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The most frequent reason is voltage drop as a result of connecting and disconnecting large loads resulting in some level of flicker being present in the majority of mains systems. Disregarding the unfavourable effect on people, flicker does not need to be and usually is not a symptom of a malfunction of an installation. However, if a rather abrupt and unexplainable flicker level increase is observed in the mains (an increase of PST and PLT), this should not be ignored. It may turn out that the flicker is caused by bad connections in the installation.
in Flicker