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Earth for Oscilloscope

Discussion in 'Electronic Design' started by Myauk, Mar 8, 2007.

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  1. Myauk

    Myauk Guest

    For connecting the power chord of an Oscilloscope to the AC manins
    (220/110 Vrms), what if the socket without earth (phase and neutral
    only) is used?
    Will it effect the signal integrity?
    I understand the general measurements like measuring the presence of
    CBVS signal while generating a standard test pattern like full colour
    bar won't be much problematic.
    What kind of 'sensitive' measurements require the power chord of the
    AC mains to have appropriate Earthing???
  2. D from BC

    D from BC Guest

    I'm a little rusty in this area but I'll try to answer..
    You'll have to check...
    Earth ground exists to prevent shock.
    Ideally, no current should be flowing through to earth ground unless
    there's a fault.
    All metal boxes are at the same potential.
    For some equipment, removing the ground connection can allow the
    chassis to be hot either by an internal fault or by an external
    source.. Could be 100's of volts. Touch this and let's say a earthed
    pipe and zap!
    I know that somewhere down the electrical line the neutral and earth
    ground get connected.
    About the measurement performance without an earthed scope....
    I'd guess none.but don't know for sure..
    D from BC
  3. It all depends, I had the scope earth removed for a long time.
    The company moved to a different building one day, and I got
    my scope back in a box with its leads I thought.
    Installed it, connected lead.
    Big boss came in, asked me to measure something for him that had mains power.
    I connected scope earth to it and all computers on that floor crashed -no power-
    big bang and spark too.
    Somebody swapped the mains leads.

    Twenty ears before that I did the same thing on something like a 400V 100A
    thyristor controlled rectifier, came back from holiday, connected scope to
    thyristor gate, switched on power, BANG.
    When I was on holiday somebody re-connected scope ground.

    So, the official lesson is: Use scope ground, do a differential measurement.
    (ch1 ch2 substract).
    It also protects you and others from getting electrocuted.

    The not so official lesson is that I do not always follow the official concensus.
  4. Twenty ears before that I did the same thing on something like a 400V 100A
    A lesson learned as a student lab assistant at University: Connected probe
    earth lead to mains hot lead in a circuit (was trying to measure mains
    voltage, or such) and discovered the meaning of "common earth" of scope
    chassis (input to output). Bang. Smoke. Suddenly an audience. Grins,
    resetting of breakers. All was well, thankfully.

    An important lesson, not too expensive, except maybe for my pride...
  5. Rich Grise

    Rich Grise Guest

    Is the clip still usable? Usually when I do that, it practically destroys
    the clip.

  6. Is the clip still usable? Usually when I do that, it practically destroys
    If I recall, black, spot weld on the teeth where the clip touched, but still
    usable. But it was a long time ago. Scope kept working.

    Tek 365, the Volkswagen of scopes. Not pretty, but did everything you needed
    it to do and well. Many are lasting well into the 21st century. Probably
    should make a monument to this scope in the Electronics Hall of Fame.
  7. In that case of the thyristor fluid metal was spraying around...
    I was not hit.
    For measuring high voltages a few millivolt can safely be ignored, and I
    simply use a separate testlead for gound, not the probe crocodile, in
    fact I remove that, as it may hang against something.
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