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TECHNICAL SPECIFICATION
CLC/TS 61643-12
SPÉCIFICATION TECHNIQUE
August 2006
TECHNISCHE SPEZIFIKATION
ICS 29.240;29.240.10
English version
Low-voltage surge protective devices
Part 12: Surge protective devices connected
to low-voltage power systems –
Selection and application principles
(IEC 61643-12:2002, modified)
Parafoudres basse tension  Überspannungsschutzgeräte für
Partie 12: Parafoudres connectés aux Niederspannung
réseaux de distribution basse tension – Teil 12: Überspannungsschutzgeräte für
Principes de choix et d'application den Einsatz in Niederspannungsanlagen –
(CEI 61643-12:2002, modifiée)
Auswahl und Anwendungsgrundsätze
(IEC 61643-12:2002, modifiziert)

This Technical Specification was approved by CENELEC on 2006-04-15.

CENELEC members are required to announce the existence of this TS in the same way as for an EN and to
make the TS available promptly at national level in an appropriate form. It is permissible to keep conflicting
national standards in force.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, the Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2006 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. CLC/TS 61643-12:2006 E

Foreword
The text of the International Standard IEC 61643-12:2002, prepared by SC 37A, Low-voltage
surge protective devices, of IEC TC 37, Surge arresters, together with the common
modifications prepared by CLC 37A was submitted to the formal vote. The combined text was
approved by CENELEC as CLC/TS 61643-12 on 2006-04-15.
The following date was fixed:
– latest date by which the existence of the CLC/TS
has to be announced at national level (doa) 2006-07-01
Annex ZA has been added by CENELEC.

– 3 – CLC/TS 61643-12:2006
CONTENTS
Introduction.6
Keys to understanding the structure of this standard.6
List of variables and abbreviations used in this standard.8
1 Scope.10
2 Normative references .10
3 Definitions .10
4 Systems and equipment to be protected .15
4.1 Low-voltage power distribution systems.15
4.1.1 Lightning overvoltages and currents .16
4.1.2 Switching overvoltages .16
4.1.3 Temporary overvoltages U .17
TOV
4.2 Characteristics of the equipment to be protected .18
5 Surge protective devices .18
5.1 Basic functions of SPDs .18
5.2 Additional requirements.18
5.3 Classification of SPDs .19
5.3.1 SPD: classification.19
5.3.2 Typical design and topologies.19
5.4 Characteristics of SPDs.20
5.4.1 Service conditions described in EN 61643-11 .20
5.4.2 List of parameters for SPD selection.21
5.5 Additional information on characteristics of SPDs.22
5.5.1 Information related to power-frequency voltages.22
5.5.2 Information related to surge currents .22
5.5.3 Information related to voltage protection level provided by SPDs.24
5.5.4 Information related to SPD failure modes .25
5.5.5 Information related to short-circuit withstand .26
5.5.6 Information related to load current I and to voltage drop (for two-
L
port SPDs or one-port SPDs with separate input and output
terminals) .26
5.5.7 Information related to change of characteristics of SPDs .26
6 Application of SPDs in low-voltage power distribution systems .26
6.1 Installation and its effect on the protection given by SPDs.26
6.1.1 Possible modes of protection and installation .27
6.1.2 Influence of the oscillation phenomena on the protective distance .30
6.1.3 Influence of the connecting lead length.30
6.1.4 Need for additional protection.32
6.1.5 Consideration regarding location of the SPD depending on the
classes of test .33
6.1.6 Protection zone concept .33
6.2 Selection of SPD .33
6.2.1 Selection of U , U and I /I /I /U of the SPD .35
n imp max
c T oc
System configuration of distribution network.35
6.2.2 Protective distance .37
6.2.3 Prospective life and failure mode.37
6.2.4 Interaction between SPDs and other devices.38
6.2.5 Choice of the voltage protection level U .39
p
6.2.6 Co-ordination between the chosen SPD and other SPDs .39
6.3 Characteristics of auxiliary devices.41
6.3.1 Disconnecting devices .41
6.3.2 Event counters .42
6.3.3 Status indicator .42
7 Risk analysis .42
8 Co-ordination where equipment has both signalling and power terminals.44
Annex A (informative)  Examples of various SPD technologies .45
A.1 Examples of internal circuits for one port and two port SPDs .45
A.2 Response of SPDs to a combination wave impulse.47
Annex B (informative) Explanation of testing procedures used in EN 61643-11 .48
B.1 Determination of U for SPDs tested in accordance with class I and class II tests .48
res
B.2 Impulse waveshape for assessment of U .48
res
B.3 Influence of a back filter on determination of Ures .48
B.4 Operating duty test for SPDs .49
B.5 TOV failure test .50
B.6 Differences in the testing conditions of Type 1 (test class I), 2 (test class II) and 3
(tests class III) SPDs.50
B.7 short-circuit withstand capability test in conjunction with overcurrent protection (if
any). 51
Annex C (informative) Partial lightning current calculations.52
Annex D (informative) Examples of application of TS 61643-12 .54
D.1 Domestic application .54
D.2 Industrial application .56
D.3 Presence of a lightning protection system.60
Annex E (informative) Examples of application of the risk analysis .61
Annex F (informative)  Consideration for SPDs when Type 1 SPDs are to be applied.64
Annex G (informative)  Immunity vs insulation withstand.65
Annex H (informative) Examples of SPD installation in power distribution boards in
some countries .67
Annex ZA (normative) Normative references to international publications with their
corresponding European publications .70
Bibliography.72

Figure 1 – Examples of components and combinations of components .20
Figure 2 – Relationship between U , U , U and U .22
p 0 c cs
Figure 3 – Typical curve of U versus I for ZnO varistors.24
res
Figure 4 – Typical curve for a spark gap .25
Figure 5 – Flowchart for SPD application .27
Figure 6 – Connection Type 1 .28
Figure 7 – Connection Type 2 .29
Figure 8 – Influence of SPD connecting lead lengths .31
Figure 9 – Need for additional protection .32
Figure 10 – Flowchart for the selection of an SPD .34
Figure 11 – Typical use of two SPDs – Electrical drawing .40

– 5 – CLC/TS 61643-12:2006
Figure A.1 Examples of one-port SPDs .45
Figure A.2 Examples of two-port SPDs .46
Figure A.3 Response of one-port and two-port SPDs to a combination wave impulse.47
Figure C.1 – Simple calculation of the sum of partial lightning currents into the power
distribution system.52
Figure D.1 – Domestic installation.55
Figure D.2 – Industrial installation.58
Figure D.3 – Industrial installation circuitry .59
Figure D.4 – example for a lightning protection system .60
Figure F.1 – General distribution of lightning current.64
Figure H.1 – A wiring diagram of an SPD connected on the load side of the main
incoming isolator via a separate isolator (which could be included in SPD enclosure) .67
Figure H.2 – SPD connected to the nearest available outgoing way (MCB) to the
incoming supply (TNS installation typically seen in the UK).68
Figure H.3 – A single line-wiring diagram of an SPD connected in shunt on the first
outgoing way of the distribution panel via a fuse (or MCB). .69

Table 1 – Maximum TOV values as given in IEC 60634-4-44 .17
Table 2 – Preferred values of I .23
imp
Table 3 – Possible modes of protection for various LV systems .30
Table 4 – Minimum required Uc of the SPD dependent on supply system configuration .35
Table 5 – Typical TOV values .36

Introduction
This TS is to be used with the European standard EN 61643-11:2001, Low-voltage surge
protective devices – Part 11: Surge protective devices connected to low voltage power
systems – Requirements and tests.
Surge protective devices (SPDs) are used to protect, under specified conditions, electrical
systems and equipment against various overvoltages and impulse currents, such as lightning
and switching surges.
SPDs shall be selected in accordance with their environmental conditions and the acceptable
failure rate of the equipment and the SPDs.
This TS provides information :
• to the user about characteristics useful for the selection of an SPD.
• to evaluate the need for using SPDs in low-voltage systems.
• on selection and co-ordination of SPDs, while taking into account the entire
environment in which they are applied. Some examples are: equipment to be protected
and system characteristics, insulation levels, overvoltages, method of installation,
location of SPDs, co-ordination of SPDs, failure mode of SPDs and equipment failure
consequences.
• and provides guidance to perform a risk analysis.
The HD 384 series of harmonised documents provides direct information for contractors on
the installation of SPDs.
For the purpose of having a usable and complete working document, parts from existing
documents have been duplicated where necessary. Such parts are explicitly mentioned in the
text and attention is drawn to the reader that these parts may change in future.
Keys to understanding the structure of this standard
The list below summarizes the structure of this standard and provides a summary of the
information covered in each clause and annex. The main clauses provide basic information on
the factors used for SPD selection. Readers who wish to obtain more detail on the information
provided in Clauses 4 to 7 should refer to the relevant annexes.
Clause 1 describes the scope of this standard.
Clause 2 lists the normative references where additional information may be found.
Clause 3 provides definitions useful for the comprehension of this standard.
Clause 4 addresses the parameters of systems and equipment relevant to SPDs. In addition
to the stresses created by lightning, those created by the network itself as temporary
overvoltages and switching surges are described.
Clause 5 lists the electrical parameters used in the selection of an SPD and gives some
explanation regarding these parameters. These are related to the data given in EN 61643-11.

– 7 – CLC/TS 61643-12:2006
Clause 6 is the core of this standard. It relates the stresses coming from the network (as
discussed in Clause 4) to the characteristics of the SPD (as discussed in Clause 5). It outlines
how the protection given by SPDs may be affected by its installation. The different steps for
the selection of an SPD are presented including the problems of co-ordination when more
than one SPD is used in an installation (details about co-ordination may be found in Annex F).
Clause 7 is an introduction to the risk analysis (considerations of when the use of SPDs is
beneficial).
Clause 8 deals with co-ordination between signalling and power lines (under consideration).
Annex A gives examples of various SPD technologies
Annex B deals with explanations of testing procedures used in EN 1643-11
Annex C deals with the calculation of the sharing of lightning current between different
earthing systems.
Annex D provides specific examples on the use of this TS.
Annex E provides specific examples of the use of the risk analysis.
Annex F deals with consideration when Type ! SPDs are to be applied
Annex G discusses differences between immunity level and insulation withstand of
equipments
Annex H provides practical examples of SPD installation as used in some countries

List of variables and abbreviations used in this standard
List of variables
EMAX Maximum energy withstand
I Continuous operating current
c
I Follow current
f
I Follow current interrupting rating
fi
Iimp Impulse current for class I test
I Rated load current
L
Imax Maximum discharge current for class II test
I Nominal discharge current
n
Ip Prospective short circuit current of the power supply
Ipeak Current peak value of impulse current
I Residual current
PE
I Short-circuit current of the CWG
sc
N Ground flash density
g
N Keraunic level
k
U Maximum continuous operating voltage
c
U
Maximum continuous operating voltage of the power system
cs
Udyn Dynamic sparkover voltage of a gap
U Measured limiting voltage
m
U
Nominal voltage of the system phase to earth
n
U Line-to-neutral voltage of the system
U Open-circuit voltage for class III test
oc
U Voltage protection level
p
Uref Reference voltage of a varistor
Ures Residual voltage
U Temporary overvoltage
T
UTOV Temporary overvoltage of the power system
UTOV,HV Temporary overvoltage of the network inside the high-voltage system
UTOV,LV Temporary overvoltage of the network inside the low-voltage system
U Voltage withstand
W
– 9 – CLC/TS 61643-12:2006
List of abbreviations
CWG Combination wave generator
EMC Electromagnetic compatibility
GDT Gas discharge tubes
HV High voltage
IP Degrees of protection provided by the enclosure
L Inductance
LPS Lightning protection system
LPZ Lightning protection zone
LV Low voltage
MEB Main equipotential bonding
MOV Metal oxide varistor
HVA High voltage A (medium voltage, <50 kV), called sometimes improperly MV
PE Protective Earth
Q Charge of impulse current
RCD Residual current device
TOV Temporary overvoltage
SPD Surge protective device
ZnO Zinc oxide
LOW-VOLTAGE SURGE PROTECTIVE DEVICES –

Part 12: Surge protective devices connected to low-voltage
power distribution systems –
Selection and application principles

1 Scope
This part of IEC 61643 describes the principles for selection, operation, location and
co-ordination of SPDs to be connected to 50 Hz to 60 Hz a.c. power circuits and equipment
rated up to 1 000 V r.m.s.
NOTE 1 This technical specification deals only with SPDs and not with SPDs components integrated inside
equipment.
2 Normative references
See Annex ZA.
3 Definitions
For the purposes of this Technical Specification the following definitions apply.
NOTE These definitions are for the most part reproduced from EN 61643-11 (the definition number being
indicated within square brackets). Where necessary a note has been added for better understanding regarding
application of SPDs.
3.1
surge protective device
SPD
device that is intended to limit transient overvoltages and divert surge currents. It contains at
least one non-linear component
[definition 3.1 of EN 61643-11]
3.2
continuous operating current
I
c
current flowing through each mode of protection of the SPD when energized at the maximum
continuous operating voltage (U ) for each mode
c
3.3
maximum continuous operating voltage
U
c
maximum r.m.s. voltage which may be continuously applied to the SPD's mode of protection.
This is equal to the rated voltage
[definition 3.11 of EN 61643-11]
3.4
voltage protection level
U
p
parameter that characterizes the performance of the SPD in limiting the voltage across its
terminals, which is selected from a list of preferred values. This value is greater than the
highest value of the measured limiting voltages
[definition 3.15 of EN 61643-11]

– 11 – CLC/TS 61643-12:2006
3.5
measured limiting voltage
maximum magnitude of voltage that is measured across the terminals of the SPD during the
application of impulses of specified waveshape and amplitude
[definition 3.16 of EN 61643-11]
3.6
residual voltage
U
res
peak value of voltage that appears between the terminals of an SPD due to the passage of
discharge current
[definition 3.17 of EN 61643-11]
3.7
temporary overvoltage test value
U
T
test voltage applied for a specific duration, to the SPD to simulate the stress under TOV
conditions
NOTE 1 Adapted from 3.18 of EN 61643-11 by adding the following note.
NOTE 2 It is a characteristic declared by the manufacturer that gives information about the behaviour of the SPD
when stressed with voltages U above Uc for a given specific duration t (this behaviour may either be no change in
T T
the performance after application of the temporary overvoltage or a defined failure without hazard for either
personnel, equipment or facility)
3.8
temporary overvoltage of the network
U
TOV
power frequency overvoltage occurring on the network at a given location, of relatively long
duration. TOVs may be caused by faults inside the LV system (U ) or inside the HV
TOV,LV
system (U )
TOV,HV
NOTE Temporary overvoltages, typically lasting up to several seconds, usually originate from switching
operations or faults (for example, sudden load rejection, single-phase faults, etc.) and/or from non-linearity
(ferroresonance effects, harmonics, etc.)
3.9
nominal discharge current
I
n
crest value of the current through the SPD having a current waveshape of 8/20. This is used
for the classification of the SPD for class II test and also for preconditioning of the SPD
for class I and II tests
[definition 3.8 of EN 61643-11]
3.10
impulse current
I
imp
it is defined by three parameters, a current peak value Ipeak, a charge Q and a specific
energy W/R. Tested in accordance with the test sequence of the operating duty test. This is
used for the classification of the SPD for class I test
[definition 3.9 of EN 61643-11]
3.11
thermal runaway
operational condition when the sustained power dissipation of an SPD exceeds the thermal
dissipation capability of the housing and connections, leading to a cumulative increase in the
temperature of the internal elements culminating in failure
[definition 3.25 of EN 61643-11]

3.12
thermal stability
an SPD is thermally stable if after the operating duty test causing temperature rise, the
temperature of the SPD decreases with time when the SPD is energized at specified
maximum continuous operating voltage and at specified ambient temperature conditions
[definition 3.26 of EN 61643-11]
3.13
SPD disconnector
device (internal and/or external) required for disconnecting an SPD from the power system
NOTE This disconnecting device is not required to have isolating capability. It is to prevent a persistent fault on the
system and is used to give indication of the SPD failure.
There may be more than one disconnector function for example, an overcurrent protection function and a thermal
protection function. These functions may be integrated into one unit or performed in separate units.
[definition 3.29 of EN 61643-11]
3.14
short-circuit withstand
maximum prospective short-circuit current that the SPD is able to withstand
[definition 3.28 of EN 61643-11]
3.15
one-port SPD
SPD connected in shunt with the circuit to be protected. A one-port device may have separate
input and output terminals without a specified series impedance between these terminals
NOTE 1 Adapted from 3.2 of EN 61643-11 by adding the following note.
NOTE 2 Annex A shows some typical one-port SPDs and the generic drawing for a one-port SPD. A one-port SPD
may be connected in shunt, Figure A.1 a), or in line with the power supply. In the first case the load current is not
flowing through the SPD. In the second case, the load current is flowing through the SPD and the temperature rise
under load current and the associated maximum admissible load current may be determined as for a two-port SPD.
Figure A.3 shows the response of various types of one-port SPD to an 8/20 impulse applied via a combination
wave generator.
3.16
two-port SPD
SPD with two sets of terminals, input and output. A specific series impedance is inserted
between these terminals
NOTE 1 Adapted from 3.3 of EN 61643-11 by adding the following note.
NOTE 2 The measured limiting voltage may be higher at the input terminals than at the output terminals.
Therefore, equipment to be protected shall be connected to the output terminals. Figure A.2 shows typical two-port
SPDs. Figure A.3 shows the response of a two-port SPD to an 8/20 impulse applied via a combination wave
generator.
3.17
voltage switching type SPD
SPD that has a high impedance when no surge is present, but can have a sudden change in
impedance to a low value in response to a voltage surge. Common examples of components
used as voltage-switching devices are spark-gaps, gas discharge tubes (GDT), thyristors
(silicon-controlled rectifiers) and triacs. These SPDs are sometimes called "crowbar type"
NOTE 1 Adapted from 3.4 of EN 61643-11 by adding the following note.
NOTE 2 A voltage-switching device has a discontinuous U versus I characteristic. Figure 3c shows the response
of a typical voltage switching SPD to an impulse applied via a combination wave generator.

– 13 – CLC/TS 61643-12:2006
3.18
voltage limiting type SPD
SPD that has a high impedance when no surge is present, but will reduce it continuously with
increased surge current and voltage. Common examples of components used as non-linear
devices are: varistors and suppressor diodes. These SPDs are sometimes called "clamping
type"
NOTE 1 Adapted from 3.5 of EN 61643-11 by adding the following note.
NOTE 2 A voltage-limiting device has a continuous U versus I characteristic. Figure 3b shows the response of a
typical voltage-limiting SPD to an impulse applied via a combination wave generator.
3.19
combination type SPD
SPD that incorporates both voltage switching type components and voltage limiting type
components may exhibit voltage-switching, voltage-limiting, or both voltage-switching and
voltage-limiting behaviour depending upon the characteristics of the applied voltage
NOTE 1 Adapted from 3.6 of EN 61643-11 by adding the following note.
NOTE 2 Figure A.3 shows the response of various typical combination type SPDs to a combination wave impulse.
3.20
modes of protection
SPD protective components may be connected line to line or line to earth or line to neutral or
neutral to earth and combination thereof. These paths are referred to as modes of protection
[definition 3.7 of EN 61643-11]
3.21
follow current
I
f
current supplied by the electrical power system and flowing through the SPD after a discharge
current impulse. The follow current is significantly different from the continuous operating
current (I )
c
[definition 3.13 of EN 61643-11]
3.22
maximum discharge current for class II test
(I )
max
crest value of a current through the SPD having an 8/20 waveshape and magnitude in
accordance with the test sequence of the class II operating duty test. I is greater than I
max n
[definition 3.10 of EN 61643-11]
3.23
degradation
change of original performance parameters as a result of exposure of the SPD to surge,
service or unfavourable environment
NOTE 1 Adapted from 3.27 of EN 61643-11 by adding the following note.
NOTE 2 Degradation is a measure of the ability of an SPD to withstand the conditions for which it is designed
throughout its service life. Two type tests are applied to provide confidence with respect to degradation. The first
one is the operating duty test and the second is the ageing test. However, these two tests may be combined.
The operating duty test is conducted by applying a specified number of defined current waveshapes to the SPD.
Permitted changes in the SPD characteristics are given in EN 61643-11.
The ageing test is carried out at a specified temperature with a voltage of specified magnitude and duration applied
to the SPD. Permitted changes in the SPD characteristics are given in this standard (this test is under
consideration).
This can be used to determine the SPD prospective installed life which should also consider the following:
– replacement policy;
– location and accessibility;
– acceptable failure rate;
– operating practices.
3.24
residual current device
RCD
mechanical switching device or association of devices intended to cause the opening of the
contacts when the residual or unbalanced current attains a given value under specified
conditions
[definition 3.37 of EN 61643-11]
3.25
nominal voltage of the system phase to earth
U
n
voltage by which a system or equipment is designated and to which certain operating
characteristics are referred (for example, 230/400 V).
Under normal system conditions, the voltage at the supply terminals may differ from the
nominal voltage as determined by the tolerances of the supply systems
3.26
line to neutral voltage of the system
U
o
line to neutral voltage (r.m.s. value of the a.c. voltage) of the system, derived from the
nominal system voltage (the voltage by which the system is designated).
[definition 3.46 of EN 61643-11]
3.27
rated load current (I )
L
maximum continuous rated r.m.s. current that can be supplied to a load connected to the
protected output of an SPD
NOTE 1 Adapted from 3.14 of EN 61643-11 by adding the following note.
NOTE 2 This is only relevant to SPD(s) having separate input and output terminals.
3.28
overcurrent protection
overcurrent device (e.g. circuit breaker or fuse), which could be part of the electrical
installation located externally up-stream of the SPD
[definition 3.36 of EN 61643-11]
3.29
maximum continuous operating voltage of the power system at the SPD location
U
cs
maximum r.m.s. voltage to which the SPD may be permanently subjected at the point of
application of the SPD.
NOTE 1 Adapted from Clause 3.49 EN 61643-11:2001 by adding the following notes
NOTE 2 This takes into account only voltage regulation and/or voltage drop or increase. It is also called actual
maximum system voltage (see Figure 6) and is directly linked to U
NOTE 3 This voltage does not take into account harmonics, faults, TOVs or transient conditions.
3.30
sparkover voltage of a voltage-switching SPD
maximum voltage value before disruptive discharge between the electrodes of the gap of a
SPD
NOTE 1 Adapted from 3.38 of EN 61643-11 by adding the following note.
NOTE 2 A voltage-switching SPD may be based on components other than gaps (for example, silicon-based
components).
– 15 – CLC/TS 61643-12:2006
3.31
lightning protection system
LPS
complete system used to protect a structure and its contents against the effects of lightning
3.32
multiservice SPD
a surge protective device providing protection for two or more services such as power,
telecommunication and signalling in a single enclosure in which a reference bond is provided
between services during surge conditions.
3.33
residual current
I
PE
current flowing through the PE terminal, when the SPD is energized at the maximum
continuous operating voltage (U ) when connected in accordance with the manufacturer
c
instructions
[definition 3.42 of EN 61643-11]
3.34
prospective short-circuit current of a power supply
I
p
current which would flow at a given location in a circuit if it were short-circuited at that
location by a link of negligible impedance
[definition 3.40 of EN 61643-11]
3.35
follow current interrupting rating
I
fi
prospective short-circuit current that an SPD is able to interrupt by itself
[definition 3.41 of EN 61643-11]
4 Systems and equipment to be protected
When evaluating an installation with regard to the use of an SPD, two factors need to be
considered:
• the characteristics of the low-voltage power distribution system on which it will be used,
including expected types and levels of overvoltage and current;
• the characteristics of the equipment requiring protection.
4.1 Low-voltage power distribution systems
Low-voltage power distribution systems are basically characterized by the type of system
earthing (TNC, TNS, TNC-S, TT, IT) and the nominal voltage (see 3.35). Various types of
overvoltages and currents may occur. In this standard, the overvoltages are classified into
three groups:
• lightning;
• switching;
• temporary overvoltages.
4.1.1 Lightning overvoltages and currents
In most cases lightning stress is the main factor for the selection of an SPD’s class of test and
associated current or voltage values (I , I or U , in accordance with EN 61643-11).
imp max oc
Evaluation of the waveshape and current (or voltage) amplitude of the lightning surges is
necessary for the proper selection of an SPD. It is important to determine if the voltage
protection level of the SPD will be adequate to protect the equipment in such circumstances.
NOTE For example, areas prone to frequent lightning strikes may require an SPD suitable to withstand class I or
class II tests.
Generally (for example, in the case of direct strike to the lines or induced surges on the lines),
higher stresses occur on the electrical installation external to the structure. Within the
structure, the stresses are decreased when moving from the installation’s entrance to internal
circuits. The decrease is due to the change of circuit configuration and impedances.
The need for protection against lightning surges depends on
• the local ground flash density N (average annual ground flash density, in lightning flashes per
g
km per year, concerning the region where the structure is located). Modern lightning location
systems can provide information on N with reasonable accuracy;
g
• the exposure of the electrical installation, including incoming services. Underground systems
are generally considered to be less exposed than overhead systems.
• Even if the supply is provided by an underground cable, the use of an SPD may be recom-
mended to provide protection. To determine if surge protection is needed, the following are
some items that should be considered:
– the installation has a lightning protection system in its vicinity;
– the length of the cable is not sufficient to provide adequate separation (attenuation)
of the installation from the overhead part of the network;
– high surges of atmospheric origin can be expected on the overhead line supplying the
HV (high voltage) side of the transformer connected to the installation;
– the underground cable may be affected by direct lightning in the presence of high soil
resistivity;
– the size or height of the building powered by the cable is large enough to significantly
increase the risk for direct strikes to the building. The risk for direct strikes to other
incoming (outgoing) services (telephone lines, antenna systems, etc.) that may affect
the power system and equipment;
– other overhead services are present.
When many buildings are supplied from a single supply system, those buildings which do not
have SPDs may have high stresses on their electrical systems.
For SPD installations in a structure which is equipped with an external lightning protection
system, it is (in case of direct lightning to the structure) generally sufficient to make
calculations using earthing d.c. resistance readings (for example, earthing of the building and
power distribution system, pipes, etc), to determine the current distribution through the SPDs.
4.1.2 Switching overvoltages
These stresses, in terms of peak current and voltage, are usually lower than lightning
stresses but may have longer duration. However, in some cases, particularly deep inside a
structure or close to switching overvoltage sources, the switching stress can be higher than
the stresses caused by lightning. The energy related to these switching surges needs to be
known to permit the choice of appropriate SPDs. The time duration of the switching surges,
including transients due to faults and fuse operations, can be much longer than the lightning
surge duration.
– 17 – CLC/TS 61643-12:2006
4.1.3 Temporary overvoltages U
TOV
4.1.3.1 General
Any SPD can be exposed to a temporary overvoltage U during its lifetime that exceeds the
TOV
maximum continuous operating voltage of the power system.
A temporary overvoltage has two dimensions, magnitude and time. The time duration of the
overvoltage primarily depends upon the earthing of the supply system (this includes both the
high-voltage supply system as well as the low-voltage system to which the SPD is connected).
In determining the temporary overvoltages, consideration should be given to the maximum
continuous operating voltage of the power system (U ).
cs
4.1.3.2 Standardized values
IEC 60364-4-44 gives the maximum values of U to be expected in low-voltage power
TOV
systems (for a more detailed calculation of these values, refer to Annex E).
Lower values are possible depending on many factors such as the location of the SPD,
the type of power systems, etc.
The maximum values given in Table 1 are at the consumer installation (for transformer
location see Table 1, Note 2).
Table 1 – Maximum TOV values as given in IEC 60634-4-44
Occurrence of U System Maximum values for U
TOV TOV,HV
U + 250 V for duration >5 s
Between phase and earth TT, IT
U + 1 200 V for duration up to 5 s
250 V for duration >5 s
Between neutral and earth TT, IT
1 200 V for duration up to 5 s
The above values are extreme values related to faults in the high-voltage power systems and may be calculated
depending on the type of power systems in accordance with Annex E.
Occurrence of U System Maximum values for U
TOV TOV
Between phase and neutral TT and TN √3 × U
The above value is related to a loss of the neutral conductor in the low-voltage system.
Between phase and earth IT system (TT √3 × U
system:
see Note 1)
The above value is related to accidental earthing of the phase conductor in the low-voltage system.
Between phase and neutral TT, IT and TN
1,45 × U for a duration up to 5 s
The above value is related to short circuit between a line conductor and the neutral conductor.
NOTE 1 It has been demonstrated that such high TOVs can also occur in TT systems for durations up to 5 s.
See Annex E for more details. This is not addressed in IEC 60364-4-44.
NOTE 2 Maximum TOV values at the transformer location may be different from the table above (higher or
lower). See Annex E for more details.
NOTE 3 Loss of neutral is not used for selection of SPDs.

4.2 Characteristics of the equipment to be protected
Characteristics of the equipment to be protected under transient conditions are determined by
two test methods, they are:
• The impulse withstand of the equipment tested in accordance with IEC 60664-1. This is only
an insulation co-ordination test. During the test the equipment is unenergized.
• The impulse immunity of the equipment tested in accordance with IEC 61000-4-5. This test
evaluates the equipment’s operational immunity capabilities. The test is mainly performed with
a combination wave generator (1,2/50, 8/20) at different levels. It determines where a
malfunction, error or failure may occur during energized operation.
A comparison of the impulse withstand and impulse immunity levels with respect to the
transient environment where the equipment is to be used, determines the potential need for
SPDs. For more information see Annex G.
NOTE The selected SPDs should provide a protective level Up lower than the impulse withstand capability of the
equipment or, in some cases where the continuous operation of the equipment is critical, lower than the impulse
immunity of the equipment. Up should be selected in accordance with 6.2.2 and 6.2.5. In addition, due to possible
reaction between the equipment under test and the generator, the immunity of the equipment is a function not only
of Up but of the waveshape of the applied surge.
5 Surge protective devices
5.1 Basic functions of SPDs
The SPDs considered in this standard are those installed external to the equipment to be
protected.
Their function can be described as follows.
• In power systems in the absence of surges: the SPD shall not have a significant influence on
the operational characteristics of the system to which it is applied.
• In power systems during the occurrence of surges: the SPD responds to surges by lowering
its impedance and thus diverting surge current through it to limit the voltage to its protective
level. The surges may initiate a power follow current through the SPD.
• In power systems after the occurrence o
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