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Circuit for earth conductor presence detection

Reading a [Tesla Wall Adapter installation manual](https://www.tesla.com/sites/default/files/support/charging/Gen3_WallConnector_Installation_Manual.pdf) , their brand of an AC EV charger, chapter about “Ground Assurance” and here is the quote from there:
![Quote from Tesla Wall adapter](https://electrical.codidact.com/uploads/7wcmllwvcrl44w5vp4f30wr1inow)

Although the equipment can work with L1 and L2 connected only, because of the safety reason I assume they require Earth terminal of the charger to be grounded as well with the ability to monitor it(apparently). 

Mostly referring to the US power grid where L1 and L2 are taken out from the split phase transformer with neutral grounded at one place in the system. 


How the circuitry for something like that conceptually look like?

I was thinking about inducing DC current into the ground, small enough in value to not trip the RCD and measure the voltage drop in the ground/earth itself(I guess??) - current source and shunt for voltage sensing? Wondering if inducing any additional resistance(shunt) in series with earth is legal?

For what is worth their input power terminals are: L1, L2, and PE - there is no additional I/O solely for this function to work.