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2 Amplitude disturbancesNetworks are disturbed not only by loadfluctuations in normal operation, but also andabove all by random phenomena of accidentalorigin, for example short-circuits. A LV networkis subjected to these two causes of disturbanceswhatever their origin (LV, MV or HV). The twomain amplitude disturbances are:c voltage dips and short interruptions,c overvoltages.2.1 Voltage dips and short interruptionsA voltage dip is normally defined as a drop from10% to nearly 100% of the nominal voltage valuefor a time lasting between 10 ms and a fewseconds (see fig. 1 ). When power is off (100 %voltage drop), for 1 s to 1 min, we talk of a “shortinterruption”, and of a “very short interruption”between 10 ms and 1 s. When duration isi 10 ms, the interruption is normally due to“transient” or voluntary phenomena, such as useof dimmers.Voltage amplitude∆ v10 % 10 msn transformerTime constant(s)50 15 0.1100 14 0.15160 12 0.2250 12 0.22400 12 0.25630 11 0.3800 10 0.31000 10 0.351250 9 0.351600 9 0.42000 8 0.45InterruptionFig. 2 : value of the energising currents and timeconstant of the MV/LV transformers on energisation onthe MV side (on energisation on the LV side, “ne” istwice as high).Cahier Technique Schneider Electric no. 141 / p.5