Flicker and voltage fluctuations

In addition to loads that cause harmonic currents on the ac mains, there are also loads that have automatic turn on/turn off controls such as thermostats and timers. Kitchen appliances, space heaters, air conditioners, copiers and other equipment include these type of controls. These loads are frequently resistive in nature, which means that they draw sinusoidal currents and generate no harmonics, except during transient events.

When automatic controls cycle on and off, they cause frequent changes of the load to the supply. When the fluctuating load is in a branch circuit with other loads, these changes cause rms voltage fluctuations that effect all of the loads in the branch. In particular, variations in voltage amplitude cause changes in the light output of any filament lamps in the branch circuit. Because the output of a filament lamp is proportional to the square of the applied voltage, changes in light intensities can be significant even for small changes in voltage.

This effect of variation in light output as perceived by the human observer is referred to as flicker. Because flicker is annoying, and for some individuals presents a health hazard (persons that have epilepsy for example), the standard seeks to regulate flicker generation to an acceptable level. The figures below illustrate an rms voltage variation on the ac mains and the way the standard defines such a voltage change characteristic. The voltage change characteristic defines changes in rms voltage levels, U(t), in terms of percentage of nominal line voltage, Un.

These measurements ensures that voltage disturbances in the electrical distribution system do not interfere with other equipment connected to the ac mains incandescent lights to visibly flicker in a way that causes an annoyance or health risk to a human observer.