Earth and Ground are perhaps the most misunderstood terms in electronics. The difference comes down to a matter of qualifying a reference point.
The term “earth” literally indicates a reference point to planet earth. The planet earth is essentially an infinite reservoir of electrons and thus the best place to “drain” excess electrons from a system.
The point of contact with earth reference is generally achieved by driving a conductive spike several feet into the ground, ensuring a solid earth connection.
Thus the term “earth ground”denotes a connection to an actual earth reference point (see Figure).
Earth Ground and Chassis Ground
Figure: The “ground” point on the power supply is a chassis ground while the load has an earth ground reference.
The term “chassis ground” (Figure) indicates a ground reference on the dead metal enclosure points of an electronic device.
It is possible for this ground reference to sit at earth potential or at some other value if it is not grounded to earth. In the case where the chassis is not earth grounded, it is said to be “floating” at some potential other than earth ground.