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Isolated and Floating Circuits

  1. May 20, 2007 #1
    Ladies and gents,

    My electrical knowledge is not very complete and I have always had gaps which I'm slowly hoping to fill in! My current question is regarding Floating Circuits.

    As far as I understand, a floating circuit is a circuit, isolated from say the mains because it has no physical point of contact between it and a circuit of higher voltage. I.e. it gets its power via a transformer. (please interupt me where I start talking tosh)

    Now the reason these are considered 'safe' is because in the event of a malfunction you couldn't electrocute yourself because the circuit is running off a lower volatage. Is that right? I've always been told that 'it's the amps that kills you and not the voltage'. But I was also under the impression that stepping down the voltage with a transformer would increase current, so why is it safer?

    Is it the case that a lower voltage on the isolated circuit means that it doesn't have the electrical potential to flow through your body to ground?

    Help would be appreciated!!!

    Many thanks.

    P.S. my background is mathematical and computing though only high-school physics education.
     
  2. jcsd
  3. May 20, 2007 #2
    Got it!
    Assume in case of malfunction a part of the curcuit which is under tension (sorry if this is not the correct term, I hope you understood, I'm italian) comes in contact with the metallic container, let's say the computer's case. You touch it, and:
    1. if it was the normal 220 V without protection, current floows through your body to the ground;
    2. if it was, even the same 220 V, but coming from a transformer, nothing happens to you, until you also touch, simultaneously, the other pole of the circuit.
     
  4. May 20, 2007 #3

    ranger

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    The "shock paths" of what lightarrow is talking about are called pole-to-ground-to-opposite pole and pole-to-opposite pole, respectively. In the latter case, the current is only limited by the impedance that your body imposes on the isolated circuit.
    Here is a further elaboration:
    http://www.ewh.ieee.org/soc/emcs/pstc/TechSpk/floating.html
     
    Last edited: May 20, 2007
  5. May 20, 2007 #4
    Thanks guys, lightarrow, ranger.

    I actually saw the link you posted, ranger. I was just a little confused as to what exactly a pole and an opposite pole are in terms of a(n) (isolate) circuit?

    Thanks again!
     
  6. May 20, 2007 #5

    ranger

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    Hi Nightmage82,

    Pole in this sense can be roughly thought of as the connection made to either end of the transformer. Figure 2b illustrates the concept of pole-to-ground-to-opposite pole (follow the red arrow). The current flows from one pole of transformer, through the man, into the ground, and pack to the opposite pole of he transformer.
     
  7. May 20, 2007 #6
    Ah right I see, (please forgive my ignorance but) in that case, in figure 2b as the current flows through the chap, he will be electrocuted, is that right? Also, would this only happen if he was standing barefoot or say outside on soil? Or is 'ground' just literally anything connected to Earth such as a building?

    Also in figure 2a, is he being electrocuted because he is touching the leads on either side of the load, and if he were to touch the lead at a point where both points of contact were to be of a similar potential, he would be happy and safe?

    If that's the case then I think I've understood it, I'm just a little unsure about figure 2b, as to why the current would flow through the chap and through the ground and to the other pole of the transformer unless the voltage on the isolated circuit was very high?

    Thank you,

    Paul
     
  8. May 20, 2007 #7

    ranger

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    Notice that in figure 2b, the current indicated by the red arrow is returning to the opposite pole of the transformer. However, if you pay attention to figure 5 , with the added insulation (basic insulation), it seems that there is not current flow returning to the opposite pole. If this layer of basic insulation is removed, there is no protection and current passes through the man.

    Well he would be happy and safe if he was not standing on the ground. When he touches one side, is feet and hand are are different potentials and this is cause for electric current to flow. Now if he had no connection with the earth (ground) and he touches two sections at the same potential, then he would be safe. How you think birds are able to sit of high voltage wires!

    In the context of the digram, there isnt any "basic insulation" (in 2b) as there is in figure five. And the paper also opens with the "pole-to-pole voltage of the floating circuit is hazardous voltage."
    So I've probably mislead you a little. Your initial question was a concerning isolated circuits at low voltages. But the same principle applies. Your body has resistance. If you where to touch a live wire, and you where standing on a surface of lower potential, and the potential difference was great enough, you'd get a shock. Hence the signs - "DANGER -- HIGH VOLTAGE!". The more the potential difference the more electric current will flow.
     
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