AC current behaves in a rhythmic fashion which resembles the behavior of sound waves, and any form of energy with wave-like characteristics will have an additional characteristic referred to as phase.

An image of the smooth appearance of a test tone's sound wave on an oscilloscope can be a useful guide to understanding phase. A standard test tone shown on an oscilloscope displays a smooth rise to a peak, a fall to baseline (the middle of the oscilloscope display), then a smooth drop below peak and another rise to the baseline. This standard image actually depicts two phases which comprise one complete cycle. The part of the wave rising above baseline is one phase, and the part of the wave falling below the baseline is a second phase in the cycle.

In 120 volt household current, only one phase is used. Voltage in one of the two wires rises rapidly from baseline (zero volts) to 120 volts and falls rapidly again to zero a short time later. Then the other wire rises to 120 volts before falling again to baseline. Voltage never drops below the baseline in this type of current. In 240 volt circuits, commonly used for electric ranges and commercial installations, a third wire is used, and two- or three-phase power is produced. Each phase corresponds visually with one peak or valley on an oscilloscope.

In DC power (direct current), phase never changes. One conductor always carries peak voltage, and the other is always neutral or ground. Polarity, not phase, is the critical characteristic of DC current, because a reversal of polarity could cause catastrophic failure of a connected device. Both conductors in household AC current take turns going through an identical phase, so polarity doesn't usually matter with AC current.

A single device connected to AC current doesn't normally care whether one conductor or the other goes through its phase first. But phase accuracy becomes particularly important when you have two or more interconnected electronic devices draw energy from different electrical outlets. Audio and video devices in particular are highly sensitive to phase accuracy, and when the power plug is turned around in the socket, annoying effects ranging from drops in signal level to an annoying buzz can occur. The increasing prevalence of sensitive electronic devices in daily life, from component stereos to computer systems, is one of the main reasons why AC plugs and sockets now use connectors that force the plug to be inserted in a particular way. This insures that multiple interconnected devices all receive power in the same phase.

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