Hi,
Apparently one can be shocked if an unearthed metal case of a device shorts with an active wire in the device and a person touches the case (assuming there are no safety devices such as RCDs, etc in play). The implication here is that current can pass through the person into the physical earth and back to the power system.
If you're wearing shoes of "rubber" or standing on something properly isolated and isn't touching anything else except for the hot case, you won't get shocked!
It's all a question of
potential difference and if your body doesn't see any potential difference, no shock is felt (your entire body will be 120VAC (or whatever touches the case) - like birds resting on HT air wires of a much larger magnitude without feeling a thing.
How is this possible? I would have thought that the physical earth was non-conductive.
Physical earth, in the meaning dirt isn't conductive in itself, but with even the slightest bit of moisture, all the minerals in the earth (and in the moisture) will be fairly conductive.
With a high enough tension, you don't need the material to be conductive, as currents can "leak", as is the case when HT wires breaks and hang down touching eg. the pavement of a road.
Rescue workers are tought to take very small steps in such an area, as there will be a potential of nnV/m and a regular step will mean a large potential difference between right and left foot (and isolated shoes/boots won't save you in that instance).
You can stay in a single spot, but cover a larger distance (in the direction towards/away from the fallen cable) will result in a large potential difference.
"In Single Wire Earth Return (SWER) AC electrical distribution systems, costs are saved by using just a single high voltage conductor for the power grid, while routing the AC return current through the earth. This system is mostly used in rural areas where large earth currents will not otherwise cause hazards."
This just sounds absurd to me!
You drive a copper rod (or even a galvanized steel tube) a couple of meters into the rod (and usually water it right after to get the currents start flowing).
Earth cannot compare to copper in terms of specific resistivity, but considering the square of the conductor, the total resistivity is quite low.
You can use the earth as the sole medium for communication, if both "transmitter" and "receiver" has its output and ground connected to rods driven into the ground a good distance apart and it will result in a potential difference a certain distance away (related to how far apart one set of electrodes can be placed). It was used as far back as in WW1.
It's cheap, a homemade audio amplifier of say 5W and a sensitive (mic) preamplifier should give you a possible range of at least some hundred meters (much depending on soil resistivity and electrical noise in the area).
