HD 60364-4-442:2012

SLOVENSKI STANDARD
01-marec-2012
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Low-voltage electrical installations - Part 4-442: Protection for safety - Protection of low-
voltage installations against temporary overvoltages due to earth faults in the high-
voltage system and due to faults in the low voltage system
Errichten von Niederspannungsanlagen - Teil 4-442: Schutzmaßnahmen - Schutz von
Niederspannungsanlagen bei vorübergehenden Überspannungen infolge von
Erdschlüssen im Hochspannungssystem und infolge von Fehlern im
Niederspannungssystem
Installations électriques à basse tension - Partie 4-442: Protection pour assurer la
sécurité - Protection des installations électriques à basse tension contre les surtensions
temporaires dues à des défauts à la terre dans le réseau haute tension et dues à des
défauts dans le réseau basse tension
Ta slovenski standard je istoveten z: HD 60364-4-442:2012
ICS:
91.140.50 Sistemi za oskrbo z elektriko Electricity supply systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

HARMONIZATION DOCUMENT
HD 60364-4-442
DOCUMENT D'HARMONISATION
January 2012
HARMONISIERUNGSDOKUMENT
ICS 91.140.50
English version
Low-voltage electrical installations -
Part 4-442: Protection for safety -
Protection of low-voltage installations against temporary overvoltages
due to earth faults in the high-voltage system and due to faults in the low
voltage system
(IEC 60364-4-44:2007 (CLAUSE 442), modified)

Installations électriques à basse tension - Errichten von Niederspannungsanlagen -
Partie 4-442: Protection pour assurer la Teil 4-442: Schutzmaßnahmen -
sécurité - Schutz von Niederspannungsanlagen bei
Protection des installations électriques à vorübergehenden Überspannungen
basse tension contre les surtensions infolge von Erdschlüssen im
temporaires dues à des défauts à la terre Hochspannungssystem und infolge von
dans le réseau haute tension et dues à Fehlern im Niederspannungssystem
des défauts dans le réseau basse tension (IEC 60364-4-44:2007 (CLAUSE 442),
(CEI 60364-4-44:2007 (CLAUSE 442), modifiziert)
modifiée)
This Harmonization Document was approved by CENELEC on 2011-12-07. CENELEC members are bound to
comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for implementation of this
Harmonization Document at national level.

Up-to-date lists and bibliographical references concerning such national implementations may be obtained on
application to the CEN-CENELEC Management Centre or to any CENELEC member.

This Harmonization Document exists in three official versions (English, French, German).

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, 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, Turkey and the United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2012 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. HD 60364-4-442:2012 E

Foreword
This document (HD 60364-4-442:2012) consists of the text of IEC 60364-4-44:2007 prepared by IEC/TC 64,
"Electrical installations and protection against electric shock", together with the common modifications
prepared by CLC/TC 64 "Electrical installations and protection against electric shock".
The following dates are fixed:
– latest date by which the HD has to be implemented

at national level by publication of a harmonized

national standard or by endorsement
(dop) 2012-12-07
– latest date by which the national standards conflicting

with the HD have to be withdrawn
(dow) 2014-12-07
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent rights.
__________
Endorsement notice
The text of the International Standard IEC 60364-4-44:2007 was approved by CENELEC as a Harmonization
document with common modifications.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60364-5-51:2005 NOTE  Harmonized as HD 60364-5-51:2009 (modified).
IEC 61000-2 series NOTE  Harmonized in EN 61000-2 series (not modified).
IEC 61000-5 series NOTE  Harmonized in EN 61000-5 series (not modified).
IEC 61386 series NOTE  Harmonized in EN 61386 series (not modified).
IEC 61663-1 NOTE  Harmonized as EN 61663-1.
IEC 62020:1998 NOTE  Harmonized as EN 62020:1998 (not modified).
COMMON MODIFICATIONS
Title:
Replace the title of HD 60364-4-442 as follows:
Low-voltage electrical installations - Part 4-442: Protection for safety - Protection of low-voltage installations
against temporary overvoltages due to earth faults in the high-voltage system and due to faults in the low
voltage system
Table 44.A1 - Power-frequency stress voltages and power-frequency fault voltage in low-voltage
system
In the footnote**) delete “second paragraph”.
442.2.1 Magnitude and duration of power-frequency fault voltage

- 3 - HD 60364-4-442:2012
Replace 442.2.1 and its contents as follows:
442.2.1 Power-frequency fault voltage
The fault voltage U as calculated in Table 44.A1 which appears in the LV installation between exposed
f
conductive parts and earth shall not exceed a dangerous level, see HD 60364-4-41.
In a TN system where R and R are connected together (see Table 44.A1), their connection to a L.V. global
E B
earthing system as described in EN 50522:2010, 3.4.19 can then be considered to be a safety measure
against dangerous fault voltages.
NOTE 1  A global earthing system is an earthing system created by the interconnection of local earthing systems that ensures, by the
proximity of these earthing systems, that there are no dangerous touch voltages.
Such systems cause the division of the earth fault current in a way that results in a reduction of the earth
potential rise at the local earthing system. Such a system could be said to form a quasi equipotential surface.
In installations outside a global earthing system, the PEN conductor shall be earthed additionally at least one
more time.
NOTE 2  The existence of a global earthing system may be determined by sample measurements or calculation for typical systems.
Typical examples of global earthing systems are in city centres; urban or industrial areas with distributed low- and high-voltage earthing.

Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.
Publication Year Title EN/HD Year
- - Earthing of power installations exceeding EN 50522 2010
1 Kv a.c.
IEC 60038 1983 Nominal voltages for low-voltage public HD 472 S1 1989
(mod) electricity supply systems + corr. February 2002
+ A1 1995
IEC 60050-604 1987 International Electrotechnical Vocabulary (IEV) - -
- Chapter 604: Generation, transmission and
distribution of electricity - Operation

IEC 60364-1 - Low-voltage electrical installations - HD 60364-1 -
Part 1: Fundamental principles, assessment of
general characteristics, definitions

IEC 60364-4-41 2005 Low-voltage electrical installations - HD 60364-4-41 2007
(mod) Part 4-41: Protection for safety - Protection + corr. July 2007
against electric shock
IEC 60364-5-54 2002 Electrical installations of buildings - HD 60364-5-54 2007
(mod) Part 5-54: Selection and erection of electrical
equipment - Earthing arrangements, protective
conductors and protective bonding conductors

IEC/TS 60479-1 2005 Effects of current on human beings and - -
livestock -
Part 1: General aspects
IEC 60664-1 2007 Insulation coordination for equipment within EN 60664-1 2007
low-voltage systems -
Part 1: Principles, requirements and tests

IEC 60950-1 - Information technology equipment - Safety - EN 60950-1 -
Part 1: General requirements
- 5 - HD 60364-4-442:2012
Publication Year Title EN/HD Year
IEC/TS 61000-2-5 1995 Electromagnetic compatibility (EMC) -
Part 2-5: Environment - Classification of
electromagnetic environments - Basic EMC
publication
1)
IEC 61000-6-1 - Electromagnetic compatibility (EMC) - EN 61000-6-1 2007
Part 6-1: Generic standards - Immunity for
residential, commercial and light-industrial
environments
1)
IEC 61000-6-2 - Electromagnetic compatibility (EMC) - EN 61000-6-2
Part 6-2: Generic standards - Immunity for + corr. September
industrial environments
1)
IEC 61000-6-3 - Electromagnetic compatibility (EMC) - EN 61000-6-3
Part 6-3: Generic standards - Emission
standard for residential, commercial and light-
industrial environments
IEC 61000-6-4 - Electromagnetic compatibility (EMC) - EN 61000-6-4 -
Part 6-4: Generic standards - Emission
standard for industrial environments

IEC 61558-2-1 - Safety of power transformers, power supplies, EN 61558-2-1 -
reactors and similar products -
Part 2-1: Particular requirements and tests for
separating transformers and power supplies
incorporating separating transformers for
general applications
IEC 61558-2-4 - Safety of transformers, reactors, power supply EN 61558-2-4 -
units and similar products for supply voltages
up to 1 100 V -
Part 2-4: Particular requirements and tests for
isolating transformers and power supply units
incorporating isolating transformers

IEC 61558-2-6 - Safety of transformers, reactors, power supply EN 61558-2-6 -
units and similar products for supply voltages
up to 1 100 V -
Part 2-6: Particular requirements and tests for
safety isolating transformers and power supply
units incorporating safety isolating transformers

———————
1)
Dated reference in accordance with CLC Guide 25.

Publication Year Title EN/HD Year
IEC 61558-2-15 - Safety of power transformers, power supply EN 61558-2-15 -
units and similar -
Part 2-15: Particular requirements for isolating
transformers for the supply of medical locations

IEC 61643 series Low-voltage surge protective devices EN 61643/ series
CLC/TS 61643
IEC 61936-1 - Power installations exceeding 1 kV a.c. - EN 61936-1 -
Part 1: Common rules
IEC 62305-1 - Protection against lightning - EN 62305-1 -
Part 1: General principles
IEC 62305-3 - Protection against lightning - EN 62305-3 -
Part 3: Physical damage to structures and life
hazard
IEC 62305-4 - Protection against lightning - EN 62305-4 -
Part 4: Electrical and electronic systems within
structures
- 7 - HD 60364-4-442:2012
Annex ZB
(normative)
Special national conditions
Special national condition: National characteristic or practice that cannot be changed even over a long
period, e.g. climatic conditions, electrical earthing conditions.
NOTE  If it affects harmonization, it forms part of the Harmonization Document.
For the countries in which the relevant special national conditions apply these provisions are normative, for
other countries they are informative.
Clause Special national condition
442.2 Italy
Table 44.A1
In Italy the permissible power-frequency stress voltage on equipment in low-voltage
installations in case of a duration of the earth fault in the high-voltage system ≤ 5 s
is U + 500 V.
o
__________
IEC 60364-4-44
Edition 2.0 2007-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Low-voltage electrical installations –
Part 4-44: Protection for safety – Protection against voltage disturbances and
electromagnetic disturbances
Installations électriques à basse tension –
Partie 4-44: Protection pour assurer la sécurité – Protection contre les
perturbations de tension et les perturbations électromagnétiques

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
XA
CODE PRIX
ICS 91.140.50 ISBN 2-8318-9261-9

– 2 – 60364-4-44 © IEC:2007
CONTENTS
FOREWORD.5
INTRODUCTION.7

440.1 Scope .8
440.2 Normative references.8
441 (Vacant).9
442 Protection of low-voltage installations against temporary overvoltages due to earth
faults in the high-voltage system and due to faults in the low-voltage system .
442.1 Scope and object .9
442.1.1 General.9
442.1.2 Symbols .10
442.2 Overvoltages in LV-systems during a high-voltage earth fault .11
442.2.1 Magnitude and duration of power frequency fault-voltage .12
442.2.2 Magnitude and duration of power frequency stress-voltages.13
442.2.3 Requirements for calculation of limits .14
442.3 Power frequency stress voltage in case of loss of the neutral conductor in a
TN and TT system .14
442.4 Power frequency stress voltage in the event of an earth fault in an IT system
with distributed neutral .14
442.5 Power frequency stress voltage in the event of short-circuit between a line
conductor and the neutral conductor .14
443 Protection against overvoltages of atmospheric origin or due to switching.14
443.1 General .14
443.2 Classification of impulse withstand voltages (overvoltage categories) .15
443.2.1 Purpose of classification of impulse withstand voltages
(overvoltage categories) .15
443.2.2 Relationship between impulse withstand voltages of equipment and
overvoltage categories .15
443.3 Arrangements for overvoltage control.16
443.3.1 Inherent overvoltage control .16
443.3.2 Protective overvoltage control.16
443.4 Required impulse withstand voltage of equipment.18
444 Measures against electromagnetic influences.18
444.1 General .18
444.2 (void) .19
444.3 Definitions .19
444.4 Mitigation of Electromagnetic Interference (EMI) .20
444.4.1 Sources of EMI.20
444.4.2 Measures to reduce EMI.21
444.4.3 TN-system .22
444.4.4 TT system .26
444.4.5 IT system .27
444.4.6 Multiple-source supply.28
444.4.7 Transfer of supply .31
444.4.8 Services entering a building.33
444.4.9 Separate buildings.34
444.4.10 Inside buildings .34
444.4.11 Protective devices .36

60364-4-44 © IEC:2007 – 3 –
444.4.12 Signal cables.36
444.5 Earthing and equipotential bonding.36
444.5.1 Interconnection of earth electrodes.36
444.5.2 Interconnection of incoming networks and earthing arrangements.37
444.5.3 Different structures for the network of equipotential conductors and
earthing conductors.37
444.5.4 Equipotential bonding networks in buildings with several floors .39
444.5.5 Functional earthing conductor.40
444.5.6 Commercial or industrial buildings containing significant amounts
of information technology equipment.41
444.5.7 Earthing arrangements and equipotential bonding of information
technology installations for functional purposes .41
444.6 Segregation of circuits .42
444.6.1 General.42
444.6.2 Design guidelines .42
444.6.3 Installation guidelines .43
444.7 Cable management systems .44
444.7.1 General.44
444.7.2 Design guidelines .44
444.7.3 Installation guidelines .45
445 Protection against undervoltage .47
445.1 General requirements .47

Annex A (informative) Explanatory notes concerning 442.1 and 442.2.48
Annex B (informative) Guidance for overvoltage control by SPDs applied to overhead
lines.50
Annex C (normative) Determination of the conventional length, d .52

Bibliography .54

Figure 44.A1 – Representative schematic sketch for possible connections to earth in
substation and LV-Installation and occurring overvoltages in case of faults .11
Figure 44.A2 – Tolerable fault-voltage due to an earth-fault in the HV system .13
Figure 44.R1 − By-pass conductor for screen reinforcement to provide a common
equipotential bonding system .21
Figure 44.R2 − Example of a substitute or by-pass equipotential bonding conductor in a
TT-system.22
Figure 44.R3A − Avoidance of neutral conductor currents in a bonded structure by
using the TN-S system from the origin of the public supply up to and including the final
circuit within a building .23
Figure 44.R3B − Avoidance of neutral conductor currents in a bonded structure by using
a TN-S system downstream of a consumer’s private supply transformer .24
Figure 44.R4 − TN-C-S system within an existing building installation .25
Figure 44.R5 – TT system within a building installation .26
Figure 44.R6 – IT system within a building installation .27
Figure 44.R7A – TN multiple-source power supply with a non-suitable multiple
connection between PEN and earth .28
Figure 44.R7B – TN multiple source power supplies to an installation with connection to
earth of the star points at one and the same point .29

– 4 – 60364-4-44 © IEC:2007
Figure 44.R8 – TT multiple-source power supplies to an installation with connection to
earth of the star points at one and the same point .30
Figure 44.R9A − Three-phase alternative power supply with a 4-pole switch .31
Figure 44.R9B – Neutral current flow in a three-phase alternative power supply with an
unsuitable 3-pole switch .32
Figure 44.R9C − Single-phase alternative power supply with 2-pole switch.33
Figure 44.R10 − Armoured cables and metal pipes entering the buildings (examples) .34
Figure 44.R11 − Illustration of measures in an existing building .35
Figure 44.R12 – Interconnected earth electrodes.36
Figure 44.R13 – Examples of protective conductors in star network .37
Figure 44.R14 – Example of multiple meshed bonding star network .38
Figure 44.R15 – Example of a common meshed bonding star network .39
Figure 44.R16 – Example of equipotential bonding networks in structures without
lightning protection systems .40
Figure 44.R17A − Separation between power and information technology cables for
cable route lengths ≤ 35 m .43
Figure 44.R17B − Separation between power and information technology cables for
cable route lengths > 35 m .43
Figure 44.R18 – Separation of cables in wiring systems.44
Figure 44.R19 – Cable arrangements in metal cable-trays .45
Figure 44.R20 – Continuity of metallic system components .46
Figure 44.R21 – Location of cables inside metallic construction elements .46
Figure 44.R22 – Connection of metallic sections.47
Figure 44.Q – Examples of how to apply d , d and d for the determination of d .53
1 2 3
Table 44.A1 – Power frequency stress voltages and power frequency fault voltage in the
low voltage system .12
Table 44.A2 – Permissible power frequency stress-voltage.13
Table 44.B – Required rated impulse withstand voltage of equipment .18
Table B.1 – Different possibilities for IT systems.51

60364-4-44 © IEC:2007 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
_______________
LOW-VOLTAGE ELECTRICAL INSTALLATIONS –

Part 4-44: Protection for safety –
Protection against voltage disturbances and
electromagnetic disturbances
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60364-4-44 has been prepared by IEC technical committee 64:
Electrical installations and protection against electric shock.
This second edition of IEC 60364-4-44 cancels and replaces the first edition published in 2001,
amendment 1 (2003) and amendment 2 (2006).
The document 64/1600/FDIS, circulated to the National Committees as Amendment 3, led to
the publication of the new edition.

– 6 – 60364-4-44 © IEC:2007
The text of this standard is based on the first edition, its Amendment 1, Amendment 2 and the
following documents:
FDIS Report on voting
64/1600/FDIS 64/1609/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all the parts in the IEC 60364 series, under the general title Low-voltage electrical
installations, can be found on the IEC website.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.
The committee has decided that the contents of this publication will remain unchanged until the
maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
60364-4-44 © IEC:2007 – 7 –
INTRODUCTION
Part 4-44 of IEC 60364 covers the protection of electrical installations and measures against
voltage disturbances and electromagnetic disturbances.
The requirements are arranged into four clauses as follows:
Clause 442 Protection of low-voltage installations against temporary overvoltages due to
earth faults in the high-voltage system and due to faults in the low-voltage
system
Clause 443 Protection against overvoltages of atmospheric origin or due to switching
Clause 444 Measures against electromagnetic influences
Clause 445 Protection against undervoltage

– 8 – 60364-4-44 © IEC:2007
LOW-VOLTAGE ELECTRICAL INSTALLATIONS –

Part 4-44: Protection for safety –
Protection against voltage disturbances and
electromagnetic disturbances
440.1 Scope
The rules of this Part of IEC 60364 are intended to provide requirements for the safety of
electrical installations in the event of voltage disturbances and electromagnetic disturbances
generated for different specified reasons.
The rules of this part are not intended to apply to systems for distribution of energy to the
public, or power generation and transmission for such systems (see the scope of IEC 60364-1)
although such disturbances may be conducted into or between electrical installations via these
supply systems.
440.2 Normative references
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
IEC 60038:1983, IEC standard voltages
IEC 60050-604:1987, International Electrotechnical Vocabulary – Chapter 604: Generation,
transmission and distribution of electricity – Operation
IEC 60364-1, Low-voltage electrical installations – Part 1: Fundamental principles, assessment
of general characteristics, definitions
IEC 60364-4-41:2005, Electrical installations of buildings – Part 4-41: Protection for safety –
Protection against electric shock
IEC 60364-5-54:2002, Electrical installations of buildings − Part 5-54: Selection and erection of
electrical equipment – Earthing arrangements and protective bonding conductors
IEC 60479-1:2005, Effects of current on human beings and livestock – Part 1: General aspects
IEC 60664-1:2007, Insulation co-ordination for equipment within low-voltage systems – Part 1:
Principles, requirements and tests
IEC 60950-1, Information technology equipment – Safety – Part 1: General requirements
IEC 61000-2-5:1995, Electromagnetic compatibility (EMC) − Part 2: Environment − Section 5:
Classification of electromagnetic environments – Basic EMC publication
IEC 61000-6-1, Electromagnetic compatibility (EMC) – Part 6-1: Generic standards – Immunity
for residential, commercial and light-industrial environments
IEC 61000-6-2, Electromagnetic compatibility (EMC) – Part 6-2: Generic standards – Immunity
for industrial environments
___________
A third edition is currently in preparation.

60364-4-44 © IEC:2007 – 9 –
IEC 61000-6-3, Electromagnetic compatibility (EMC) – Part 6-3: Generic standards – Emission
standard for residential, commercial and light-industrial environments
IEC 61000-6-4, Electromagnetic compatibility (EMC) – Part 6-4: Generic standards – Emission
standard for industrial environments

IEC 61558-2-1, Safety of power transformers, power supplies, reactors and similar products –
Part 2-1: Particular requirements for tests for separating transformers and power supplies
incorporating separating transformers for general applications
IEC 61558-2-4, Safety of power transformers, power supply units and similar – Part 2-4:
Particular requirements for isolating transformers for general use
IEC 61558-2-6, Safety of power transformers, power supply units and similar – Part 2-6:
Particular requirements for safety isolating transformers for general use
IEC 61558-2-15, Safety of power transformers, power supply units and similar – Part 2-15:
Particular requirements for isolating transformers for the supply of medical locations
IEC 61643 (all parts), Low-voltage surge protective devices
IEC 61936-1, Power installations exceeding 1 kV a.c. – Part 1: Common rules
IEC 62305-1, Protection against lightning – Part 1: General principles
IEC 62305-3, Protection against lightning – Part 3: Physical damage to structures and life
hazard
IEC 62305-4, Protection against lightning – Part 4: Electrical and electronic systems within
structures
441 (Vacant)
442 Protection of low-voltage installations against temporary overvoltages due
to earth faults in the high-voltage system and due to faults in the low-
voltage system
442.1 Field of application
The rules of this clause provide requirements for the safety of low-voltage installation in the
event of
– a fault between the high-voltage system and earth in the transformer substation that
supplies the low-voltage installation,
– a loss of the supply neutral in the low-voltage system,
– a short-circuit between a line conductor and neutral,
– an accidental earthing of a line conductor of a low-voltage IT-system.
The requirements for the earthing arrangement at the transformer substation are given in
IEC 61936-1.
442.1.1 General requirements
As Clause 442 covers faults between a high-voltage line and the earth in the HV/LV substation,
it gives rules for the designer and installer of the substation. It is necessary to have the
following information concerning the high-voltage system:
– quality of the system earthing;

– 10 – 60364-4-44 © IEC:2007
– maximum level of earth fault current;
– resistance of the earthing arrangement.
The following subclauses consider four situations as proposed in 442.1, which generally cause
the most severe temporary overvoltages such as defined in IEC 60050-604:
– fault between the high-voltage system(s) and earth (see 442.2);
– loss of the neutral in a low-voltage system (see 442.3);
– accidental earthing of a low-voltage IT system (see 442.4);
– short-circuit in the low-voltage installation (see 442.5).
442.1.2 Symbols
In Clause 442 the following symbols are used (see Figure 44.A1):
I part of the earth fault current in the high-voltage system that flows through the earthing
E
arrangement of the transformer substation.
resistance of the earthing arrangement of the transformer substation.
R
E
R resistance of the earthing arrangement of the exposed-conductive-parts of the
A
equipment of the low-voltage installation.
R resistance of the earthing arrangement of the low-voltage system neutral, for low-
B
voltage systems in which the earthing arrangements of the transformer substation and
of the low-voltage system neutral are electrically independent.
U in TN- and TT-systems: nominal a.c. r.m.s. line voltage to earth
o
in IT-systems: nominal a.c. voltage between line conductor and neutral conductor or
mid point conductor, as appropriate
U power-frequency fault voltage that appears in the low-voltage system between exposed-
f
conductive-parts and earth for the duration of the fault.
power-frequency stress voltage between the line conductor and the exposed-
U
conductive-parts of the low-voltage equipment of the transformer substation during the
fault.
U power-frequency stress voltage between the line conductor and the exposed-
conductive-parts of the low-voltage equipment of the low-voltage installation during the
fault.
NOTE 1 The power-frequency stress voltage (U and U ) is the voltage that appears across the insulation of low-
1 2
voltage equipment and across surge protective devices connected to the low-voltage system.
The following additional symbols are used in respect of IT-systems in which the exposed-
conductive-parts of the equipment of the low-voltage installation are connected to an earthing
arrangement that is electrically independent of the earthing arrangement of the transformer
substation.
I fault current that flows through the earthing arrangement of the exposed-conductive-
h
parts of the equipment of the low-voltage installation during a period when there is a
high-voltage fault and a first fault in the low-voltage installation (see Table 44.A1).
I fault current, in accordance with 411.6.2, that flows through the earthing arrangement
d
of the exposed-conductive-parts of the low-voltage installation during the first fault in a
low-voltage system (see Table 44.A1).

60364-4-44 © IEC:2007 – 11 –
Z impedance (e.g. IMD internal impedance, artificial neutral impedance) between the low-
voltage system and an earthing arrangement.
NOTE 2 An earthing arrangement may be considered electrically independent of another earthing arrangement if a
rise of potential with respect to earth in one earthing arrangement does not cause an unacceptable rise of potential
with respect to earth in the other earthing arrangement. See IEC 61936-1.
442.2 Overvoltages in LV-systems during a high-voltage earth fault
In case of a fault to earth on the HV-side of the substation, the following types of overvoltage
may affect the LV-installation:
• power frequency fault-voltage (U );
f
• power frequency stress-voltages (U and U ).
1 2
Table 44.A1 provides the relevant methods of calculation for the different types of
overvoltages.
NOTE 1 Table 44.A1 deals with IT systems with a neutral point only. For IT systems with no neutral point, the
formulae should be adjusted accordingly.

Substation
U LV Installation
U
HV LV
L1
L2
L3
or N or PEN
Z
I R R R U
E E B A f
IEC  1379/07
Figure 44.A1 – Representative schematic sketch for possible connections to earth
in substation and LV-installation and occurring overvoltages in case of faults
Where high- and low-voltage earthing systems exist in proximity to each other, two practices
are presently used:
– interconnection of all high-voltage (R ) and low-voltage (R ) earthing systems;
E B
– separation of high-voltage (R ) from low-voltage (R ) earthing systems.
E B
The general method used is interconnection. The high- and low-voltage earthing systems shall
be interconnected if the low-voltage system is totally confined within the area covered by the
high-voltage earthing system (see IEC 61936-1).
NOTE 2 Details of the different types of system earthing (TN, TT, IT) are shown in IEC 60364-1.

– 12 – 60364-4-44 © IEC:2007
Table 44.A1 – Power-frequency stress voltages and power-frequency fault
voltage in low-voltage system
Types of
Types of earth
system U U U
1 2 f
connections
earthing
R and R connected U *) R × I + U 0 *)
E B
o E E o
TT
R and R separated R × I + U U *) 0 *)
E B
E E o o
R and R connected U *) U *) R × I **)
E B
o o E E
TN
R and R separated R × I + U U*) 0 *)
E B
E E o o
U *) R × I + U 0 *)
o E E o
R and Z connected
E
R and R separated
E A
R × I
U × 3 R × I + U × 3
A h
o E E o
U *) U *)
R × I
o o
E E
R and Z connected
E
IT R and R interconnected
E A
R × I
U × 3 U × 3
E E
o o
R × I + U U*) 0 *)
E E o o
R and Z separated
E
R and R separated
E A
R × I
R × I + U × 3 U × 3
A d
E E o o
*) No consideration needs to be given.
**) See 442.2.1 second paragraph.
With existing earth fault in the installation.

NOTE 3 The requirements for U and U are derived from design criteria for insulation of low-voltage equipment
1 2
with regard to temporary power-frequency overvoltage (see also Table 44.A2).
NOTE 4 In a system whose neutral is connected to the earthing arrangement of the transformer substation, such
temporary power-frequency overvoltage is also to be expected across insulation which is not in an earthed
enclosure when the equipment is outside a building.
NOTE 5 In TT- and TN-systems the statement “connected” and “separated” refers to the electrical connection
between RE and RB. For IT-systems it refers to the electrical connection between RE and Z
...