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| Re: Fourth Rails and Safety (832245) | |||
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Re: Fourth Rails and Safety |
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Posted by Bill West on Fri Sep 11 21:53:29 2009, in response to Re: Fourth Rails and Safety, posted by MGL on Fri Sep 11 19:30:45 2009. Boy is this getting off course....Michael, 1. electrolysis as a hair treatment is nothing to do with electrolysis as a corrosion mechanism. Lots of English words have multiple uses. 2a. the leads from a simple DC power supply are usually marked for their polarity ie positive and negative. This shows the direction of current flow. So if you measure from + to - a meter will show +600 volts but if you swap the meter leads the display will say -600 volts. That’s all that is meant by negative voltage. 2b. Selkirk’s sketch is for 3 wire supplies that are intended to provide 2 or 3 voltages to different loads at the same time. Many applications ground the center connection but it does not always have be, it depends on what the rest of the design is doing. 2c. Lionel track is 2 running rails and the “third” rail in the middle, there is only one voltage. It’s not grounded either. 2d. 4th rail traction systems do not use the running rails for power at all and they do not have to ground either of the two power carrying rails, although they could. What most do is put a ground somewhere near the center of the power supply. This may not be able to supply power the way Selkirk’s sketch does however. The ground may just be a sensing relay that spots insulation failures on the 2 power rails and the power rectifier is just connected to the 2 power rails. The reason for using the 4th rail is indeed likely to be historical corrosion concerns. 3. To answer your original question a. if the power supply is completely ungrounded, 4th rail would be safer as you would have to touch both power rails at once to get hurt. For other technical reasons this is not a good system to use, it introduces other safety hazards the most basic of which is that half your insulation can fail without anyone knowing about it. b. if the power supply is grounded on one side the system is the same as a 3rd rail and the 4th rail is a waste of time. c. if the power supply has either a power ground or a sensing ground near the center, 4th rail would be poorer because there are more places you can accidentally touch. Even a sensing relay would pass enough power to stop your heart. Whether the voltage you touch is 210 or 420 is not an issue either. But as you’ve found there doesn’t seem to be any data on 4th rail, possibly because it was not likely an original reason for choosing it and because the statistics would probably be too small to make an argument against the corrosion concerns. Sort of like asking would being hit in the head at 30mph by a metal cover over the coupler be safer than being hit directly by the coupler. Being on the roadbed is extremely risky no matter how you rate the individual hazards so I doubt if anyone cares about the extra power rail. Bill |
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