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I recently encountered an electric fence in the countryside. The fence was made from wooden poles in ground which were connected by 3 wires from top to bottom. At one of the poles, however, the middle wire just ended right at that pole (I assume that the wire had broken there and thus was severed while the two others continued on to the rest of the fence). See diagram below: enter image description here

I wondered what would happen to the current in this middle wire as it couldn't complete the circuit (all of the wires were disconnected from eachother), so I picked a long grass straw and put the tip of it on top of the wire while holding at the other end of the straw. At first, I couldn't feel anything, but as I moved the straw closer to the wire, I began to feel an electric pulse. My conclusion was that the wood must have been acting as a sort of earthing agent, but as wood isn't conductive, I didn't think that would be possible. Could someone explain this to me?

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    $\begingroup$ Lets think about how this fence could work. How does the animal get the shock in the first place? i.e. what circuit is the animal completing? $\endgroup$
    – shai horowitz
    Commented Jun 28, 2022 at 21:15
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    $\begingroup$ Hint: If the fence is in good working order, then there will be no current unless and until an animal (or a human with a blade of grass) touches the fence wire. $\endgroup$
    – Solomon Slow
    Commented Jun 28, 2022 at 23:33
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    $\begingroup$ @SolomonSlow Yeah that's what I thought as well, and why I was surprised to find electrical current in the wire. $\endgroup$
    – untitled
    Commented Jun 29, 2022 at 4:22
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    $\begingroup$ But what I'm saying is, there was no current until you touched it. There was only a high voltage with respect to ground. (Hint2: You were standing on the ground.) $\endgroup$
    – Solomon Slow
    Commented Jun 29, 2022 at 13:18

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An electric fence works by having an open circuit in normal conditions, and then only having a closed circuit when an animal touches the fence. So the end of the wire is not particularly problematic since the circuit is designed to be open in the normal state.

The way these are wired is that all of the wires are connected to the positive terminal of the fence energizer, so they are at the same voltage. The negative terminal of the fence energizer is then connected to one or more grounding rods sunk well into the earth. This configuration does not make a closed circuit so the current does not flow even when the energizer puts out the high voltage. Electric fence diagram

To make current flow, the animal must make the connection between the fence wire and the ground. Once that happens then the animal is part of the circuit connecting the positive terminal to the negative terminal. When you touched the wire with your straw you were presumably also touching the ground, and therefore you completed the circuit and were able to feel the shocks.

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  • $\begingroup$ And perhaps the straw was humid (not too dry) and therefore conductive? $\endgroup$
    – Roger V.
    Commented Jun 29, 2022 at 15:12
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    $\begingroup$ Yes, and the voltage is several kilovolts specifically to provide provide enough current through relatively poor conductors like hair or feathers. So straw would generally be sufficiently conductive to notice even if it does reduce the current. $\endgroup$
    – Dale
    Commented Jun 29, 2022 at 15:17

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