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measuring resistance in a live circuit

Discussion in 'Electronic Design' started by panfilero, May 1, 2013.

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  1. panfilero

    panfilero Guest

    how do you feel about measuring resistance or continuity in circuits that are hot, i see people doing it all the time, and it seems to work, but something about it doesn't seem right... is it an ok thing to do? or is it ok only in so and so situation, or you should never do that?

    thoughts?
     
  2. seems like a bad idea, unless you have a good meter that doesn't fail if
    you abuse it. Even then, the readings you can get could be questionable.

    I should probably test what it takes to trick a fluke meter into beeping
    on the continuity test, it's probably not very much.
     
  3. mike

    mike Guest

     
  4. mike

    mike Guest

    Here's a thought experiment for you.
    What's the resistance of a battery?
    Put your Fluke on it.
    How many ohms does it measure?
    Does that agree with your expectation?
    What does that say about the wisdom of trusting ohms
    measurements on a live system with a Fluke?
     

  5. If you need to ask, don't do it.
     
  6. panfilero

    panfilero Guest

    yeah i don't think the people doing it are asking, but good advice
     
  7. panfilero

    panfilero Guest

    you know, it's really more the continuity test I see people do with live circuits, not the resistance measurement... and it's the continuity test that seems to work most of the time.... scratch resistance out of it, just continuity
     
  8. It's even more spectacular to measure the resistance of the mains with
    an AVO. (I'm told the flames are purple.)
     

  9. I don't want to encourage dangerous behavior (so, don't do this!), but
    consider a live* circuit with a series switch of some kind. When the
    switch is open, you can measure the load R, perhaps even measure it
    accurately depending on common-mode voltage and rejection. Close it,
    deliberately or accidentally, and you may well experience a "Joerg
    moment" or much worse. Google 'arc flash' and 'arc blast' for
    examples.

    * I'm using 'live' in the electrician sense- power has not been
    switched off (and it should also be locked out) at the breaker panel.
     
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