@JamesW you will not have current flow at all without a difference in potential.
There is a difference in potential between the source of the current and the destination.
The current runs through the ground and it travels through everything in its path.
The bonding grid is just one of the paths that the current travels through.
Even if you have a perfect bonding grid, you will always have differences in potential depending on the amount of current and the resistance.
If you check the voltage at a pump breaker load terminals and at the pump switch line terminals, the voltage will be the same if the pump is off.
However, if the pump is on, there will be a voltage difference between the breaker and the switch due to voltage drop, which depends on the amperage and the resistance.
The same thing happens in a pipe with pressure and flow.
When the valve is off, the pressure will be the same in the pipe at both ends.
However, when the valve is open and water begins to flow, the pressure at both ends of the pipe will be different due to head loss.