Question

earth is neutral Prove????

Answer 1

Earth is not fully neutral.
At the surface it has some (+) current.

Answer 2

Neutral means there is no net charge.

Earth or Ground means that it is at lower voltage than your other stuff and will not change voltage when you dump charge in or take it out. The ground is like sea level…it doesn’t have to be a true zero, just below you and not likely to change much if you send a river into it.

So, you could have a neutral piece of metal that would make a very poor common ground, because it’s small and would rapidly change potential. Ideally your grounds that aren’t the actual planetary surface should be neutral, but the planetary surface may have a slight net charge due to solar wind, weather patterns, etc.