The values of transient overvoltages depend on the nature of the supply distribution
system (underground or overhead) and the possible existence of a low-voltage protective
device upstream of the origin of the installation. IEC 60364-4-443 provides guidance
where protection against overvoltages is covered by inherent control or is assured by
protective control.
Inherent control is the condition within an electrical system wherein the characteristics
of the system itself can be expected to limit the prospective transient overvoltages to a
defined level.

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