Overvoltage definitions

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 [1]

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. J1 – Examples 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. J2 – Main 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.