The most common operating mode for AC power sources is to provide controlled power to electrical products under test. In this mode, the power source simply replaces utility-supplied AC line power, whether 120V-60Hz “North American type” power, 220/230V-50Hz used in most of Asia, South America and Europe, or 100V-50/60Hz used in Japan.

Many AC-powered products, such as switching power supplies and electronic lighting ballasts, draw high start-up currents to charge capacitive circuitry. Excessive inrush currents not only cause lights to flicker, but can also damage the unit. It's important, therefore, to be able to measure inrush currents to get a handle on this problem.

The crest factor of an AC current waveform is the ratio of waveform's peak value to its rms value: