Shielding, Grounding, Noise Suppression, etc.

Grounding

There are generally three types of gounds: earth ground, signal ground, and chassis ground. Their respective symbols are shown below:

Each of these symbols represents a common node, but there are some differences.

The earth ground denotes a physical connection to the earth, which is conventially defined to be 0 volts. On electrical appliances, this is the third pin on the power plug. However, many circuit diagram will use this symbol to designate a 0V reference even if there is no physical connection with the ground.

The second symbol, the signal ground, is used as a reference for data transmission lines. The signal ground is generally not connected to the earth to isolate it from electrical noise and interference from electric currents and fields in the earth. Thus, there is often a significant voltage difference between the signal ground and the earth ground.

The third symbol is known as the chassis ground, and is usually connected to the conducting shell or chassis of that particular piece of equipment. For example, the negative termial of a car battery is wired to the metallic shell of the car. This provides a reference voltage that may or may not be connected to the earth, although for most appliances it usually is. Not connecting the chassis ground to the earth ground can result in a voltage difference between the chassis ground and the earth ground, which leaves the risk of electrical current (including static electricity) finding a path between the chassis and earth through something...or someone. This is the reason why fuel trucks must be grounded while dispensing fuel—the voltage difference can result in a spark and explosion.