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Protection
against
overvoltage
Section 443 of BS 7671 deals with the
protection of electrical installations against
transient overvoltages. These can be from
the supply distribution system or generated
by equipment.
Overvoltage protection by surge protection devices (SPDs)
is not generally needed for a distribution board where a
suitable rated impulse withstand voltage is declared by the
manufacturer.
Table 44.4 in BS 7671 provides examples of various impulse
categories for equipment and table 44.3 gives the corresponding
minimum impulse withstand voltage.
For distribution boards where the nominal voltage of the
installation is 230/240V or 277/480V category III, 4kV would be
appropriate.
The designer or installer may choose to apply the requirements
of regulation 443.2.4. This uses a risk assessment method to
determine whether SPDs are required.