Shock current paths.

Discussion in 'General Electronics Chat' started by Orwell, Aug 31, 2014.

  1. Orwell

    Thread Starter New Member

    Aug 31, 2014
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    Hi everyone. I'm a 3rd year electrician but please don't be "shocked" by some of my questions. I'm finding that a lot of journeymen don't know the answers to theory type questions, and I'm the sort of thinker that needs to understand WHY something works before I can install it properly... I marvel at how guys can install large equipment without a clue as to how it works. Line in, load out I suppose. Anyway...

    I'm reading some of the shock current path literature on the site here and have a few questions.

    First, I work with a welder at an industrial site, installing/welding supports for cable tray. His welder has me perplexed. He can select the polarity of his welder, which he has set to positive, so the positive lead the the "hot lead."

    The welder itself has a ground terminal which is not used. He has a positive lead (attached to his stick) and a negative lead (the working lead or clamp, which some welders erroneously call the "ground clamp") To weld, the working clamp has to be touching the work piece, and the positive electrode (stick) is sending DC current through the work piece and back to the welder on the negative work clamp, correct?

    So I'm thinking of the different shock current paths.
    • A ground fault anywhere in his welding system will lead to a shock for a person touching the grounded section of the circuit **while he's welding**, because he isn't using the ground terminal on his welder.
    • While he is not welding (incomplete circuit), a ground fault on the positive side of his system will lead to a shock (but this would be the case whether the negative side of his system was grounded or not, right?).
    • A person can be shocked if they themselves are the only path for the current to flow through - say, by grabbing the positive and negative leads at the same time.
    I'm trying to think - why doesn't he use his ground? I asked him and he honestly doesn't know why he doesn't, so it must just be an industry thing where welders don't use the ground?

    One more question:
    Does the theory behind DC shock current paths apply perfectly to AC systems? Or can AC systems shock in different scenarios than DC and vice versa?
     
    Last edited: Aug 31, 2014
  2. #12

    Expert

    Nov 30, 2010
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    A welding machine has a big transformer in it. That isolates its output from the typical planetary ground (bond). The current path for welding is a fairly small circle compared to the wiring in the building. If the secondary of the welding machine is not bonded, the rest of the planet and all the people standing on it are not concerned with the voltage or current used in the welding circuit. Touching any one welding conductor will not form a path to ground because no other part of the secondary is bonded to the planet. When you start bringing in the idea of ground faults you have to be specific about where the accidental connection is.

    The thing being welded is often in contact with the earth. That forms one connection to ground. You could get shocked by standing on the ground and touching the "hot" lead of the welder, and it doesn't matter whether "hot" is positive or negative.

    I guess that's a partial answer.
     
  3. Orwell

    Thread Starter New Member

    Aug 31, 2014
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    "When you start bringing in the idea of ground faults you have to be specific about where the accidental connection is."
    I tried to be specific....

    Let me clarify something I didn't mention. His welder runs on diesel and sits in the pan of a pickup truck.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    The ground connection is to protect against a fault from the input power to the welder chassis. That ground connection is likely not connected to either of the welding electrodes so would have no effect on those.
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    There is no way of knowing what voltage the positive and negative terminals of the rectified secondary of the welder's transformer is vs. Earth ground.

    The ground strap on the transformer could be connected to almost anything - I am guessing it is connected to the frame of the transformer (connected to the chassis of the welder). The thing is, once the power is transformed to the secondary, there is no physical connection between secondary and primary so there is no reason the ground strap needs to be connected to the piece getting welded.

    On the other hand, the DC welder may be grounded by the fact that the negative cable is connected to something grounded (frame of industrial equipment mounted to the building -or- it could be connected to something that is floating and not grounded (auto frame with vehicle on rubber tires). Then, the part could be 10s, hundreds, thousands or 10s of thousands of volts above or below ground. The little static spark will bring the two in line as the welder is connecting the electrode to the part getting welded.

    If there is exceptional capacitance on the part, you could end up with significant static discharge as the electrode approaches the part...


    Look at the 1:00 time in video.
     
  6. Orwell

    Thread Starter New Member

    Aug 31, 2014
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    Okay, I think I'm getting it.

    Can you explain a floating ground to me?
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    All voltage is measured relative to some other part of the circuit. When you have two circuits that are not attached by a conductor, there is no reason that the negative battery terminals are at the same voltage. Even the secondary of a transformer is not connected to the primary so that can be floating above or below (or even oscillating) compared to the primary.

    Connecting some parts of the two circuits together (usually zero volt terminals a.k.a. Common), will now allow you to measure between the two circuits. Although, as in the video, you may get a spark as the two circuits are initially connected if several thousand volts delta to begin with.

    This effect is why you (must/should) wear a grounding strap when working with CMOS components.
     
  8. Orwell

    Thread Starter New Member

    Aug 31, 2014
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    Quickly, I want you to know your effort in teaching me isn't being wasted. I VERY much appreciate your help. I haven't even read your last post yet, but I know you spent some time at it, as it appears decently long. My girlfriend is staring at me angrily, she's hungry so I'm on the way out to a restaurant. But I can't wait to get back and read the comment you left.

    After this thread, I'm going to dive into the textbooks before asking more questions, as I need to do my homework before expecting you guys to answer all my stupid questions. Again, I'm appalled at how many Red Seal journeymen electricians cannot answer simple theory elated questions. This is why I think I'm stuck on the simple stuff as a 3rd year.
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I've been sitting in front of the TV today attending to a sick relative. Bored out of my mind. No problems. Enjoy your dinner
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    First off, in N.A. the term Ground can either mean Earth Ground, or chassis or work ground, as opposed to the more definitive label Earth as used in U.K. and Europe etc, with ground meaning simply a circuit common.
    Floating ground to me means that the Common terminal is not at Earth Ground potential.
    In a portable welding system the work ground clamp, maybe or not be connected to earth ground as in working on a fixture or equipment etc that is isolated from earth ground.
    Max.
     
  11. MaxHeadRoom

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    Jul 18, 2013
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