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.