IEC TS 62257-5:2015

IEC TS 62257-5 ®
Edition 2.0 2015-12
TECHNICAL
SPECIFICATION
colour
inside
Recommendations for renewable energy and hybrid systems for rural
electrification –
Part 5: Protection against electrical hazards
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IEC TS 62257-5 ®
Edition 2.0 2015-12
TECHNICAL
SPECIFICATION
colour
inside
Recommendations for renewable energy and hybrid systems for rural

electrification –
Part 5: Protection against electrical hazards

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-3070-1

– 2 – IEC TS 62257-5:2015  IEC 2015
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 Classification of decentralised rural electrification systems . 11
5 Protection against electric shock . 11
5.1 General . 11
5.2 Requirements on the d.c. side of a DRES . 11
5.3 Requirements on the a.c. side of a DRES . 12
5.3.1 General . 12
5.3.2 TT system. 12
5.3.3 TN system . 12
6 Protection against overcurrent . 13
6.1 General . 13
6.2 Protection against overload currents . 13
6.3 Protection against short-circuits . 13
7 Protection against risk of fire . 13
8 Protection against effects of lightning . 14
8.1 Principle . 14
8.2 Examples . 14
8.3 Protection against overvoltage . 14
8.4 Protection against direct lightning . 14
9 Determination of the pick up area of a rod or wire (see IEC 62305-3:2010) . 14
9.1 General . 14
9.2 Operational conditions and external influences . 14
9.3 Wiring system . 15
9.4 Isolation and switching . 15
9.4.1 Isolation . 15
9.4.2 Over-current protective devices . 16
9.4.3 Residual Current Devices (RCD) . 17
9.5 Surge protective devices . 17
9.6 Earthing arrangement, protective conductors and protective bonding
conductors . 18
9.6.1 Earth electrodes . 18
9.6.2 Protective bonding conductors . 19
10 Verification . 19
11 Operation and maintenance . 19
Annex A (informative) Protection against electric shock in electrical installations . 20
A.1 Protection against electric shock . 20
A.2 Automatic disconnection of supply . 20
A.2.1 General . 20
A.2.2 In TN systems . 21
A.2.3 In TT systems . 21
A.3 Double or reinforced insulation . 22

A.4 Extra-low-voltage (SELV and PELV) . 22
A.5 Electrical separation . 22
A.6 Additional protection . 23
Annex B (informative) Types of LV distribution systems earthing . 24
B.1 Terms and definitions . 24
B.2 Types of system earthing used in DRES (Figures are from IEC 60364-1:2005) . 25
B.2.1 General . 25
B.2.2 AC TN systems . 27
B.2.3 AC TT systems . 34
B.2.4 DC distribution systems . 36
Annex C (informative) Classification of electrical equipment . 41
C.1 Classification of residual current devices (RCDs) (see IEC 61008,
IEC 61009, IEC 60755, IEC 60947-2, IEC 62423) . 41
C.2 Classification of circuit breakers for a.c. operation (see IEC 60898-1,
IEC 60947-2) . 42
C.3 Classification of surge protective devices (see IEC 61643-11) . 43
Annex D (informative)  General information concerning protection against lightning . 44
D.1 General . 44
D.2 Protection against lightning – Principles . 45
Bibliography . 46

Figure B.1 – General outline of the distribution system . 24
Figure B.2 – Distribution system of the smallest type . 25
Figure B.3 – TN-S system 3-phase, 4-wire with separate neutral conductor and
protective conductor throughout the distribution system . 28
Figure B.4 – TN-S system 3-phase, 3-wire with separate earthed line conductor and
protective conductor throughout the distribution system . 29
Figure B.5 – TN-S system 3-phase, 3-wire with protective conductor and no distributed
neutral conductor throughout the distribution system . 30
Figure B.6 – TN-C-S system 3-phase, 4-wire where the PEN conductor is separated
into the protective conductor PE and the neutral conductor N elsewhere in the
electrical installation . 31
Figure B.7 – TN-C-S system 3-phase, 4-wire where the PEN conductor is separated
into the protective conductor PE and the neutral conductor N at the origin of the
electrical installation . 32
Figure B.8 – TN-C-S system – single-phase, 2-wire where the PEN conductor is
separated into the protective conductor PE and the neutral conductor N at the origin of
the electrical installation . 32
Figure B.9 – TN-C system 3-phase, 4-wire with neutral and protective conductor
functions combined in a single conductor throughout the distribution system . 33
Figure B.10 – TN-S multiple source system 3-phase, 4-wire with separate protective
conductor and neutral conductor to current using equipment . 34
Figure B.11 – TT system 3-phase, 4-wire with earthed protective conductor and neutral
conductor throughout the distribution system . 35
Figure B.12 – TT system 3-phase, 3-wire with earthed protective conductor and no
distributed neutral conductor throughout the distribution system . 35
Figure B.13 – TN-S d.c. system . 37
Figure B.14 – TN-C d.c. system . 38
Figure B.15 – TN-C-S d.c. system . 39

– 4 – IEC TS 62257-5:2015  IEC 2015
Figure B.16 – TT d.c. system . 40
Figure D.1 – Example of effects of a lightning stroke . 44

Table 1 – Typology of decentralized electrification systems . 11
Table 2 – Rated operating residual current of the protective device depending on the
value of the earthing resistance . 12
Table 3 – Number of protected poles with regard to the characteristics of the
distribution system . 16

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RECOMMENDATIONS FOR RENEWABLE ENERGY
AND HYBRID SYSTEMS FOR RURAL ELECTRIFICATION –

Part 5: Protection against electrical hazards

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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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.
The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a technical
specification when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• the subject is still under technical development or where, for any other reason, there is the
future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC 62257-5, which is a technical specification, has been prepared by IEC technical
committee 82: Solar photovoltaic energy systems.

– 6 – IEC TS 62257-5:2015  IEC 2015
This second edition cancels and replaces the first edition issued in 2005. It constitutes a
technical revision.
The main technical changes with regard to the previous edition are as follows:
– redefine the maximum AC voltage from 500 V to 1 000 V, the maximum DC voltage from
750 V to 1 500 V;
– removal of the limitation of 100 kVA system size. Hence the removal of the word “small” in
the title and related references in this technical specification.
This technical specification is to be used in conjunction with the IEC 62257 series (specifically
IEC TS 62257-1 to IEC TS 62257-6).
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
82/950/DTS 82/1001A/RVC
Full information on the voting for the approval of this technical specification 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 parts in the IEC 62257 series, published under the general title Recommendations
for renewable energy and hybrid systems for rural electrification, 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 stability date indicated on the IEC website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
INTRODUCTION
The IEC 62257 series intends to provide to different players involved in rural electrification
projects (such as project implementers, project contractors, project supervisors, installers,
etc.) documents for the setting up of renewable energy and hybrid systems with AC voltage
below 1 000 V and DC voltage below 1 500 V.
These documents are recommendations:
• to choose the right system for the right place;
• to design the system;
• to operate and maintain the system.
These documents are focused only on rural electrification, concentrating on, but not specific
to developing countries. They should not be considered as all inclusive to rural electrification.
The documents try to promote the use of renewable energies in rural electrification; they do
not deal with clean mechanisms developments at this time (CO emission, carbon credit, etc.).
Further developments in this field could be introduced in future steps.
This consistent set of documents is best considered as a whole with different parts
corresponding to items for safety, sustainability of systems aiming at the lowest life cycle cost
as possible. One of the main objectives is to provide the minimum sufficient requirements,
relevant to the field of application, that is: renewable energy and hybrid off-grid systems.

– 8 – IEC TS 62257-5:2015  IEC 2015
RECOMMENDATIONS FOR RENEWABLE ENERGY
AND HYBRID SYSTEMS FOR RURAL ELECTRIFICATION –

Part 5: Protection against electrical hazards

1 Scope
This part of IEC 62257 specifies the general requirements for the protection of persons and
equipment against electrical hazards to be applied in decentralised rural electrification
systems. Requirements dealing with protection against electric shock are based on basic
rules from IEC 61140 and IEC 60364.
Decentralized Rural Electrification Systems (DRES) are designed to supply electric power for
sites which are not connected to a large interconnected system, or a national grid, in order to
meet basic needs.
The majority of these sites are:
• isolated dwellings,
• village houses,
• community services (public lighting, pumping, health centers, places of worship or cultural
activities, administrative buildings, etc.),
• economic activities (workshops, micro-industry, etc.).
The DRE systems fall into three categories:
• process electrification systems (for example for pumping),
• individual electrification systems (IES) for single users,
• collective electrification systems (CES) for multiple users.
Process or individual electrification systems exclusively consist of two subsystems:
• an electric energy generation subsystem,
• the user's electrical installation.
Collective electrification systems, however, consist of three subsystems:
• an electric energy generation subsystem,
• a distribution subsystem, also called microgrid,
• user’s electrical installations including interface equipment between the installations and
the microgrid.
The general requirements specified in this part of IEC 62257 should be applied to all the
identified categories of DRES. Application to each subsystem of a DRES is dealt within a
specific subpart of IEC TS 62257-9.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60364-4-41, Low-voltage electrical installations – Part 4-41: Protection for safety –
Protection against electric shock
IEC 60364-4-43, Low-voltage electrical installations – Part 4-43: Protection for safety –
Protection against overcurrent
IEC 60364-4-44:2007, Low-voltage electrical installations – Part 4-44: Protection for safety –
Protection against voltage disturbances and electromagnetic disturbances
IEC 60364-4-44:2007/AMD1:2015
IEC 60364-5-52:2009, Low-voltage electrical installations – Part 5-52: Selection and erection
of electrical equipment – Wiring systems
IEC 60364-5-53:2001, Electrical installations of buildings – Part 5-53: Selection and erection
of electrical equipment – Isolation, switching and control
IEC 60364-5-53:2001/AMD1:2002
IEC 60364-5-53:2001/AMD2:2015
IEC 60364-5-54, Low-voltage electrical installations – Part 5-54: Selection and erection of
electrical equipment – Earthing arrangements and protective conductors
IEC 60364-7-712, Electrical installations of buildings – Part 7-712: Requirements for special
installations or locations – Solar photovoltaic (PV) power supply systems
IEC 62305-2:2010, Protection against lightning – Part 2: Risk management
IEC 62305-3:2010, Protection against lightning – Part 3: Physical damage to structures and
life hazard
IEC 61140:2015, Protection against electric shock – Common aspects for installation and
equipment
IEC TS 62257-1, Recommendations for renewable energy and hybrid systems for rural
electrification – Part 1: General introduction to IEC 62257 series and rural electrification
IEC TS 62257-2, Recommendations for renewable energy and hybrid systems for rural
electrification – Part 2: From requirements to a range of electrification systems
IEC TS 62257-3: Recommendations for renewable energy and hybrid systems for rural
electrification – Part 3: Project development and management
IEC TS 62257-4, Recommendations for renewable energy and hybrid systems for rural
electrification – Part 4: System selection and design
IEC TS 62257-6, Recommendations for renewable energy and hybrid systems for rural
electrification – Part 6: Acceptance, operation, maintenance and replacement
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

– 10 – IEC TS 62257-5:2015  IEC 2015
3.1
decentralized rural electrification system
DRES
any electrical power system that is stand alone and not connected to the grid
3.2
Renewable Energy
RE
energy from a source that is not depleted when used
3.3
mini-grid
subsystem of a DRES intended for power distribution
3.4
mini-powerplant
energy source of a DRES
3.5
Surge Protection Device
SPD
device intended to limit transient overvoltages and divert surge currents; contains at least one
non-linear component. An appliance/device designed to protect electrical devices from voltage
spikes.
3.6
protection against electric shock
provision of measures reducing the risk of electric shock
3.7
basic protection
protection against electric shock under normal conditions
3.8
fault protection
protection against electric shock under single-fault conditions
3.9
hazardous-live-part
live part which, under certain conditions, can give a harmful electric shock
3.10
lightning protection system
LPS
complete system used to reduce physical damage due to lightning flashes to a structure
3.11
external lightning protection system
part of the LPS consisting of an air-termination system, a down-conductor system and an
earth-termination system
3.12
earthing arrangement
grounding arrangement, US
all the electric connections and devices involved in the earthing of a system, an installation
and equipment
3.13
equipotential bonding system
EBS
interconnection of conductive parts providing equipotential bonding between those parts
Note 1 to entry: If an equipotential bonding system is earthed, it forms part of an earthing arrangement.
4 Classification of decentralised rural electrification systems
DRESs are classified into six different types. See Table 1.
Table 1 – Typology of decentralized electrification systems
Type of generator Classification of associated systems
Individual Collective
REN only, hybrid or not no storage T .I T .C
1 1
REN only, hybrid or not storage T .I T .C
2 2
REN, hybrid or not no storage T .I T .C
3 3
plus Genset
REN, hybrid or not storage T .I T .C
4 4
plus Genset
Genset only no storage T .I T .C
5 5
Genset only storage T .I T .C
6 6
Notation principle: Ti.I = individual system, type i; Tj.C = collective system, type j.
Storage: storage of energy produced by one of the generator of the system and which can be reconverted.

Architecture and characteristics of the different electrification system types are developed in
Clause 6 of IEC TS 62257-2:2015.
5 Protection against electric shock
5.1 General
Basic rules for protection against electric shock are given in IEC 61140 and IEC 60364-4-41.
Information is also available in Annex A.
5.2 Requirements on the d.c. side of a DRES
The principles for the design and erection of a d.c. electrical circuit are similar to those for an
a.c. circuit. The main differences concern short-circuit current calculation and the selection of
the protective devices.
Protection by extra-low voltage (SELV and PELV systems) or protection by double or
reinforced insulation should preferably be adopted on the d.c. side of DRES.
Simple separation, at least, should be provided between the a.c. side and the d.c. side unless
the inverter is not able, by construction, to feed d.c. fault current into the a.c. installation.
Earthing of one of the live conductors of the d.c. side is permitted, if there is at least simple
separation between the d.c. side and the a.c. side.

– 12 – IEC TS 62257-5:2015  IEC 2015
5.3 Requirements on the a.c. side of a DRES
5.3.1 General
Protection by use of automatic disconnection of supply should preferably be adopted on the
a.c. side of a DRES. For each circuit, maximum disconnecting times given in IEC 60364-4-41
should apply.
TN-S or TN-C-S system should preferably be used for decentralized rural electrification
system, TT system is acceptable. IT system is normally not used for DRES and has hence not
been dealt with in this specification.
A residual current protective device, with a rated residual operating current not exceeding
30 mA, should be provided as additional protection for each installation.
5.3.2 TT system
Basic protection is provided by basic insulation of live parts or by barriers or enclosures. Fault
protection is provided by residual current devices regarding the resistance value of the earth
electrode to which the PE conductor is connected. The fault current should be high enough to
activate the differential current device. The rated operating residual current I of the device
∆n
should fulfil the formula:
Formula: Rated operating residual current
U
L
I ≤ with U = 50 V
Δn L
R
A
where U is the conventional maximum voltage and R is the earthing resistance.
L A
This formula results in the values shown in Table 2.
Table 2 – Rated operating residual current of the protective device
depending on the value of the earthing resistance
R I
A ∆n
Ω A
R ≤ 50
A 1
50 < R ≤ 100
A 0,5
100 < R ≤ 167
0,3
A
167 < R ≤ 300
0,1
A
300 < R ≤ 500
A 0,03
5.3.3 TN system
Basic protection is provided by basic insulation of live parts or by barriers or enclosures. Fault
protection is provided by devices protecting against over-currents.
Additional information is given in Annexes A and B.

6 Protection against overcurrent
6.1 General
Protective devices should be provided to break any over-current flowing in the circuit
conductors before such a current could cause a danger due to thermal and mechanical effects
or a temperature rise detrimental to insulation, joints, termination (see IEC 60364-4-43).
6.2 Protection against overload currents
The operating characteristics of a device protecting a cable against overload current should
satisfy the two following conditions:
I ≤ I ≤ I
B n z
I ≤ 1,45 × I
2 z
where
I is the deign current of the circuit;
B
I is the continuous current-carrying capacity of the cable;
z
I is the rated current of the protective device;
n
I is the current ensuring effective operation in the conventional time of the protective
device.
6.3 Protection against short-circuits
For cables and isolated conductors, each short-circuit protective device should meet both of
the following conditions:
• The breaking capacity should not be less than the prospective short-circuit current at the
place of its installation, except where another protective device having the necessary
breaking capacity and coordinated characteristics is installed upstream.
• All current caused by a short-circuit occurring at any point of the circuit should be
interrupted in a time not exceeding that which brings the conductors to the admissible limit
temperature. For short-circuits of duration up to 5 s, the time t, in which a given short-
circuit current will raise the conductors from the highest admissible temperature in normal
duty to the limit temperature can, as an approximation, be calculated from the formula:
t = k× S I
where
t is the duration in s;
S is the cross-sectional area, in square millimetres;
I is the effective short-circuit current, in amperes, expressed as r.m.s. value;
k is a factor taking account of the resistivity, temperature coefficient and heat capacity of
the conductor material, and the appropriate initial and final temperatures.
7 Protection against risk of fire
Where there is a risk of personal injury or property damage due to fire caused by an earth
fault in the system, a residual current protective device should be provided at least at the
origin of the user’s installation. Its rated operating residual current should be ≤300 mA. Such
a device should switch all live conductors.

– 14 – IEC TS 62257-5:2015  IEC 2015
8 Protection against effects of lightning
8.1 Principle
Information about the effects of lightning on electrical supply systems is given in Annex D.
Decision for lightning protective provision (lightning rod, surge protective devices, etc.) should
be based on risk assessment, taking account of the lightning frequency statistics, the
characteristics and position of the structures, the length of the overhead lines, if any, the cost
and the requested availability of the equipment.
8.2 Examples
Examples of risk assessment methods appropriate for lightning protection can be found in
IEC 60364-4-44:2007, Clause 443 and IEC 62305-2:2010, Provisions for lightning protection
of DRES.
8.3 Protection against overvoltage
Where protection against overvoltage (for example due to indirect lightning) is required, an
SPD(s) should be installed both at the distribution board of the micro-power plant, and at the
origin of the user's installations or associated with each socket-outlet.
Installation of SPD should comply with IEC 60364-5-53:2001, Clause 534.
To minimize voltages induced by lightning, the area of all wiring loops should be as small as
possible.
8.4 Protection against direct lightning
Where protection against direct lightning is required, the following provisions apply:
• In case of wind powered generation, the lightning rod should be installed at the summit of
the mast.
• Where PV generation coexists with wind-powered generation, protection against direct
lightning is generally achieved by placing the panels inside the pick-up zone of the wind-
powered generator mast.
• Where PV generation is alone, the panels can be protected by installing a protective wire
above the PV panel or lightning rod/s with an appropriate pick-up area.
• Protection should be completed by the installation of SPDs between conductors and
between conductors and earth, with appropriate characteristics (see IEC 60364-5-53:2001,
Clause 534).
9 Determination of the pick up area of a rod or wire (see IEC 62305-3:2010)
9.1 General
All equipment should be selected according to the rules of IEC 60364-5-53.
9.2 Operational conditions and external influences
Every item of equipment should be selected and erected in compliance with the appropriate
standards.
Equipment should be suitable for the nominal voltage (r.m.s. value for a.c.) of the circuit
concerned and for the overvoltages which could occur.

Equipment should be selected for the design current (r.m.s. value for a.c.) which it has to
carry in normal service.
Equipment on the d.c. side should be suitable for direct voltage and direct current.
Equipment should also be capable of carrying the currents likely to flow in abnormal
conditions for such periods of time as are determined by the characteristics of the protective
devices.
If frequency has an influence on the characteristics of equipment, the rated frequency of the
equipment should correspond to the frequency and frequency variations which could occur in
the circuit concerned.
The electrical equipment should withstand the expected external influences such as wind, ice
formation, temperature and solar radiation, etc. If a piece of equipment does not have, by
construction, the necessary qualities corresponding to the location in which it is installed,
appropriate additional protection should be provided, forming part of the installation.
Electrical equipment should be selected and erected so that it does not produce, in normal
service, any interference with the other equipment in the system. The causes of interference
include:
• power factor;
• inrush current;
• phase unbalance (three-phase systems);
• harmonics.
9.3 Wiring system
The minimum cross-sectional area of protective conductors should be determined according
IEC 60364-5-54.
The minimum cross sectional area of conductors should be determined according to:
• The current-carrying capacity of conductors taking account of external influences and of
the methods of installation. See tables in IEC 60364-5-52.
• The acceptable voltage drop in conductors of the user’s installations should be determined
according IEC 60364-5-54. Voltage values should comply with the following limits at the
terminals of any user’s electrical equipment:
For a.c. voltage,
0,90 × 230 V < U < 1,10 × 230 V
a.c.
For d.c. voltage,
0,85 × 12 V < U < 1,20 × 12 V or
d.c.
0,85 × 24 V < U < 1,20 × 24 V
d.c.
9.4 Isolation and switching
9.4.1 Isolation
The purpose of isolation is to separate a circuit or equipment unit from the rest of the system
in order to guarantee the safety of persons who may have to work on, to maintain or repair it.
Every circuit should be capable of being isolated.

– 16 – IEC TS 62257-5:2015  IEC 2015
In TN-C-S systems, the PEN conductor should not be interrupted (broken, switched or
disconnected).
In TN-S systems, the neutral conductor needs not be interrupted.
Suitable means (padlocking, location within lockable enclosure, etc.) should be provided to
prevent any equipment from being unintentionally energised.
The isolating distance between open contacts should be visible or clearly and reliably
indicated.
9.4.2 Over-current protective devices
9.4.2.1 General
Fuses (gPV type) or circuit-breakers with appropriate range of instantaneous tripping should
be used.
The range of instantaneous tripping for a circuit-breaker should be selected according to the
prospective short-circuit current.
Overcurrent protective devices should be preferably of a type ensuring protection against both
overload and short-circuit currents and capable of acting as isolating switch in the open
position.
Special attention should be paid to over-current protective devices installed in series, to
ensure that an appropriate coordination is achieved. Selectivity between protective devices in
series should preferably be total.
9.4.2.2 AC over-current protective devices
The number of protected poles depends on the neutral earthing distribution system and on the
cross-sectional area the neutral conductor, in accordance with Table 3.
NOTE A protected pole is a pole provided with an over-current release.
Table 3 – Number of protected poles with regard to the characteristics
of the distribution system
Distribution system Conductors Cross-sectional area Protected poles Conditions
of the neutral, PEN
or PEL conductor
TT or TN-S 3 L  3 L
S = S
3 L + N 3 L or 3 L + N
N L
L + N S = S L or L + N
N L
3 L
3 L + N S < S 1 + 2 + 3 + 4
N L
3 L + N S < S 3 L + N 1 + 2 + 3
N L
TN-C-S S = S 3 L
3 L + PEN
PEN L
3 L + PEN S < S 3 L
1 + 2 + 3 + 4
PEN L
S = S L
L + PEL
PEL L
Conditions:
2 2
1: The cross-sectional area of the conductors is >16 mm Cu or >25 mm Al.
2: The power consumed between phases and neutral is <10 % of the total power transmitted by the mains.
3: The maximum current expected to flow in the neutral conductor is less than its permissible current.
4: The neutral conductor is protected against short-circuits by the steps taken to protect the phase conductors.

9.4.2.3 DC over-current protective devices
For the selection of d.c. overcurrent protective devices, it is recommended to be assisted by
the manufacturer after having determined and transmitted the characteristics of the circuit
(short-circuit current, rated current, time constant).
NOTE For calculation of the short-circuit current in case of a battery whose internal resistance is not known, the
following formula can be used:
I = 10 × C
k
where C is in A/h.
For calculation of the short-circuit current at the terminals of a d.c. generator, the following formula can be used:
I = 1,1 × U / R
k n i
where R is the internal resistance of the generator.
i
For calculation of the short-circuit current at any point of the installation, the following formula can be used:
I = 1,1 × U /R + 2R
k n i L
where R is the line resistance.
L
And in case of the presence of a d.c. motor, the value of I , here above is increased by the value of 6I of the
k N
motor.
9.4.3 Residual Current Devices (RCD)
Residual current devices should be so selected, and the electric circuits so subdivided that
any earth leakage current which may be expected to occur during normal operation of the
connected load(s) will be unlikely to cause unnecessary tripping of the device.
NOTE Residual current protective devices can operate at any value of residual current in excess of 50 % of the
rated operating current.
Residual current protective devices in d.c. systems should be specially designed for detection
of d.c. residual currents, and t
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