Safety Illustration on grounding- Chapter 3

Discussion in 'Feedback and Suggestions' started by tone_b, Nov 29, 2009.

  1. tone_b

    Thread Starter New Member

    Nov 8, 2009
    Earth grounding the equipment case is an important safety feature in household appliances.
    Last edited: Feb 7, 2010
  2. Dave

    Retired Moderator

    Nov 17, 2003
    I'm a little confused. Are you suggesting that it isn't clear when there is a "shock" from the sequence of pictures hence current path through (or not) the character is necessary?

  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    Think about this carefully. Why would this happen if both hands are placed on an object, which can only be at one potential?
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    When commenting on the E-Book it is always useful to make a link to the part concerned. I presume you are referring to this sequence in the DC section.

    It is unfortunate that this safety section has come before AC electricity is introduced in the E-Book. As a consequence the AC mains is shown as originating from a battery.

    Now there are major differences between DC and AC electricity. One fundamental one is that there is no (permanent) positive and negative in AC, like the terminals of a battery.

    However both have the characteristic that current flows from high voltage to low voltage. No curent flows between two points at the same voltage.

    Now look at the attached picture again.

    No current will flow between however many points of contact you make with the case if it becomes 'hot'. So if you grab a hot case with both hands a current path will not exist in one hand , through the body, and out of the other.
    So you can touch a hot case and live to tell the tale, if you are not touching anything else. Circus performers did this at one time, but don't try it yourself.

    Normally people are touching something else ( which is not hot) so a current path exists from the point or points of contact through the body and out to the something else.

    So in the diagram the case is shown grounded so that it can never become hot to prevent this happening. In the accidental event of a hot connection wire coming loose and toucvhing the case a large current will instantaneously flow through the case to earth, rupturing a fuse and disconnecting the electricity supply.