AC is one of those things we never think about-even most electronics textbooks consider it to be a simple, 60-Hertz 120-volt RMS sine wave or 50-Herz 230-volt RMS sine wave (depends on where you live). Many books mention any impurities only in passing. But it's not that simple. The mains power can contain radio frequency interference, fluctuations in the voltage, high voltage spikes and even short breaks in power distribution.