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Metal Case Safety

Discussion in 'Electronic Basics' started by Fred, Nov 22, 2004.

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  1. Fred

    Fred Guest

    Hey Everyone

    If a transformer is used in a design is it safer to enclose it in a metal
    box or a plastic box? Most lab equipment (power supplies, scopes etc) use a
    metal case. Wouldn't plastic be safer? Is metal only used for strength?
  2. Depends. If the metal case is properly grounded, it is at least as safe.
    If it is not grounded, plastic is safer.
    Because if it is grounded, and a failure happends, current will pass to
    gnd, and the ground relay will sense that, turning off power. So, if you
    intend to use it in an *dry* envirorment without earthed wall outlets,
    pick plastic. Otherwise, I'd stick to metal.
    Also reffer to faradays cage...It won't hurt you to touch the metal case
    when it is powered, as long as no current is passing trough you, but
    rather trough gnd connection which will likely have less resistance than
    humans. In norway, the voltage upon touch to an earthed part *shall*
    never be above 50V, so then you must adjust ground resistance according
    to the maximum current which can pass in a single-phase short (short to
    earth). This was slightly offtopic, but hope it explains a bit:)
  3. Fred

    Fred Guest

    That helps, thanks.

    The secondary output of the transformer is rectified to produce DC. Should
    the ground of the DC be connected to the case, which is also connected to
    the ground (earth) in the plug?
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