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AC is being produced at a power plant using, let's say, a giant dynamo. A coil is being rotated rapidly through a magnetic field thus generating AC current. This current is then regulated, transformed and sent through the power lines to my town.

Now, if I touched one of the lines while being on the ground, I'd get electrocuted and die. Why? The current flows from one side of the coil of the generator to the other side of the coil (I'm imagining a dynamo here), so why do I close a circuit if I touch the ground, a large (infinite?) resistance? I only touch one cable of the dynamo coil. Don't I need to also touch the other side of the generator for current to flow through me?

It would seem more logical if I only got electrocuted if I touched two cables on a power line, both leading to different sides of the dynamo at the power plant. It doesn't seem like I'm closing a circuit. And would this (electrocution) also happen if the line was transporting High Voltage DC?

What am I missing?

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    \$\begingroup\$ Why do you think the (earth) ground is a large resistance? Many places it isn't - and the distance to the nearest point where the neutral is tied to ground may be quite short. \$\endgroup\$ – Chris Stratton Sep 2 '13 at 16:46
  • \$\begingroup\$ Because if I plug a power supply into the ground chances are I won't close the circuit. \$\endgroup\$ – Calin Sep 2 '13 at 16:53
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Electrocution can be a low current phenomena - milliamps across the heart can trigger ventricular fibrillation and death. If you're talking about mains voltage in the hundreds or thousands of volts, even a large resistance will allow enough current to flow to do damage.

Earth provides a path because there are points in the distribution network where parts of the network are tied to earth. These points are generally 'solidly' earthed with large conductors buried deeply, such that local environmental variation (rainfall, water table, etc.) don't appreciably affect the 'quality' of the earthing.

In your hypothetical situation of touching a conductor while touching earth, the path of least resistance will likely be from yourself to the nearest earth connection, then through that connection via the earth back to the generator. As Chris Stratton said in his comment, it will likely be very close by, and the connection through the earth between the solid earth points will be surprisingly low. The limiting factor will be the resistance between you and the nearest earthing point.

Consider the 'TN' earthing system:

enter image description here

Also, consider the 'TT' earthing system:

enter image description here

In both of these scenarios, there's a local earth reference at the generator. This allows both usage of the earth as a low-noise reference (for EMI filtering and other things) as well as allowing Earth to be a fault current path (allowing high enough current flow to trip an interrupting device like a fuse or a breaker.)

Your 'two side' scenario (touching the high side with one hand and the low side with the other) is worse, yes, since there isn't that limiting resistance between yourself and the local earthing point knocking some of the steam out of the hit. The absolute worst-case earthing situtation, though, may not actually be so different from a line-to-line hit - imagine it happening close to the generator, with the earthing point only a few meters away.

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  • \$\begingroup\$ But in my generator case there is no ground wire, no 0 potential. In reality, however, there is. And if my connection to a rod in the earth allows current to flown, then why do I not get electrocuted when I touch the ground wire? Shouldn't that wire also lead current to the other side of the coil of the generator? \$\endgroup\$ – Calin Sep 2 '13 at 17:09
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    \$\begingroup\$ Are you 100% certain that there is no earthing anywhere between yourself and the generator? (If so, the earth would not be a path.) \$\endgroup\$ – Adam Lawrence Sep 2 '13 at 17:14
  • \$\begingroup\$ My case is hypothetical, I am assuming how a generator at a power plant works. \$\endgroup\$ – Calin Sep 2 '13 at 17:25
  • \$\begingroup\$ The question is, why is earth a path in my towns power grid? Where does the current that flows through me in the earth go? Where is the ground wire on the power lines connected to at the power plant? \$\endgroup\$ – Calin Sep 2 '13 at 17:27
  • \$\begingroup\$ I can't imagine a generator being floated from the earth. Having earth available allows you to do things like earthing enclosures, allowing a safe path for fault energy to flow without it coming into contact with a person. \$\endgroup\$ – Adam Lawrence Sep 2 '13 at 17:27

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