- #31
GENIERE
sheldon said:
Simple question, but not a simple answer. E fields exist between the Earth and the electrical wiring in the home. When you close a light switch, the electrons in the wire are set in motion and produce an electro-magnetic field exactly in the same manner, as does a radio station when it generates an EM field in the antenna. There also exists the natural electro static field between the Earth’s surface and any point in the atmosphere. If memory serves, there may be several hundred volts difference in potential measurable between your head and feet when standing upright in a pasture.
When measuring these potentials, the results will vary depending on how you make the measurement. If you wish to measure the natural static E-field, you will need to use a DC volt meter with an extremely high input resistance, the lower the input resistance, the lower the reading. Of course all depends on the atmospheric conditions at the time of measurement. Avoid doing this if there’s an anvil shaped cloud overhead.
Your question really concerns the EM fields generated by the household wiring. Again, the result will depend on the measurement. Let's assume you have two 10 foot ground rod buried in the ground separated by 5 feet and make a resistance measurement. The resistance will vary considerably dependant primarily on the moisture content of the Earth. One could expect readings of tens of ohms per foot to megohms per foot. Generally speaking, the Earth is a poor conductor. If you measure the potential difference between one of the conductors in the home to a ground rod and the home wiring is not referenced (connected) to the Earth, what do you measure? The 60Hz radiated EM field will produce a potential difference between the wiring and the Earth. When you connect the voltmeter between the wiring and the Earth, the internal impedance of the meter will be in parallel to the impedance of the Earth and the radiated field. In other words the impedance of the meter greatly influences the measurement. With a 20,000 ohms per volt AC meter, I’d guess 20 –70vac. With a 1000 ohms per voltmeter, I’d guess only a few volts.
What about the safety aspect in a non-earth referenced system when a person is touching the “hot” black wire of an extension cord and standing on the ground with bare feet? How much current will pass through a person’s hand, through his feet to the Earth. In this scenario with a direct connection to the hot wire, the radiated field can be ignored due to its high impedance. It’s simply a guess, but the impedance of the Earth, body impedance, and the capacitive and inductive impedance of the entire system would limit the current. This would likely be on the order of micro amps, possibly milliamps and not noticed by the individual. In hospital OR’s and in other intentionally non-earth referenced systems (gas stations?), I believe the total impedance must be maintained above one (1)megohm. The above scenario is “safer” then the Earth referenced system, wherein only the impedance of the Earth and the person’s body would limit current flow. In ideal conditions that current could be quite high, perhaps several amperes with household voltages and capable of causing death.
Why do we use the less safe Earth referenced power distribution system? IMHO the hundreds of thousands of miles of high voltage conductors from the Utility electrical generators to the home are coupled to the Earth via the total inductance and capacitance of the system. The total impedance of the system would be quite low and comparable to a direct wire connection. I believe the non-earth referenced system would require the utilities to also run an additional common conductor (“Y versus Delta configuration) besides the three phases thus adding to the expense and decreasing reliability. If the load was completely balanced, no current would flow in the additional conductor but one could not expect the load to be always balanced.