ABSTRACT
The block diagram in Figure 4 shows how grounding works. The symbol of a sine wave in a circle represents the power source. For our purposes, this is the building's main switch panel. Below the power source, the stacked series of lines represents ground. This normally consists of a heavy-gauge wire or cable that connects the switch panel to a thick copper ground rod driven into the earth. (In large, steel-framed buildings, one of the building's steel columns will serve as the ground point.)
Note that one side of the AC power wires is also connected to ground at the switch panel. This line is known as the "neutral" line, because it is at essentially earth potential. The other power-carrying wire is usually referred to as the "hot" line. It is this line that will rise and fall in voltage over the 360-degree cycle of a complete sine wave. (The neutral line can't vary significantly from 0 volts, because it is grounded at the main switch panel.)
On the right side of the figure is a block diagram of the computer, including its power supply. The computer is connected to the main switch panel via the building's wiring, with its three-wire power cord plugged into a wall outlet. Of the three wires, the two power-carrying wires (hot and neutral) go into the power supply-and nowhere else. The computer's chassis is connected to the grounding wire. The grounding wire serves two purposes: safety and signal reference. Grounding the chassis ensures that it will be at the same potential as the earth, which eliminates any possibility of electrocution if someone should happen to touch the chassis. The second function performed by grounding the chassis is to set up a reference (of 0 volts) to which all other voltages in the system can be compared.
