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This question is mostly extending on a question that has been asked here:
Single terminal of voltage source attached to earth ground

I'm learning the basics of electronics and this question has always bothered me. The post above explains that a single terminal of a voltage source, when connected to ground (earth), will have no current flowing, as there is no circuit. It then goes on to talk about how capacitance might play a small factor.

My main question is this: Why then would you get electrocuted if you touched a live wire while grounded, since it is not a proper circuit? Is it different with AC and DC?

Also, when the single terminal of a flyback transformer (usually the anode cap on a CRT monitor) is placed near the ground, it creates a bright arc to the ground. How does that happen if it's not a complete circuit? In high voltage projects, most people connect the live wire from the flyback transformer straight to ground to complete the circuit.

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  • \$\begingroup\$ Ground is a physical connection to earth for AC and a common node for DC circuits. \$\endgroup\$ – Daniel Tork Dec 23 '16 at 16:12
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Why then would you get electrocuted if you touched a live wire while grounded, since it is not a proper circuit?

Because it IS a proper circuit.

If the return wire of the high voltage source is grounded then, touching the live wire will close a circuit where your body (also grounded as per the quote above) is the "load" and a current will flow.

If the high voltage source were floating i.e. it didn't make a connection to ground then you might feel a little tingle when first touching the live wire but this is due to discharging the small amount of capacitance that the floating voltage source has (with respect to ground).

Also, when the single terminal of a flyback transformer (usually the anode cap on a CRT monitor) is placed near the ground, it creates a bright arc to the ground. How does that happen if it's not a complete circuit?

Usually, the high voltage winding of a CRT flyback transformer is grounded (or partially grounded via a resistor) and therefore the arc is completing a circuit.

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  • \$\begingroup\$ When you're not grounded and touch the wire,you still get shocked,because you're forming a capacitor with the ground and capacitors act as short circuits for AC.It depends how high above the ground you are. \$\endgroup\$ – Daniel Tork Dec 23 '16 at 16:18
  • \$\begingroup\$ @DanielTork yes I agree (but that's not what was asked as far as I can tell). \$\endgroup\$ – Andy aka Dec 23 '16 at 16:20
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    \$\begingroup\$ This might be off topic, but if the power source wasn't grounded, wouldn't that mean that there is 0% chance of getting electrocuted by just touching the live wire? What is the purpose of grounding power sources then? Also, how significant would the current flow be, if someone touched a live wire while grounded? \$\endgroup\$ – WeavingBird1917 Dec 23 '16 at 17:05
  • \$\begingroup\$ If the power source is AC then you can still pass current due to capacitance but it won't be as noticable unless the AC frequency is high. If it is high then current can be proportionately bigger but will only flow on surface of the skin and give burns rather than electrocution. \$\endgroup\$ – Andy aka Dec 23 '16 at 17:19
  • \$\begingroup\$ The purpose of grounding a supply ironically is for safety - if you touch live wire you get a shock but not a full-blown one and, in modern day circuits, you have residual current devices that will trip on a few tens of mA. They won't work if the supply is floating and you get very little indication that danger is approaching if touching one wire of a floating supply. Next thing you do is touch both wires (thinking the circuit is dead) and then you might also become dead! \$\endgroup\$ – Andy aka Dec 23 '16 at 17:22
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AC generators are connected to ground by convention at the source end. This completes the circuit back to you. The fact that the earth connection has been described as not a proper circuit means that it is impossible to determine its exact impedance so it is difficult to use in conventional circuits as a connection. This does not mean that it does not have a low enough impedance to complete a Line-Earth circuit and kill you.

The anode cap on a crt is effectively one end of a capacitor and you find that the other end is connected to ground somehow.

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A closed circuit is ALWAYS required for current to flow. You can assume that is ALWAYS the case. There are no exceptions. Full stop.

So your hypothetical cases where you don't think there is a circuit are simply incorrect. If you think there isn't a closed circuit there, then your perception is incorrect and you should keep looking until you see the circuit which is always there.

Connection to earth could be a red-herring. Unless the earth itself if part of the closed circuit. And that is very possible especially in cases where you can get shocked or electrocuted from touching only ONE energized conductor. Remember that the circuit is completed through your body and down into the earth. And the source of the electricity is also connected to earth, perhaps a long distance away.

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  • \$\begingroup\$ Could you explain or predict the amount of current that is flowing when touching earth? Would there be a substantial amount of current flowing if someone (grounded) touched a 240V live wire, and assuming that the voltage source was groundwd 50 feet away? \$\endgroup\$ – WeavingBird1917 Dec 23 '16 at 16:53
  • \$\begingroup\$ I'm wondering how much current was actually flowing, since an arc can be seen connecting to ground (voltage was around 25KV). \$\endgroup\$ – WeavingBird1917 Dec 23 '16 at 16:55
  • \$\begingroup\$ The answer to BOTH of your questions depends on the RESISTANCE of the circuit. If you are standing bare-foot in salt-water, you have a really low-impedance (excellent) connection to the earth. But if you are wearing rubber booth and standing in an insulated bucket on the end of a fiberglass boom lift, then you have very high-impedance to ground and touching high-voltage (even 1000s of volts) is not dangerous (by design). \$\endgroup\$ – Richard Crowley Dec 23 '16 at 17:01

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