The energy in Europe’s high-voltage grids is transmitted at a constant frequency of 50 Hertz. The grid frequency must not fall significantly below this figure otherwise there is the risk, in the worst case, of an interregional or cross-border power outage. Automatic load shedding is the measure of last resort for preventing this sort of failure and is only used in the most extreme situations. Fortunately this "emergency brake" is hardly ever required.
An underfrequency can occur when consumption increases dramatically or a power plant suddenly fails. In order to prevent an underfrequency, individual supply regions have to be disconnected from the grid: this results in power outages. The load on the grid is reduced, however, and a constant frequency of 50 Hertz is restored.
... through automatic load shedding
Frequency-dependent load shedding means that individual grid sections are automatically disconnected when certain measured values are reached. The supply regions to be disconnected are selected randomly, so the risk of a power failure is the same for all. This emergency brake re-stabilises the electricity grid. After only a few hours, all consumers are supplied with electricity again.