The gauge showing 0 - 0.7 , in a quick look at the manual this should be amps, so reading 0 - 0.7 amps on a clamp meter maybe asking a bit much, on AC it might not matter, but make sure you have it installed it in the current flow direction, from earth toward neutral(meter should have an arrow on the clamp showing current direction). You may need to get a more sensitive meter.
Don't start unbolting circuit earths and getting between the earth conductor and the earth bar, remember that a healthy heart is supposed to be able to handle a 30 ma shock, you have up to 700ma, so just because it is an Earth it can kill you.