SIST EN 61472:2013

Live working - Minimum approach distances for AC systems in the voltage range 72,5 kV to 800 kV - A method of calculation

This International Standard describes a method for calculating the minimum approach distances for live working, at maximum voltages between 72,5 kV and 800 kV. This standard addresses system overvoltages and the working air distances or tool insulation between parts and/or workers at different electric potentials. The required withstand voltage and minimum approach distances calculated by the method described in this standard are evaluated taking into consideration the following: - workers are trained for, and skilled in, working in the live working zone; - the anticipated overvoltages do not exceed the value selected for the determination of the required minimum approach distance; - transient overvoltages are the determining overvoltages; - tool insulation has no continuous film of moisture or measurable contamination present on the surface; - no lightning is seen or heard within 10 km of the work site; - allowance is made for the effect of conducting components of tools; - the effect of altitude, insulators in the gap, etc, on the electric strength is taken into consideration. For conditions other than the above, the evaluation of the minimum approach distances may require specific data, derived by other calculation or obtained from additional laboratory investigations on the actual situation.

Arbeiten unter Spannung - Mindest-Arbeitsabstände für Wechselspannungsnetze im Spannungsbereich von 72,5 kV bis 800 kV - Berechnungsverfahren

Travaux sous tension - Distances minimales d'approche pour des réseaux à courant alternatif de tension comprise entre 72,5 kV et 800 kV - Une méthode de calcul

La CEI 61472:2013 décrit une méthode de calcul des distances minimales d'approche pour des travaux sous tension réalisés à des tensions maximales comprises entre 72,5 kV et 800 kV. La présente norme traite des surtensions de réseau et des distances de travail dans l'air ou de l'isolation des outils entre des pièces et/ou des travailleurs à des potentiels électriques différents. La tension de tenue requise et les distances minimales d'approche calculées suivant la méthode décrite dans la présente norme sont évaluées en prenant en compte  - si les travailleurs sont formés et qualifiés pour travailler dans la zone de travail sous tension;  - si les surtensions attendues ne dépassent pas la valeur choisie pour la détermination de la distance minimale d'approche requise;  - si les surtensions transitoires sont les surtensions déterminantes;  - si l'isolation des outils ne présente pas sur la surface un film continu d'humidité ou un niveau de pollution mesurable;  - si aucun éclair n'est vu ni entendu à moins de 10 km du lieu de travail;  - si l'influence des parties conductrices des outils est prise en compte;  - et si l'effet de l'altitude, de la présence d'isolateurs dans l'intervalle, etc. sur la tenue électrique est pris en compte. Pour des conditions autres que celles ci-dessus, l'évaluation des distances minimales d'approche peut exiger des données spécifiques, provenant d'autres calculs ou obtenues à partir d'études additionnelles en laboratoire sur la situation concrète. Cette troisième édition annule et remplace la deuxième édition de la CEI 61472, publiée en 2004. Elle constitue une révision technique. Les modifications techniques majeures par rapport à la deuxième édition sont:  - la clarification du domaine d'application;  - la revue des définitions;  - la clarification de la méthodologie pour déterminer si le travail sous tension est acceptable et pour calculer les distances minimales d'approche;  - la modification de l'équation de base pour calculer la distance minimale d'approche;  - et l'introduction du Tableau 1 pour une simplification du facteur de correction en fonction de l'altitude k_a.

Delo pod napetostjo - Najmanjše razdalje za dostop do sistemov z izmenično napetostjo v območju od 72,5 kV do 800 kV - Metoda izračuna (IEC 61472:2013)

Ta mednarodni standard opisuje metodo za izračun najmanjših razdalj za dostop za delo pod napetostjo pri največjih napetostih med 72,5 kV in 800 kV. Ta standard obravnava sistemske prenapetosti in zračne razdalje pri delu ali izolacijo orodja med deli in/ali delavci pri različnih električnih potencialih. Zahtevana napetostna trdnost in najmanjše razdalje za dostop, ki se izračunajo z metodo, opisano v tem standardu, se ovrednotijo ob upoštevanju naslednjega: – delavci so usposobljeni za delo na območju pod napetostjo; – pričakovane prenapetosti ne presegajo vrednosti, izbrane za določitev zahtevane najmanjše razdalje za dostop; – tranzientne prenapetosti so določilne prenapetosti; – izolacija orodja na površini nima neprekinjenega filma vlage ali merljive kontaminacije; – 10 km od delovišča ni mogoče videti ali slišati bliskanja; – predvidi se učinek prevodnih komponent orodij; – upošteva se učinek nadmorske višine, izolatorjev v reži itd. na električno moč. Za pogoje, ki niso navedeni zgoraj, so lahko za vrednotenje najmanjših razdalj za dostop potrebni posebni podatki, izpeljani iz drugih izračunov ali pridobljeni z dodatnimi laboratorijskimi preiskavami dejanske situacije.

General Information

• Status: Published
• Publication Date: 19-Aug-2013
• ICS: 13.260 - Protection against electric shock. Live working
• Technical Committee: DPN - Live Working
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 30-Jul-2013
• Due Date: 04-Oct-2013
• Completion Date: 20-Aug-2013
• Ref Project: EN 61472:2013 - Live working - Minimum approach distances for a.c systems in the voltage range 72,5 kV to 800 kV - A method of calculation

Relations

• Replaces: SIST EN 61472:2007 - Live working - Minimum approach distances for a.c. systems in the voltage range 72,5 kV to 800 kV - A method of calculation
• Effective Date: 01-Sep-2013
• Replaces: SIST EN 61472:2007 - Live working - Minimum approach distances for a.c. systems in the voltage range 72,5 kV to 800 kV - A method of calculation
• Effective Date: 28-Jan-2023
• Corrected By: SIST EN 61472:2013/AC:2016 - Live working - Minimum approach distances for a.c systems in the voltage range 72,5 kV to 800 kV - A method of calculation
• Effective Date: 01-Jan-2016

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN 61472:2013
01-september-2013
1DGRPHãþD
SIST EN 61472:2007
'HORSRGQDSHWRVWMR1DMPDQMãHUD]GDOMH]DGRVWRSGRVLVWHPRY]L]PHQLþQR
QDSHWRVWMRYREPRþMXRGN9GRN90HWRGDL]UDþXQD,(&
Live working - Minimum approach distances for AC systems in the voltage range 72,5 kV
to 800 kV - A method of calculation
Travaux sous tension - Distances minimales d'approche pour des réseaux à courant
alternatif de tension comprise entre 72,5 kV et 800 kV - Une méthode de calcul
Ta slovenski standard je istoveten z: EN 61472:2013
ICS:
13.260 9DUVWYRSUHGHOHNWULþQLP Protection against electric
XGDURP'HORSRGQDSHWRVWMR shock. Live working
SIST EN 61472:2013 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 61472:2013

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SIST EN 61472:2013

EUROPEAN STANDARD
EN 61472

NORME EUROPÉENNE
July 2013
EUROPÄISCHE NORM

ICS 13.260; 29.240.20; 29.260.99 Supersedes EN 61472:2004


English version


Live working -
Minimum approach distances for a.c systems in the voltage range
72,5 kV to 800 kV -
A method of calculation
(IEC 61472:2013)


Travaux sous tension -  Arbeiten unter Spannung -
Distances minimales d'approche pour des Mindest-Arbeitsabstände für
réseaux à courant alternatif de tension Wechselspannungsnetze im
comprise entre 72,5 kV et 800 kV - Spannungsbereich
Une méthode de calcul von 72,5 kV bis 800 kV -
(CEI 61472:2013) Berechnungsverfahren
(IEC 61472:2013)





This European Standard was approved by CENELEC on 2013-05-16. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

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

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the CEN-CENELEC Management Centre has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, 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


© 2013 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61472:2013 E

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SIST EN 61472:2013
EN 61472:2013 - 2 -

Foreword
The text of document 78/1004/FDIS, future edition 3 of IEC 61472, prepared by IEC/TC 78 "Live working"
was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as EN 61472:2013.
The following dates are fixed:
(dop) 2014-02-16
• latest date by which the document has
to be implemented at national level by
publication of an identical national
standard or by endorsement
• latest date by which the national (dow) 2016-05-16
standards conflicting with the

document have to be withdrawn

This document supersedes EN 61472:2004.
This document has been prepared according to the requirements of EN 61477: Live working – Minimum
requirements for the utilization of tools, devices and equipment, where applicable.
EN 61472:2013 includes the following significant technical changes with respect to EN 61472:2004:
– clarification of the scope;
– review of the definitions;
– clarification of the methodology of determining whether live working is permissible and the calculation of
the minimum approach distances;
– modification of the basic equation for calculation of the minimum approach distance;
– introduction of Table 1 for altitude correction factor simplification ka;
– introduction of criteria in presence of composite insulator and clarification on the use of insulator factor
ki;
– review of the informative Annex F on the influence of floating conductive objects on the dielectric
strength;
– review of the informative Annex G on live working near contaminated, damaged or moist insulation.
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 61472:2013 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60060-1:2010 NOTE  Harmonised as EN 60060-1:2010 (not modified).
IEC 60071-1:2006 NOTE  Harmonised as EN 60071-1:2006 (not modified).
IEC 60071-2:1996 NOTE  Harmonised as EN 60071-2:1997 (not modified).
IEC 60743 NOTE  Harmonised as EN 60743.
IEC 61477:2009 NOTE  Harmonised as EN 61477:2009 (not modified).

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SIST EN 61472:2013




IEC 61472

®


Edition 3.0 2013-04




INTERNATIONAL



STANDARD




NORME



INTERNATIONALE
colour

inside










Live working – Minimum approach distances for a.c. systems in the voltage

range 72,5 kV to 800 kV – A method of calculation




Travaux sous tension – Distances minimales d'approche pour des réseaux à

courant alternatif de tension comprise entre 72,5 kV et 800 kV – Une méthode de

calcul















INTERNATIONAL

ELECTROTECHNICAL

COMMISSION


COMMISSION

ELECTROTECHNIQUE

PRICE CODE
INTERNATIONALE

CODE PRIX X


ICS 13.260; 29.240.20; 29.260.99 ISBN 978-2-83220-717-8



Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

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SIST EN 61472:2013
– 2 – 61472 © IEC:2013
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Terms, definitions and symbols . 6
2.1 Terms and definitions . 6
2.2 Symbols used in the normative part of the document . 8
3 Methodology . 9
4 Factors influencing calculations . 10
4.1 Statistical overvoltage . 10
4.2 Gap strength . 10
4.3 Calculation of electrical distance D . 11
U
4.3.1 General equation . 11
4.3.2 Factors affecting gap strength . 11
5 Evaluation of risks . 16
6 Calculation of minimum approach distance D . 17
A
Annex A (informative) Ergonomic distance . 18
Annex B (informative) Overvoltages . 20
Annex C (informative) Dielectric strength of air . 24
Annex D (informative) Gap factor k . 26
g
Annex E (informative) Allowing for atmospheric conditions . 28
Annex F (informative) Influence of floating conductive objects on the dielectric
strength . 32
Annex G (informative) Live working near contaminated, damaged or moist insulation . 40
Bibliography . 45

Figure 1 – Illustration of two floating conductive objects of different dimensions and at
different distances from the axis of the gap . 13
Figure 2 – Typical live working tasks . 15
Figure B.1 – Ranges of u at the open ended line due to closing and reclosing
e2
according to the type of network (meshed or antenna) with and without closing
resistors and shunt reactors . 22
Figure F.1 – Influence of the length of the floating conductive objects – phase to earth
rod-rod configuration – 250 µs /2 500 µs impulse . 35
Figure F.2 – Influence of the length of the floating conductive objects – phase to phase
conductor-conductor configuration – 250 µs /2 500 µs impulse . 36
Figure F.3 – Reduction of the dielectric strength as a function of the length D for
constant values of β – Phase to earth rod-rod configuration . 37
Figure F.4 – Reduction of the dielectric strength as a function of the length P for
constant values of β – Phase to phase conductor-conductor configuration . 37
Figure G.1 – Strength of composite insulators affected by simulated conductive and
semi-conductive defects . 43

Table 1 – Average k values . 12
a
Table 2 – Floating conductive object factor k . 14
f
Table B.1 – Classification of overvoltages according to IEC 60071-1 . 20

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SIST EN 61472:2013
61472 © IEC:2013 – 3 –
Table D.1 – Gap factors for some actual phase to earth configurations . 27
Table E.1 – Atmospheric factor k for different reference altitudes and values of U . 30
a 90
Table G.1 – Example of maximum number of damaged insulators calculation (gap
factor 1,4) . 41
Table G.2 – Example of maximum number of damaged insulators calculation (gap
factor 1,2) . 42

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SIST EN 61472:2013
– 4 – 61472 © IEC:2013
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

LIVE WORKING –
MINIMUM APPROACH DISTANCES FOR A.C. SYSTEMS
IN THE VOLTAGE RANGE 72,5 kV TO 800 kV –
A METHOD OF CALCULATION


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.
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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
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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 61472 has been prepared by technical committee 78: Live
working.
This third edition cancels and replaces the second edition of IEC 61472 published in 2004. It
constitutes a technical revision.
This document has been prepared according to the requirements of IEC 61477: Live working
– Minimum requirements for the utilization of tools, devices and equipment, where applicable.
Significant changes with regard to the second edition are the following:
– clarification of the scope;
– review of the definitions;
– clarification of the methodology of determining whether live working is permissible and the
calculation of the minimum approach distances;

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SIST EN 61472:2013
61472 © IEC:2013 – 5 –
– modification of the basic equation for calculation of the minimum approach distance;
– introduction of Table 1 for altitude correction factor simplification k ;
a
– introduction of criteria in presence of composite insulator and clarification on the use of
insulator factor k ;
i
– review of the informative Annex F on the influence of floating conductive objects on the
dielectric strength;
– review of the informative Annex G on live working near contaminated, damaged or moist
insulation.
The text of this standard is based on the following documents:
FDIS Report on voting
78/1004/FDIS 78/1010/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.
The committee has decided that the contents of this publication will remain unchanged until
the stability 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.

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.

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SIST EN 61472:2013
– 6 – 61472 © IEC:2013
LIVE WORKING –
MINIMUM APPROACH DISTANCES FOR A.C. SYSTEMS
IN THE VOLTAGE RANGE 72,5 kV TO 800 kV –
A METHOD OF CALCULATION



1 Scope
This International Standard describes a method for calculating the minimum approach
distances for live working, at maximum voltages between 72,5 kV and 800 kV. This standard
addresses system overvoltages and the working air distances or tool insulation between parts
and/or workers at different electric potentials.
The required withstand voltage and minimum approach distances calculated by the method
described in this standard are evaluated taking into consideration the following:
– workers are trained for, and skilled in, working in the live working zone;
– the anticipated overvoltages do not exceed the value selected for the determination of the
required minimum approach distance;
– transient overvoltages are the determining overvoltages;
– tool insulation has no continuous film of moisture or measurable contamination present on
the surface;
– no lightning is seen or heard within 10 km of the work site;
– allowance is made for the effect of conducting components of tools;
– the effect of altitude, insulators in the gap, etc, on the electric strength is taken into
consideration.
For conditions other than the above, the evaluation of the minimum approach distances may
require specific data, derived by other calculation or obtained from additional laboratory
investigations on the actual situation.
2 Terms, definitions and symbols
For the purpose of this document, the following terms, definitions and symbols apply.
2.1 Terms and definitions
2.1.1
damaged insulator
insulator having any type of manufacturing defect or in-service deterioration which affects its
insulating performance
2.1.2
electrical distance
D
U
distance in air required to prevent a disruptive discharge between energized parts or between
energized parts and earthed parts during live working
[SOURCE: IEC 60050-651:–, 651-21-12]

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SIST EN 61472:2013
61472 © IEC:2013 – 7 –
2.1.3
ergonomic distance
ergonomic component of distance
D
E
distance in air added to the electrical distance, to take into account inadvertent movement
and errors in judgement of distances while performing work
[SOURCE: IEC 60050-651:–, 651-21-13]
2.1.4
fifty per cent disruptive discharge voltage
U
50
peak value of an impulse test voltage having a fifty per cent probability of initiating a
disruptive discharge each time the dielectric testing is performed
[SOURCE: IEC 60050-604:1987, 604-03-43]
2.1.5
highest voltage of a system
U
s
highest value of operating voltage which occurs under normal operating conditions at any time
and any point in the system (phase to phase voltage)
Note 1 to entry: Transient overvoltages due e.g. to switching operations and abnormal temporary variations of
voltage are not taken into account.
[SOURCE: IEC 60050-601:1985, 601-01-23, modified – A reference to phase to phase voltage
has been added.]
2.1.6
minimum approach distance
minimum working distance
D
A
minimum distance in air to be maintained between any part of the body of a worker, including
any object (except tools appropriate for live working) being handled directly, and any part(s)
at different electric potential(s)
Note 1 to entry: The minimum approach distance is the sum of the electrical distance appropriate for the
maximum nominal voltage and of the selected ergonomic distance.
[SOURCE: IEC 60050-651:–, 651-21-11]
2.1.7
minimum residual insulation length
D
Lins
insulation length required to prevent a disruptive discharge between energized parts and
earthed parts measured along the insulator length, taking into account the presence of
floating conductive objects and of damaged insulator portions
2.1.8
ninety per cent statistical impulse withstand voltage
U
90
peak value of an impulse test voltage at which insulation exhibits, under specified conditions,
a ninety per cent probability of withstand
Note 1 to entry: This concept is applicable to self-restoring insulation.
[SOURCE: IEC 60050-604:1987, 604-03-42, modified – The definition has been changed to
refer specifically to a ninety per cent probability of withstand.]

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SIST EN 61472:2013
– 8 – 61472 © IEC:2013
2.1.9
part
any element present in the work location, other than workers, live working tools and system
insulation
2.1.10
per unit value
u
expression of the per unit value of the amplitude of an overvoltage (or of a voltage) referred to
U 2 / 3
s
Note 1 to entry: This applies to u and u defined in Clause 4.
e2 p2
2.1.11
transient overvoltage
short duration overvoltage of few milliseconds or less, oscillatory or non-oscillatory, usually
highly damped
[SOURCE: IEC 60050-604:1987, 604-03-13, modified – The two notes in the original definition
have been deleted.]
2.1.12
two per cent statistical overvoltage
U
2
peak value of a transient overvoltage having a 2 per cent statistical probability of being
exceeded
2.1.13
work location
any site, place or area where a work activity is to be, is being, or has been carried out
[SOURCE: IEC 60050-651:–, 651-26-03]
2.2 Symbols used in the normative part of the document
A length of damaged insulator or number of damaged units in an insulator of length A ,
d o
not shunted by long arcing horn or grading ring
A length of undamaged insulator or number of undamaged insulator units not shunted
o
by long arcing horn or grading ring
ratio of the total length in the direction of the gap axis of the floating conductive
β
objects (s) to the original air gap length
D length of the remaining air gap phase to earth
D minimum approach distance
A
D ergonomic distance
E
D electrical distance necessary to obtain U
U 90
D minimum residual insulation length
Lins
d , d ,
distances between the worker(s) and parts of the installation at different electric
1 2
d d potentials (see Figure 2)
3, 4
F
sum of all lengths, in the direction of the gap axis, of all floating conductive objects
in the air gap (in metres)

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SIST EN 61472:2013
61472 © IEC:2013 – 9 –
K statistical safety factor
s
K factor combining different considerations influencing the strength of the gap
t
k atmospheric factor
a
k coefficient characterizing the average state of the damaged insulators
d

k floating conductive object factor
f
k gap factor
g
k damaged insulator factor
i
k damaged composite insulator factor
ic
k damaged insulator strings factor
is
k standard statistical deviation factor
s

L original air gap length
f
P length of the remaining gap phase to phase
r distance of a conductive object from the axis of the gap
s normalized value of the standard deviation of U expressed in per cent
e
50
U two per cent statistical overvoltage
2
U fifty per cent disruptive discharge voltage
50
U ninety per cent statistical impulse withstand voltage
90
U two per cent statistical overvoltage between phase and earth
e2
U ninety per cent statistical impulse withstand voltage phase to earth
e90
U two per cent statistical overvoltage between two phases
p2
U ninety per cent statistical impulse withstand between two phases
p90
u per unit value of the two per cent statistical overvoltage phase to earth
e2
u per unit value of the two per cent statistical overvoltage between two phases
p2
U highest voltage of a system between two phases
s
3 Methodology
The methodology of determining whether live working is permissible and the calculation of the
minimum approach distances is based on the following considerations:
a) to determine the statistical overvoltage expected in the work location (U ) and from this,
2
determine the required statistical impulse withstand voltage of the insulation in the work
location (U );
90
b) to calculate the minimum residual insulation length D if working next to insulators;
Lins
c) to calculate the electrical distance D required for the impulse withstand voltage U ;
U 90
d) to add an additional distance to allow for ergonomic factors associated with live working,
such as inadvertent movement.
The minimum approach distance D is thus determined by:
A

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SIST EN 61472:2013
– 10 – 61472 © IEC:2013
(1)
D = D (K = 1,0)+ D
A U s E
where
D is the electrical distance necessary to obtain U ;
U 90
D is the ergonomic distance and is dependent on work procedures, level of training, skill of
E
the workers, type of construction, and such contingencies as inadvertent movement, and
errors in appraising distances (see Annex A for details).
Refer to Clause 5 for application of ergonomic distance.
4 Factors influencing calculations
4.1 Statistical overvoltage
The electrical stress at the work location shall be known. The electrical stress is described as
the statistical overvoltage that may be present at the work location. In a three-phase a.c.
power system the statistical overvoltage U between phase and earth is:
e2
U = ( 2/ 3) U u (2)
e2 s e2
where
U ( 2/ 3) is the highest phase to earth peak voltage, of the system expressed in kV, and
s
u is the statistical overvoltage phase to earth expressed in per unit.
e2
The statistical overvoltage U
p2 between two phases is:
U = ( 2/ 3) U u (3)
p2 s p2

where u is the statistical overvoltage phase to phase expressed in per unit.
p2
If the per unit phase to phase data are not available, an approximate value can be derived
from u by the following formula:
e2
u = 1,35 u + 0,45 (4)
p2 e2
The transient overvoltages to be considered are the maximum that can occur, either on the
installation being worked on or at the work site, whether caused by system faults or by
switching (see Annex B).
4.2 Gap strength
For the determination of the electrical distance, the required withstand voltage for live working
is taken to be equal to the voltage U , determined from the general expression
90
U = K U (5)
90 s 2
Considering the phase to earth and phase to phase voltages separately and combining
equation (5) with equations (2) and (3) gives:
U = K 2/ 3) U u (6)
e90 s ( s e2
U = K ( 2/ 3) U u (7)
p90 s s p2

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SIST EN 61472:2013
61472 © IEC:2013 – 11 –
where
K is the statistical safety factor (1,0 or 1,1 for formula (5), (6) and (7)) (see
s
Clause 5);
U and U are respectively the statistical impulse withstand voltages phase to earth and
e90 p90
phase to phase, expressed in kV.
4.3 Calculation of electrical distance DU
4.3.1 General equation
The strength of the gap is influenced by a series of considerations which can be combined in
a factor K used in the following formula for calculating D (in metres):
t U
K
U /(1 080 t)
90
D = 2,17 (e – 1) + F  (8)
U
where
F sum of all lengths, in the direction of the gap axis, of all floating conductive objects in
the air gap (in metres) (see 4.3.2.4);
U is the phase to earth (U ) or the phase to phase (U ) statistical impulse withstand
e90 p90
90
voltage in kV;
K is given by:
t
K = k k k k k (9)
t s g a f i
4.3.2 Factors affecting gap strength
4.3.2.1 Standard statistical deviation factor k
s
Factor k accounts for the statistical nature of the breakdown voltage. Unless the value of the
s
standard deviation, s , is known from tests representing the gap configuration, a value of
e
0,936, based on a standard deviation of 5 %, for positive impulses, can be used (see
Annex C).
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