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What Is a Surge Current?

By Geisha A. Legazpi
Updated May 23, 2024
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A surge current is a sudden increase in current usually caused by a voltage imbalance. In general terms, a surge is a sudden increase and electrical current is the rate of charge flows. Current is usually measured in amperes (A). If a direct current (DC) power cable carries 20 A during steady load conditions, a current surge of about 50 A could be possible during transient conditions, such as during power up. Voltage imbalance is a difference in electrical potential that causes surge currents.

There are many ways to avoid equipment failure due to surge currents. Such a current can be internally or externally generated. An internally generated surge current is usually the result of the capacitor surge current, which results when a capacitor or condenser is previously uncharged followed by a sudden flow of current that is meant to equalize the capacitor voltage to the source circuit.

An externally generated surge current usually comes from lightning. Despite attempts to protect sensitive equipment from lightning surge, there is always a way that such currents can be induced into energized equipment. Energized equipment may have power feeders and miscellaneous cables, such as interface cables and signal cables that carry radio frequency, audio, or data.

In radio frequency systems, the antenna, usually on an antenna tower elevated above the ground, is always where signal level is best. The same ideal place for tapping the radio frequency signal is also where lightning surge is most likely to occur. Lightning rods are placed at the highest point of such towers to “catch” most of the lightning surge, but some of the surge energy finds its way into the electronic equipment.

Surge current protection devices available in the market include power surge protection devices and signal surge protection devices. 110 volts alternating current (VAC) power-rated equipment can typically handle a voltage surge causing the input voltage to rise to a maximum of about 150 VAC. Signal surge protection devices are also available for all kinds of signals. These include the radio frequency surge protection devices as well as surge protection devices for audio and data.

A surge current prediction is also important to keep electronic and electrical equipment functioning. Circuit designers have to anticipate all possible surge conditions in order to build reliable equipment. These transients are capable of overloading circuit components to the point of total or partial failure.

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Discussion Comments
By Logicfest — On Jan 29, 2014

Good point about power surge prediction. It takes a lot more to protect electronics than a good surge protector -- some engineering skill in putting the components together so they can withstand surges is important, too.

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