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How do I ground a chassis with only a 2-conductor power cable?

Discussion in 'General Electronics' started by Circuit Breaker, Jul 23, 2004.

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  1. Hello again all,

    I have a polarized, 2-conductor power cable. If I use this to power a
    device that is in a metal cabinet, how do I safely ground the chassis of
    the device to prevent user shock in the event of a hot short to chassis?

    The use of a 3-conductor cable is impractical for my purposes, and the use
    of only 2-conductor polarized cables is done all the time by manufacturers
    of TVs, VCRs, and stereo equipment.

    I asked a question before and received two answers which, although helpful
    to some extent, did not provide me with what I need to know. So, I
    decided to rephrase the question. I actually am using a plastic case for
    a power supply, and I'd prefer to stick with the 2-conductor cable if at
    all possible. What I'm actually trying to do is shield the power
    transformer so that I can eliminate hum that is being picked up by nearby
    projects inductively. I've wrapped the entire box in aluminum foil and
    crimped it onto a wire in the -VDC lead, which didn't help. I've tried
    attaching just the transformer case to -VDC, which of course didn't help
    either. I need to somehow "ground" the transformer case, but I'm wary of
    tying it directly to the neutral conductor. I know there is a way to do
    this using "AC rated" capacitors, but I know nothing of what caps I need
    or what to do with them. The reason I think this should work is that it
    works for the VCR people -- they use chassis-grounded shielding to
    separate the transformer from the electronics of the receiver/tape reader,
    so this same trick should work for me. But, they somehow connected the
    chassis to the line neutral through some kind of resistor-capacitor setup.
    I *could* use an old computer power cable, but I would rather not destroy
    something just for the exclusive purpose of bringing an earth ground to
    the transformer case.

    Any thoughts? Sorry for asking the question again, but I'm sure SOMEONE
    out there knows what I'm doing and can help.

    Thanks in advance, again...

  2. I would never assume that there are NO wiring errors on hot vs
    neutral. Don't connect your chassis to either of the 2 wires. Use a
    3rd lead or forget it.
  3. Ken Weitzel

    Ken Weitzel Guest

    I'll second that!

    And add that everyone who does know what you're trying to
    do will help. By telling you not to.

  4. Gordon Youd

    Gordon Youd Guest

    Would an isolation transformer help???.

    They are used in TV workshops to work on "live" chassis..............

    Regards, Gordon.
  5. dB

    dB Guest

    The screen needs to be made of a magnetic material, such as iron or
    steel -that's why aluminium makes no difference, it is transparent to
    magnetic fields. The material used to screen the c.r.t. of an
    oscilloscope is the best. They call it, or used to call it, mu-metal.
    But perhaps this was a trade name.

    Why? If it screened, the screen doesn't need to be "grounded".
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