For loop impedance, you already have a 3 wire circuit made for you for most systems, Active, and Neutral / Ground tied back at the fusebox,
For active / neutral, measure the ground to neutral voltage difference, if this is not almost 0, then you know you have other devices on the circuit which may interfere,
You would pulse a load on the active / neutral, measure the drop in voltage between them but also measure the difference between neutral and ground, this will allow you to calculate the impedance of the active and neutral wires independently (neutral is E-N difference / Test Load current, Active is the remainder of the change in voltage.)
As for measuring the ground circuit impedance, the easiest way would be plugging in to a ground on a different circuit, and using the neutral as a reference, but i understand this is rarely an option.
In reality i would do a chopper approach to remove X/Y filter capacitance leakage, you draw some power from A-N with an isolated supply, then pulse 15mA between neutral and ground, you know the impedance of the neutral from earlier, so after subtracting you have the ground impedance,
You would measure the voltage difference between the pulse being on and being off, this would remove any filter currents.