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Overvoltage definitions

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LV switchgear: functions and selection
Overvoltage protection
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ElectroMagnetic Compatibility (EMC)
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overvoltage (in a system)

any voltage between one phase conductor and earth or between phase conductors having a peak value exceeding the corresponding peak of the highest voltage for equipment

definition from the International Electrotechnical Vocabulary (IEV 604-03-09); available on Electropedia

Various types of overvoltage

An overvoltage is a voltage pulse or wave which is superimposed on the rated voltage of the network (see Fig. J1).

Fig. J1Examples of overvoltage

This type of overvoltage is characterized by (see Fig. J2):

  • the rise time tf (in μs);
  • the gradient S (in kV/μs).

An overvoltage disturbs equipment and produces electromagnetic radiation. Moreover, the duration of the overvoltage (T) causes an energy peak in the electric circuits which could destroy equipment.

Fig. J2Main characteristics of an overvoltage

Four types of overvoltage can disturb electrical installations and loads:

  • Switching surges: high-frequency overvoltages or burst disturbance (see Fig. J1) caused by a change in the steady state in an electrical network (during operation of switchgear).
  • Power-frequency overvoltages: overvoltages of the same frequency as the network (50, 60 or 400 Hz) caused by a permanent change of state in the network (following a fault: insulation fault, breakdown of neutral conductor, etc.).
  • Overvoltages caused by electrostatic discharge: very short overvoltages (a few nanoseconds) of very high frequency caused by the discharge of accumulated electric charges (for example, a person walking on a carpet with insulating soles is electrically charged with a voltage of several kilovolts).
  • Overvoltages of atmospheric origin.