[newbie] Hi all and a question about the e-book

Discussion in 'General Electronics Chat' started by Fedaykin, Aug 1, 2012.

  1. Fedaykin

    Thread Starter New Member

    Aug 1, 2012
    Hi all
    I'm new to this forum and pretty new to electronics (i'm not so bad at IT but i can't read schematics and stuff). I choose to register 'cause i'm studying the first e-book (DC) and i like it very much, also it seems the forum's full of expert and helpful people. :)
    Now the noobish question:
    The little man at page 100 (http://www.allaboutcircuits.com/vol_1/chpt_3/7.html) it's shocked because his body has more resistance than the ground? And why 250v?
    Thanks everyone

    Last edited: Aug 1, 2012
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    Because the neutral is earthed at the substation there is effectively 2390V between the downed cable and the substation. Taking the drawing to scale call it 10m. The little chaps feet are 1m apart so he effectively has 2390*(1/10)=239V across them?
    Fedaykin likes this.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    The example is just showing voltage can be distributed along the earth itself. It's actually a not-unknown problem in farming areas, I have heard tales of dairy farmers getting shocks inside their barns from stray voltages.

    Why 250 volts? It's just an example, but say the 2390 volts is 20 feet apart, and the man has 2 feet between his legs. So the field is 2390 volts/20 feet for 119.5 volts/foot. For a 2 foot spread the voltage is 119.5 volts/foot x 2 foot = 239 volts... close enough to the 250 volts in the example to see the point.

    Also, don't take the resistance of the guy compared to the earth literally. This is just meant to show there can be a voltage where you may not expect one.

    And welcome to the forums!
    Fedaykin likes this.
  4. Fedaykin

    Thread Starter New Member

    Aug 1, 2012
    Ok, i think i got it, thanks.
    Probably i need to better understand the "electrically common" concept.