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Technical reference
Overvoltage protection to BS 7671
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 IEC/BS EN 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 of
loss of human life (including permanent injury), 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 (
U
p) and not its energy withstand (e.g.
I
imp ).
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 SPDs
connecting leads
–– Reduced risk from downstream voltage oscillations
BS 7671 follows IEC/BS EN 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 in section 11/3. For more information on
surge protection to BS 7671, please contact us.