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How does voltage in wires affect conduit and nearby materials?

  1. Jul 19, 2012 #1
    Two questions here. From real-world problems, I was hoping someone smarter than me could explain the physics behind some issues I was told about. It's been a long time since I looked at an engineering or physics book, so please bear with me.

    1) For a 3-phase motor, I was told that someone had the idea to run each conductor in its own conduit. He thought it was a good idea until the wires started to heat up and melt. Let's assume that the wires were sized for the correct full load amps of the motor. I was told that when the 3 conductors are run in the same conduit, that voltage/current offset each other. When they were run independently, it must've induced some sort of a charge in the conduit causing it to heat up? Is this possible, or is this story made up?

    2) If I have a power supply, very high DC voltage (~60kV) with very low current (15mA), do you have to keep the wiring away from all metal? This is more of an engineering installation question, so maybe this isn't appropriate here. I was of the belief that as long as things were non-ferrous, everything would be fine, perhaps even putting the cables in aluminum conduit. I was told this was bad practice. For such an installation, no metal (even non-ferrous) should be within a foot of the cable, due to the possibility of the cable inducing a charge on nearby ungrounded metal. If someone were to then touch that metal, they're liable to get a shock. Is this backed by science?

    Thanks for any explanations. It's been a long time since I even had to think about Maxwell or Faraday.
     
  2. jcsd
  3. Jul 19, 2012 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    1) When the wires are run together in the same conduit, there is no net B-field (the currents cancel). When run alone, it may be that the eddy currents induced in the conduit heat it up too much.

    2) The spacing issue is to avoid arc-over from the HV conductors to other conductors.
     
  4. Jul 20, 2012 #3
    Steel conduit experiencing inductive heating is certainly not made up. See the attached section of the National Electric Code.
     

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  5. Jul 20, 2012 #4

    jim hardy

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    he's not the first, by a long shot.

    http://media.nowpublic.net/images//97/7/977a0e06c35f94412e56c9d32e1704f5.jpg [Broken]

    Those red magnetic lines heat iron . But it takes substantial current, like to a motor or something, to make it dangerously hot.

    if the iron encircles the return conductor too then the two magnetic fields will cancel.
    That's why that paragraph in electrical code says run them all together.

    If you have a clamp-around ammeter , clamp it around both wires of a lampcord and observe zero reading. Then split the cord and clamp around either conductor observe you read lamp current.

    Then repair or replace the lampcord.

    old jim
     
    Last edited by a moderator: May 6, 2017
  6. Jul 20, 2012 #5
    That's actually a very simple, practical test. I wish I had thought to try that earlier.

    As for my second question, would arc-over be an issue since my plan would have been to run the HV cable in aluminum conduit? The cable itself would also be insulated, of course. The shock hazard posed extended to metal fasteners as well, which might be in the vicinity of where the conduit is routed.

    I have no problem using all non-metallic conduit, fasteners, etc, I'd just like to convince myself of why it'd be needed.
     
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