I once assisted a colleague in measuring the earthing of the building where I worked. He had a three-point measuring device. I imagined this diagram.
The 4 ohm resistor is the connection to the earthing that needs to be checked.
The other two resistors are actually stakes driven into the ground at a distance of at least 15m between them and the earthing.
The electrical resistances of the two stakes driven into the ground do not necessarily have to be 0 ohms. We are interested in the resistance of the building's grounding.
In this case (we assume 4 ohms), the voltage on the millivoltmeter is 4 mV at a current consumption of 1 mA. According to Ohm's law R = U / I ( 4 ohm = 4 mV / 1 mA).
The measurement voltage can be less than 100V.
Feel free to play with the resistors, mainly the grounding resistance.
Have fun! :)
