particular the power supply. I surmise that ONE rail on LU is grounded, and IS a return to the substation if not both rails, depending on how the signal plant is wired. Otherwise, the only means of determining the voltages is across the two power rails and the correct answer there would be ~600 volts of either polarity (depending on which rail the red meter lead goes to) but it would not be possible to measure a + and a - voltage with respect to common. Just me, science can be irritating at times, particularly if we're not looking at the same schematic. :) |
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