Power Events, Defined

The most common power events and their deleterious effects are described below...


The most common power events and their deleterious effects are described below. Underfrequency and overfrequency also are power-quality parameters, but they normally result from one of the following, or are directly related to generation problems:

  • Voltage sag is a momentary voltage dip of 15 percent or more and can last for a few hundred cycles. It is typically caused by fault conditions on either the transmission line or the distribution line, or it can be caused by large current in-rush due to large motor starting, large transformer magnetizing current and other large loads instantaneously applied to the distribution system. Electromechanical devices are somewhat immune to voltage sag and can continue to function throughout the duration, but the same sag can be disastrous for microprocessor-based equipment.


  • Voltage swell is similar to voltage sag both in magnitude and duration, but the voltage rises by at least 15 percent. The typical cause is a phase-to-ground fault on a three-phase, four-wire distribution system. The voltage swell occurs on the two unfaulted phases. The effect on electromechanical devices is negligible. However, voltage swells can cause microprocessors to yield an invalid output without causing physical damage to the devices.


  • Harmonics have always existed in AC power systems. Harmonics in both the voltage and current waveform components of power systems are simply integer multiples of the fundamental frequency (60 Hz, 180 Hz, 300 Hz). Switched-pulse-mode DC power supplies-such as those used in personal computers and power conversion devices similar to variable-frequency drives-create nonlinear loads that no longer result in smooth sinusoidal waveforms.

The current component of harmonics is of particular concern due to neutral heating and significantly distorted voltage waveforms. Transformer cores become saturated by the distorted currents, and the consequent heating of neutral conductors, transformers, switchboards and panelboards has a significant impact on a building's entire electrical distribution system. Unmitigated neutral heating limits overall system capacity, shortens equipment life and can damage the panelboard neutral bar.

  • Transient-voltage surge or voltage spike typically consists of a very-high-magnitude pulse of extremely short duration. Lightning is the primary source; however, utility circuit-breaker operation or capacitor switching can cause the same effect. Microprocessor-based and other electronic devices are susceptible to significant damage. Electromechanical equipment can be damaged-due to insulation breakdown-but this is less likely.


  • Undervoltage is a condition where the available voltage level is 90 percent of the nominal voltage. The duration of the condition can be from a few minutes to several hours. Undervoltages usually result from system overload or utility-initiated brownout. Microprocessor-based devices can accommodate undervoltage with few or no significant problems, but electromechanical devices may be forced to shut down.


  • Overvoltage is excess voltage of 10 percent above nominal for a sustained duration of a few minutes to several hours. The primary cause is improper voltage regulation by the utility, usually due to poor control of voltage regulators or capacitor banks. This condition generally does not cause significant problems for either microprocessor-based devices or electromechanical devices, but it does shorten incandescent lamp life and may damage some electronic fluorescent ballasts.


  • Power outages are, of course, complete interruptions of power lasting from seconds to days. Obviously, the impact of a power interruption, both for microprocessor-based and electromechanical equipment, can be a tremendous loss of business-and of considerable revenue-for commercial and industrial facilities.

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