Sorry what I meant is that if you just connect to the ground on one continent you might have a huge potential difference, compared to the ground on the original

Sorry what I meant is that if you just connect to the ground on one continent you might have a huge potential difference, compared to the ground on the original continent. So if you would want to run your system on e.g. 100 V you need to make sure that the relative difference between your two end-points is 100 V and that voltage is unrelated to the voltage to ground at either endpoint.