Current return (not MBTA specific)

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Current return (not MBTA specific)

Postby Yellowspoon » Sat Sep 16, 2017 11:40 am

Whether third rail, trolley wire, or catenary, the MBTA'S current return is through the running rails. To the best of my knowledge, it's that way throughout North America. So why does London do it differently? They have a fourth rail for current return. Does anyone else in the world use this system? Do they use this fourth rail for signals and train control as well, or do they use the running rails for that?
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Re: Current return (not MBTA specific)

Postby ExCon90 » Sat Sep 16, 2017 2:18 pm

I can't remember the source, but I believe the central return rail was adopted to avoid the need for impedance bonds (common on other systems) to allow the negative return to continue through a succession of signal blocks while containing the signal circuit within the block; thus the signal circuit and the current-return circuit are isolated from each other. From what I've observed, in territory where both commuter rail and the Underground share the same track, a central return rail is installed, since that's what the Underground trains are designed for, while impedance bonds are in place to permit "railroad" trains to use one of the running rails for current return, which is what they are equipped for. I have to assume that on most systems it is felt that impedance bonds are cheaper than a fourth rail. Another factor may be that if the Metropolitan and the Metropolitan District Railways, both originally steam-powered, were still using Manual Block when they were electrified, there would have been no signal circuits to consider and when Automatic Block was installed later, putting in a fourth rail was easier than re-doing the electrification--but I don't know. FWIW.
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Re: Current return (not MBTA specific)

Postby CRail » Sat Sep 16, 2017 3:55 pm

There was an American city, I don't recall which, that had streetcars with two poles similar to Trackless Trolleys. Stray voltage returning to ground rather than the source was found to be corrosive to nearby infrastructure so the return was redirected to a second wire rather than through the rails.
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Re: Current return (not MBTA specific)

Postby dieciduej » Sat Sep 16, 2017 4:29 pm

The London Tubes was originally a metal tube, so using the standard running rail would cause electrolytic corrosion when the traction current returned.

As for the two pole trolley that was Cincinnati to prevent issues with the stray currents affecting telephone services.

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Re: Current return (not MBTA specific)

Postby diburning » Sat Sep 16, 2017 9:07 pm

dieciduej wrote:The London Tubes was originally a metal tube, so using the standard running rail would cause electrolytic corrosion when the traction current returned.


In addition to that, some lines, such as the District Line were originally built and operated as a full sized railroad. Because the tubes are iron, it was easier to install an insulated non-weight bearing return rail than it was to attempt to insulate weight-bearing travel rails which would need frequent replacement due to the fact that they were weight bearing, and would wear out the insulation.

Is Cincinnati's current streetcar operation (using the CAF Urbos 3 LRVs) brand new? Or did it replace an older route/system that had the dual wires?
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Re: Current return (not MBTA specific)

Postby dieciduej » Sun Sep 17, 2017 5:28 am

diburning wrote:Is Cincinnati's current streetcar operation (using the CAF Urbos 3 LRVs) brand new? Or did it replace an older route/system that had the dual wires?


The dual wire system was part of the original Cincinnati Street Railway (1859) 1888 to 1951, I believe 1888 was the start of electrification, 1859 for horse car ops. Since they reused cable car and horse car tracks which did not need any special bonding or grounding, using the running rails as a current return would have incurred high electrical losses and zaps! To add the proper bonding/ground would cost $$$$, where as running a second overhead wire would cost $$. As a sidebar they were 5' 2 1⁄2".

Since the today's Cincinnati Bell Connector is a from scratch system is uses the running rail as the return. I hope that clarifies things.

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Re: Current return (not MBTA specific)

Postby CRail » Mon Sep 18, 2017 2:44 am

Thanks Joe D for clarifying! Further thought on the subject jogged my memory a bit and I got stories crossed. The Edmonton trackless trolley system used a "floating ground." Rather than grounding the return it was as isolated as the positive wire. That was because it was found that stray voltage was corrosive to nearby infrastructure.
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Re: Current return (not MBTA specific)

Postby jonnhrr » Tue Sep 19, 2017 1:41 pm

A discussion of why London uses 4 rails may be found here: http://www.trainweb.org/tubeprune/tractioncurr.htm
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Re: Current return (not MBTA specific)

Postby ExCon90 » Tue Sep 19, 2017 2:34 pm

Thanks for posting that. I looked around but didn't think of trainweb.
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