In a simple AC power distribution system there are usually three wires in the power cable that connect a computer to the wall socket. There is an active or live wire (also known as the hot wire), a neutral wire and a ground wire. Power is delivered to the load using the active and neutral wires. The ground wire is for safety purposes.
In the context of power supplies, noise is any undesirable voltage impulse which appears at the output. Noise at the output is caused by noise on the three input wires and may appear as either "common-mode" or "normal-mode." Figure 1 illustrates how common-mode and normal-mode noise appear on the power wires.
Common-mode noise is present on both the active and neutral wires and is measured with respect to ground. (The term "common" refers to the fact that identical noise appears on both the active and neutral wires.) Incorrectly, however, common-mode noise is understood to be only on the neutral wire. Common-mode noise can be created by lightning, the switching of circuit breakers or poor grounding.
Noise which can be measured between the active and neutral wire is called normal-mode and is sometimes referred to as differential-mode or transverse-mode noise. Most normal-mode noise is created as a result of large loads switching on or off, particularly big motors and power factor correction capacitors.