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Best way to disconnect?

Discussion in 'General Electronics Discussion' started by loudandgreen, May 24, 2020.

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  1. loudandgreen

    loudandgreen

    9
    9
    Dec 22, 2015
    A buddy and I were having a "discussion" about the best way to disconnect wiring whether it be a/c or d/c. He's worked power poles and most things a/c and I myself have worked around most things 12volt d/c. I've always removed the negative first and he always removes the positive (hot) side first. Is this a apples to oranges comparison and if so why?
     
  2. Minder

    Minder

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    Apr 24, 2015
    Depends on the application and the source.
    For example, when disconnecting an automotive battery, always disconnect the negative first, this is in the event that the wrench comes in contact with the frame, with the neg. it won't matter, but if disconnecting the Pos first, and the same scenario, will blow a hole in the wrench!
    Otherwise, for anything recognized as high voltage rated, I disconnect the 'live' first.
    M.
     
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  3. loudandgreen

    loudandgreen

    9
    9
    Dec 22, 2015
    Ah! I learned something! I've had accidents with tools and batteries when I was a kid like a little sparkage but nothing major even though I've been told by the older mechanics that it CAN cause a battery to explode but I've never had it happen to me.... I've seen it happen to someone else though...
     
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  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    In case you need to jump start a car this leads to the following sequence:
    • connect + first then -
    • jump start
    • disconnect - first, then +
    to avoid the issue described by @Minder .

    With AC there is no positive side. Common terms are hot and neutral with neutral being usually at earth potential (usually means not necessarily, never trust in what you think may be correct, check it out first).
    Which leaves the rest of the circuit at a safe potential (if neutral is connected correctly).
     
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  5. Minder

    Minder

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    Apr 24, 2015
    Replacing the battery and jump starting are two different issues?
    M.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    2,502
    Nov 17, 2011
    Who wants to replace a battery?
    By connect I mean use the jumper cables to connect the helping car's battery to the dead battery.
    The procedure to observe is the same in both cases, isn't it?

    Of course, once you needed to jump start a car you need to check the battery and the generator for correct operation and maybe you'll have to replace one of them.
     
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