This technical article relates to the increasingly important topic of electrical surges, which can damage sensitive electrical and electronic equipment. Surge protection devices (SPDs) play a vital role in protecting sensitive electrical and electronic equipment from damage caused by overvoltages or other electrical power surges or spikes. But what are the issues, and how do surge arrestors work? Voltimum managing editor James Hunt reports:
Overvoltages, electrical power surges or spikes are fast, short duration electrical transients in an electrical circuit. An overvoltage is a voltage pulse (or wave) that is superimposed on the rated voltage of the network, and it can disturb equipment and produce electromagnetic radiation. Moreover, the duration of the overvoltage causes an energy peak in the electric circuits which could damage or even destroy equipment.
Voltage spikes or surges are fast, short duration electrical transients in an electrical circuit. Fast, short duration electrical transients in the electrical potential of a circuit are typically caused by lightning strikes, tripped circuit breakers, short circuits, power outages, power transitions in other equipment on the same power line, malfunctions caused by the power utility, storm-damaged powerlines, and electromagnetic pulses in the 100kHz – 1MHz frequency range.
Up to 80% of such transients are generated from internal sources such as inductive load switching and normal equipment operations, while at least 20% of transients are generated from external sources such as lightning and power utility switching.
Various types of overvoltage can disturb electrical installations. These include the following:
Power-frequency overvoltages –These are of the same frequency as the network caused by a permanent change of state in that network – for example, after an insulation fault occurs, or the breakdown of the neutral conductor.
Switching surges – These are high-frequency overvoltages or burst disturbances caused by a change in an electrical network steady state – typically because of switchgear operation. The switching off of lighting and the starting and stopping of AC motors can be the cause of this type of surge.
Electrostatic discharge – This causes very short overvoltages (a few nanoseconds) of very high frequency caused by the discharge of accumulated electric charges. An example of this can be a person walking on a carpet with insulated soles, who can become electrically charged to several kV.
Lightning strike – This can produce extremely large pulsed electrical energies of several thousand amperes (typically 2000 – 400,000A) and many tens or hundreds of thousands of volts at high frequency (typically around 1 mHz) over micro- to millisecond short duration. Lightning damages electrical and electronic systems such as transformers, electricity meters and electrical appliances. It can also cause damage or disturbances to computers, telecommunications networks, and cause faults in programmable logic controller (PLC) programs and other control systems.
Some surge arrestors designed to protect three-phase systems against lightning strike are said to be able to pass currents of 320,000A/phase (equivalent to 1,200,000A total).
The overall result of various overvoltages can be cumulative equipment damage and possible premature failure, expensive system resets and down time and data losses. It can even result, especially from lightning strike, in catastrophic equipment failure resulting in immediate shutdown, as well as longer term disruption of businesses, plus expensive equipment repair and replacement costs. Lightning also causes a large number of fires, mostly in agricultural areas, but high-rise buildings are especially at risk.
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