Correction to Volume I - DC » ELECTRICAL SAFETY

Discussion in 'Feedback and Suggestions' started by Unregistered, Apr 9, 2008.

  1. Unregistered

    Thread Starter Guest

    I believe there is an error on this page.
    Scrolling down there is a picture of a man grabbing a pipe with the legend that the resistance between his body and the pipe is 2K ohms. Then it shows him grabbing the same pipe with both hands, and the using this as an example of a parallel circuit with a total circuit resistance of 1K ohms.
    Resistance is not resistance until current is flowing, and since both hands are on the same conductor, unless there is a ground that is not shown, we have a bird on a high voltage wire condition. No current will flow, no effective resistance.
    If there is a separate ground, then the calculation needs to take into account the resistance of that ground. If for example the man is barefoot, and his feet also have a 2K resistance the total circuit resistance would be 3K ohm as it is a series parallel circuit.
    If it were me, I would redraw the picture with the second hand grabbing a second 1.5" pipe so that there is a complete circuit.
    Since this is a chapter about electrical safety, it should be correct.
    Hope this helps.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    I would beg to differ, and assert that the illustration and the facts as presented are accurate. A resistance is a resistance, even with no current through it. Nothing magical goes on inside a resistor that alters its characteristics from some undefined state to one that matches the stated resistance painted on the encapsulation just as a voltage is placed across it.

    The example is to illustrate that a person in contact with a conductor by a hand grip will make a path for conduction approximately equal to a resistance of 2000 ohms. Therefore, should the pipe or conductor be electrically charged and the person be standing on a conductive surface, the voltage will cause a current through his body. And, should that person make contact with his other hand, the conduction path will have about one half the resistance of the first case.

    Nowhere does it state that a circuit is formed by the contact. It is simply to raise awareness of the potential safety hazard raised by grasping or touching a conductor.

    Never touch or grasp a conductor unless you have verified that no dangerous voltage is present in it.
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    Certainly voltage drop is not present until current flows, and the voltage drop will be directly proportional to said current. I've never before heard that resistance is not present until current flows. Perhaps there has been some confusion here.

    Resistance is an intrinsic property, as Beenthere has noted. It does not change with current flow, whether said flow increases without bound or approaches zero. (Well... resistance might increase when the thing vaporises as current increases without bound, but I think folk get my drift.)