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Overvoltage protection to BS 7671
TSC-0912 - 09.10.12
The latest amendment to the 17th Edition of
the Wiring Regulations, BS 7671:2008(+A1:2011),
in force from January 2012, establishes a
requirement for assessing protection against
transient overvoltages (surges) as an integral
part of satisfactory electrical system design.
BS 7671 assesses the need to protect AC power circuits,
although cross-references transient overvoltage
protection on other metallic services including data,
signal and telecommunications lines, as defined by
BS EN/IEC 62305 Standard for Lightning Protection.
It covers transient overvoltages of atmospheric origin
(lightning) or as a result of electrical switching,
through two sections:
Section 443 which defines the criteria for risk
assessment of transient overvoltages, considering
factors such as levels of consequential loss and the
withstand voltage/impulse immunity of installed
electronic systems
Section 534 which outlines the parameters for
selection and installation of SPDs as appropriate,
to ensure satisfactory protection of electronic
systems and electrical equipment
Risk assessment
Section 443 establishes that protection against
transient overvoltages should be expected where:
An installation includes bare overhead metallic
service lines which are at risk from lightning and
The level of transient overvoltage anticipated
would exceed the withstand voltage of sensitive
electrical equipment/impulse immunity of critical
electrical equipment, or
The risk of potential consequential loss (to life,
property or provision of service) would be
unacceptable
Whilst direct lightning strokes are not considered,
reference is made to BS EN 62305 which would require
installation of equipotential bonding SPDs where a
structural LPS is installed, or there is a risk of a direct
lightning stroke to a service line.
Factors contributing to risk include external influences
(thunderstorm days per year) and consequential levels
of protection.
Irrespective of external influences, where higher
reliability or higher risks are anticipated, protection
measures should be installed.
Considering the consequential levels of protection
defined by BS 7671, protection is required wherever
there is a risk to human life, to public services and to
commercial or industrial activity.
Selection & installation of SPDs
Section 534 provides guidance on the selection and
installation of SPDs to limit transient overvoltages.
The selection of an SPD is dependent on its location
within the installation, the withstand voltage/
impulse immunity of equipment at this location, and
the expected transient overvoltage energy that the
SPD is required to limit.
The largest transient overvoltages are expected
at the service entrance, i.e. at the origin of
the installation.
Additionally transient overvoltages can be anticipated
at sensitive and critical equipment as a result of
electrical switching within the installation.
SPDs should therefore be installed as appropriate
at main distribution board level (after the meter),
sub-distribution board level to protect sensitive
equipment, and locally to protect critical equipment.
Where multiple SPDs are installed on the same
conductor, these should coordinate with each other to
ensure protection levels are not compromised within
the system.
The most important characteristic for an SPD is its
voltage protection level (UP) and not its energy
withstand (e.g. Iimp). SPDs with lower voltage
protection levels (or let-through voltage) offer much
better protection to sensitive and critical electronic
systems, including:
Minimal equipment stress (i.e. keeping circuit
degradation to a minimum)
Reduced risk from additive inductive voltages on
the SPD’s connecting leads
Reduced risk from downstream voltage oscillations
BS 7671 follows BS EN/IEC 62305 by classifying SPDs
by Type. Equipotential bonding SPDs (Type 1) must be
installed at the service entrance where a structural LPS
is installed or there is an overhead metallic service line
at risk from a direct lightning stroke.
Type 1 SPDs however do not provide protection to
electronic systems. Transient overvoltage SPDs (Type 2
or Type 3) are required downstream to protect
sensitive and critical equipment.
These SPDs protect against the transient overvoltages
caused by indirect lightning (inductive or resistive
coupling) and the electrical switching of large
inductive loads.
They should offer full mode protection to protect
sensitive and critical electronic systems, since transients
can occur between all modes.
Specific performance parameters for SPDs are defined
in BS 7671, which are covered by Furse SPDs in this
catalogue when installation follows the selection chart
provided on page 181.
For more information on surge protection to BS 7671,
please contact us.