To ensure that aircraft electronics and other electrically-powered equipment will operate reliably, you must test them under extreme power conditions. In the military world, MIL-STD-704 (now up to rev. F), “Aircraft Electric Power Characteristics,” establishes the requirements and characteristics of aircraft electric power. It deals strictly with power quality, and does not say anything about electromagnetic interference.

While switch-mode power supplies have many advantages, one of their disadvantages is that they are a non-linear load, and because of that can inject harmonic currents into the electrical distribution system. Devices with switching supplies include domestic appliances (TVs, microwave ovens, lighting equipment and dimmers), and office equipment (PCs, printers). Motor-driven equipment can also cause harmonic distortion.

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: