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| Re: (interlocking) TA is obsessed with CBTC, and ''New'' tech for no reason. (583403) | |||
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Re: (interlocking) TA is obsessed with CBTC, and ''New'' tech for no reason. |
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Posted by Jeff H. on Sun Mar 9 00:21:43 2008, in response to Re: (interlocking) TA is obsessed with CBTC, and ''New'' tech for no reason., posted by JournalSquare-K-Car on Sat Mar 8 19:26:07 2008. You do know that in the NYC subway system (with exception ofa few odd places) the signals work on a single-rail track circuit? One rail is continuous and bonded periodically to the substation negative feeder. The other rail is insulated at each track circuit boundary. Connected across the two rails is on one end a transformer secondary with a series fuse and resistor, and on the other end a relay coil with a resistor and fuse in series. As you may know, the DC resistance of a coil is very low. Normally the resistance of the negative return rail is so, so low that almost all of the current flows through it. But if there is some high resistance obstacle, e.g. a broken bond, or rusty rail condition affecting one rail more than the other, the DC return current will attempt to complete the circuit as follows: From wheelset to signal rail, stopped at IJ, through transformer or relay winding to the negative return rail and then back to the substation. The relay operates with on the order of 1 amp AC, the track transformer is generally limited to about 20, and traction return current from one train is tens of thousands of amps, and has an open-circuit potential of 600 volts. Trust me, it blows crap up inside signal cases. |
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