"Voltage spikes or surges are typically caused by switching heavy loads... on or off. At a point in its sinusoidal cycle when the mains power-supply voltage nears its peak (a nominal 230Vrms), the actual peak instantaneous voltage is 325V. Now consider a (not uncommon) brief voltage spike of several hundred volts on top of that sine-wave crest and you can imagine how damaging it could be. The spike could also be in the opposite polarity to the mains cycle, reducing the instantaneous voltage instead of boosting it, which could cause just as many problems. Voltage spikes of up to 650V (twice the normal mains voltage) are not that uncommon"
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