Page 218 - Electrical Safety of Low Voltage Systems

CHAPTER12

Safety Against

Overvoltages

Quod Lex non dicit, non vult.
What the Law says not, wants not.
latin proverb

12.1 Introduction
Overvoltagesaredefinedastheunwantedpotentialsoccurringinelec-
trical systems between one-phase conductor and the earth (referred
to as common-mode voltage), or between phase conductors (referred to
as differential-mode,or transverse, voltage), having a peak value greater
than the peak of the largest nominal voltage of the system itself.
Overvoltages can be triggered by atmospheric lightning dis-
charges, in which case they are defined as external, or by a rapid change
of system conditions (e.g., ground faults, switching operations, large
equipment being turned off, etc.), in which case they are defined as
internal.
Internal overvoltages caused by switching events are transient
phenomena of duration of a few microseconds or less, with oscilla-
tionsusuallyhighlydamped;theirfrequencyisintheorderof100kHz.
Ground faults occurring in the primary, or secondary, side of substa-
tions may cause internal temporary overvoltages at power frequency
of relatively long duration in the order of seconds; such temporary
overvoltages are usually undamped or weakly damped.
Should the overvoltages exceed the dielectric capability of the in-
sulationacrosswhichtheyareapplied,itsprematurefailuremayoccur
and, consequently, current would circulate between live parts, or live
parts and earth. The production of heat may cause the escalation of

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