I want to measure the resistance between 2 gnd cables. System Earth and Aux Earth.
What resistance would you expect?
So over a long distance 5V would be insufficient i guess.
TLC Electrical Supplies gives resistances for cables. A 1mm conductor has a resistance of 18 MILLIOHMS per metre. Or 18 ohms per kilometer.
You CAN measure resistance EITHER by applying a current and measuring the voltage; or applying a voltage and measuring the current.
However since a TRUE voltage source can supply infinite current (or a true current source, infinite voltage) that in principle can be risky.
So we generally use a potential divider principle.
Your problem (apart from do you understand ohms law?) is that the resistance may be very small. So you will NOT need lots of volts - because big V / tiny R = massive current.
Use 5V and a potential divider circuit made up of 100 ohm to +5 and then external circuit you are measuring - zero volts.