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charlie b

Staff member
Seattle, WA
Electrical Engineer
Mike, let me bring back one key sentence from my earlier explanation:

charlie b said:
You could have a voltage of 1,120 volts from the hot wire to planet Earth, and a voltage of 1000 volts from the neutral wire to planet Earth, and the difference is still 120 volts, so therefore the coffee pot will work properly.
If that happened to the doorbell circuit, then the doorbell might also work properly. However, I suspect that the doorbell circuit?s insulation system might not be able to handle that level of voltage, nor would the internals of the doorbell itself.

But did you notice that I did not assert that it could happen, that voltage levels of 1000+ volts to ground are common occurrences in ungrounded systems? I said no such thing. Nor did I say what sorts of things would have to take place in order to bring it about. I selected the values of ?1120 volts to ground for the hot? and ?1000 volts to ground for the neutral? for no better reasons than that they made the math simple (1120 - 1000 = 120), and that there were high enough to gain the reader?s attention.

In any ungrounded system, there is no way of knowing what the voltage to ground will be at any point in the system at any point in time. It will vary from moment to moment. The amount of variation will depend on many factors. These include the capacitance between the system wires and planet Earth, the capacitance between these same wires and any nearby power source (e.g., overhead transmission lines), variations in the total load drawn by the system, start-stop cycles of large motors, and a host of other influences. I suspect, though I have done no studies and participated in no related projects, that the usual voltage variations in an ungrounded system would be no more than about 25% of the system?s voltage level. If I am right about that, then your 16 volt doorbell circuit might have a voltage to ground of perhaps 20 volts, but it is unlikely to have a dangerous level of voltage to ground. I can make that last assertion without any more facts at my disposal then the fact that the NEC does not require us to ground the doorbell circuit.

My essential points were that the installation of a ground rod will ensure that, (1) The voltage to ground of the phase conductor will be no higher than that conductor?s voltage to neutral, (2) The voltage to ground of the neutral conductor will remain close to zero, and (3) These two situations will remain constant, unchanging, ?stable.?

tom baker

First Chief Moderator
Staff member
Thank you Eric for your explaination.
Whats interesting is this was all figured out a very long time ago and put in the NEC....at one time someone knew why ground rods required 25 ohms.
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