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OT: errant DC current

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  • OT: errant DC current

    There was a write up about adding light rail to the new Montreal Champlain bridge. Acc to the article, very sophisticated engineering/insulation/? was required to prevent stray current leaking & damaging the bridge structure. System runs at 1500 volts dc( unsure if 3rd rail or catenary).
    Wondering the "how", "why" etc.

  • #2
    I work in pipeline corrosion and there are what's called cathodic protection systems which run DC current through the metal parts of the structure to make their potential negative to a sacrificial "anode" which is buried in the ground nearby. The DC current makes the corrosion occur at the anode instead of the structure components. This can be done on rebar inside concrete as long as it is electrically bonded together.

    If the current from the train system leaks to the structure, it can overpower the CP system and drive the current in reverse, actually accelerating the destruction of the bridge. This is a common issue with multiple pipelines in the same right of way or power line corridor, pipelines will fight each other as each operator keeps turning their CP system up higher and higher to counteract the effects of the other systems. Operators have to work together to minimize and mitigate the impacts they have on each other's systems.

    Also, its a waste of electricity if it's leaking to earth via the bridge structure, so it's a good idea regardless if implementing isn't too costly.

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    • #3
      Believe it or not, a lot of natural gas underground lines have a current passed thru them for corrosion protection also.

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      • #4
        I spent 3 years as a corrosion protection test station monitor, small world.
        mark costello-Low speed steel

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        • #5
          https://nationalpost.com/news/report...amplain-bridge
          Location: Helsinki, Finland, Europe

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          • #6
            "The eight-lane bridge opened in July of this year, replacing the older Champlain Bridge that was compromised by the overuse of road salt and had to be replaced before it failed."

            Umm, what they call a problem I see as an opportunity. Chase the enginincompoops and college professors away, install electrodes and employ the DC to prevent concrete spalling from salt water rusting the rebar system.

            Been done in multiple parking structures around the National Swamp for 20 years.

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            • #7
              Montreal, you say. Big time snow country, and with snow comes salt. And even if they do not use salt on the bridge and it's approaches, it will be transported there by every vehicle that crosses the bridge. I don't know if that rail line uses an overhead, catenary or a third rail: the third rail would probably be more vulnerable. And there are probably additional, feeder conductors that are used with their own insulators. But in either case, that salt can get on and coat any insulators that are used, so the current can "jump" the insulators and pass through the bridge structure. As an example, the power company in Florida routinely washes down the insulators on the long distance, high Voltage power lines in the MIDDLE of the state because the salt spray from the ocean and the gulf can carry that far (dozens of miles). That salt spray coats the insulators and the rain can convert that salt coating into salt water and an arc ensues. That is probably how the problem happens.

              As to what is done to prevent it I do not know. Any number of measures could be taken from physical barriers to cleaning systems to monitors to detect problems or many more. Also electrical bonding of the bridge members with conductive straps could be used but they would also be difficult to maintain. They probably employ a combination of two or more measures.



              Originally posted by Dunc View Post
              There was a write up about adding light rail to the new Montreal Champlain bridge. Acc to the article, very sophisticated engineering/insulation/? was required to prevent stray current leaking & damaging the bridge structure. System runs at 1500 volts dc( unsure if 3rd rail or catenary).
              Wondering the "how", "why" etc.
              Paul A.
              SE Texas

              Make it fit.
              You can't win and there IS a penalty for trying!

              Comment

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