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EUROPEAN STANDARD EN 61472
NORME EUROPÉENNE
EUROPÄISCHE NORM November 2004

ICS 13.260; 29.240.20; 29.260.99

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 61472:2004)
Travaux sous tension –  Arbeiten unter Spannung –
Distances minimales d'approche Mindest-Arbeitsabstände für
pour des réseaux à courant alternatif Wechselspannungsnetze im
de tension comprise entre 72,5 kV Spannungsbereich von 72,5 kV
et 800 kV – bis 800 kV –
Une méthode de calcul Berechnungsverfahren
(CEI 61472:2004) (IEC 61472:2004)

This European Standard was approved by CENELEC on 2004-10-01. 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 Central Secretariat 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 Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden,
Switzerland and United Kingdom.

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

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

© 2004 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. EN 61472:2004 E
Foreword
The text of document 78/582/FDIS, future edition 2 of IEC 61472, prepared by IEC TC 78, Live
working, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as
EN 61472 on 2004-10-01.
This standard has been prepared according to the requirements of EN 61477: Live working –
Minimum requirements for the utilization of tools,devices and equipement, where applicable.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2005-07-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2007-10-01
__________
Endorsement notice
The text of the International Standard IEC 61472:2004 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 NOTE Harmonized as HD 588.1 S1:1991 (not modified).
IEC 60071-1 NOTE Harmonized as EN 60071-1:1995 (not modified).
IEC 60071-2 NOTE Harmonized as EN 60071-2:1997 (not modified).
IEC 60743 NOTE Harmonized as EN 60743:2001 (not modified).
IEC 61477 NOTE Harmonized as EN 61477:2002 (not modified).

__________
NORME CEI
INTERNATIONALE IEC
INTERNATIONAL
Deuxième édition
STANDARD
Second edition
2004-07
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
Live working –
Minimum approach distances
for a.c. systems in the voltage range
72,5 kV to 800 kV –
A method of calculation
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Pour prix, voir catalogue en vigueur
For price, see current catalogue

61472 © IEC:2004 – 3 –
CONTENTS
FOREWORD.7
1 Scope.11
2 Terms, definitions and symbols .11
3 Methodology.17
4 Factors influencing calculations.19
5 Evaluation of risks.27
6 Calculation of minimum approach distance D .29
A
Annex A (informative) Ergonomic distance.37
Annex B (informative) Overvoltages.41
Annex C (informative) Dielectric strength of air .49
Annex D (informative) Gap factor k .53
g
Annex E (informative) Allowing for atmospheric conditions .57
Annex F (informative) Influence of electrically floating objects on the dielectric strength .65
Annex G (informative) Live working near contaminated, damaged or moist insulation .79
Bibliography.85
Figure 1 – Illustration of two floating objects of different dimensions and at different
distances from the axis of the gap (see 4.3.4).33
Figure 2 – Typical live working tasks (see Clause 2 and 4.3.4) .35
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 (see B.2.1.1).47
Figure F.1 – Reduction in the discharge voltage of the air gap due to alteration in the
electric field caused by the presence of a floating-potential conductive object in critical
position along the axis of the gap (phase to earth rod-rod configuration) –
250 µs /2 500 µs impulse (see F.3.1.2 et F.3.1.3) .73
Figure F.2 – Reduction in the discharge voltage of the air gap due to alteration in the
electric field caused by the presence of a floating-potential conductive object in critical
position along the axis of the gap (phase to phase conductor-conductor configuration)
– 250 µs /2 500 µs impulse (see F.3.1.2 et F.3.1.3) .75
Figure F.3 – Reduction of the dielectric strength as a function of the clearance D for
constant values of β – Phase to earth rod-rod configuration (see F.3.1.3 and F.3.2) .77
Figure F.4 – Reduction of the dielectric strength as a function of the clearance D for
constant values of β – Phase to phase conductor-conductor configuration (see F.3.1.3
and F.3.2) .77

61472 © IEC:2004 – 5 –
Table 1 – Floating object factor k .25
f
Table 2 – Example of calculation of electrical distance for some switching overvoltage
values.31
Table B.1 – Classification of overvoltages according to IEC 60071-1 .45
Table D.1 – Gap factors for some actual phase to earth configurations .55
Table E.1 – Atmospheric factor k for different reference altitudes and values of U .61
a 90
Table G.1 – Example of maximum number of damaged insulators calculation (gap
factor 1,4) .81
Table G.2 – Example of maximum number of damaged insulators calculation (gap
factor 1,2) .83

61472 © IEC:2004 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
LIVE WORKING –
MINIMUM APPROACH DISTANCES FOR AC 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
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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
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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 second edition cancels and replaces the first edition of IEC 61472 published in 1998.
This second edition 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.

61472 © IEC:2004 – 9 –
Significant changes with regard to the first edition are the following: this second edition
– revises the application range of this method of calculation to 72,5 kV and above;
– expands in a detailed manner the calculation of the influence of floating objects;
– refers closely to the relevant brochures of CIGRE and to IEC 60071-2.
The text of this standard is based on the following documents:
FDIS Report on voting
78/582/FDIS 78/586/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 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.
The contents of the corrigenda of May 2005 and November 2006 have been included in this
copy.
61472 © IEC:2004 – 11 –
LIVE WORKING –
MINIMUM APPROACH DISTANCES FOR AC 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 between parts and/or workers
at different 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 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 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
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 Transient overvoltages due e.g. to switching operations and abnormal temporary variations of voltage are
not taken into account.
[IEV 601-01-23, modified]
2.1.2
transient overvoltage
short duration overvoltage of few milliseconds or less, oscillatory or non-oscillatory, usually
highly damped
[IEV 604-03-13]
61472 © IEC:2004 – 13 –
2.1.3
fifty per cent disruptive discharge voltage
U
peak value of an impulse test voltage having a 50 per cent probability of initiating a disruptive
discharge each time the dielectric testing is performed
[IEV 604-03-43]
2.1.4
ninety per cent statistical impulse withstand voltage
U
peak value of an impulse test voltage at which insulation exhibits, under specified conditions,
a 90 % probability of withstand
NOTE This concept is applicable to self-restoring insulation.
[IEV 604-03-42, modified]
2.1.5
two per cent statistical overvoltage
U
peak value of a transient overvoltage having a 2 % statistical probability of being exceeded
[IEV 651-01-23, modified]
2.1.6
required insulation level for live working
statistical impulse withstand voltage of the insulation at the work location necessary to reduce
the risk of breakdown of this insulation to an acceptably low level
NOTE It is generally considered that an acceptable low level is reached when the value of the statistical withstand
voltage is greater or equal to the statistical overvoltage having a probability of being exceeded by no more than
2 %.
[IEV 651-01-17]
2.1.7
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 This applies to u and u defined in Clause 4.
e2 p2
2.1.8
minimum approach distance
D
A
minimum distance in air to be maintained between any part of the body of a worker, including
any object (except appropriate tools for live working) being directly handled, and any parts at
different potentials
NOTE The “appropriate tools” are tools for live working suitable for the maximum nominal voltage of the live
parts.
[Definition 2.7.1 of IEC 60743 and IEV 651-01-20, modified]
2.1.9
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
[Definition 2.7.2 of IEC 60743 and IEV 651-01-21, modified]

61472 © IEC:2004 – 15 –
2.1.10
ergonomic distance
D
E
distance in air to take into account inadvertent movement and errors in judgement of
distances while performing work
[Definition 2.7.3 of IEC 60743 and IEV 651-01-22]
2.1.11
part
any element present in the work location, other than workers, live working tools and system
insulation
2.1.12
live part
conductor or conductive part intended to be energized in normal operation, including a neutral
conductor, but by convention not a PEN conductor [IEV 195-02-12] or PEM conductor
[IEV 195-02-13] or PEL conductor [IEV 195-02-14]
NOTE This concept does not necessarily imply a risk of shock.
[Definition 2.1.2 of IEC 60743 and IEV 651-01-03, modified]
2.1.13
work location
any site, place or area where a work activity is to be, is being, or has been carried out
[IEV 651-01-08]
2.2 Symbols used in the normative part of the document
β ratio of the total length of the floating object(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 ,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 dimensions, in the direction of the gap axis, of the floating objects in the air
gap (in metres)
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 units
d
k
floating object factor
f
k gap factor
g
61472 © IEC:2004 – 17 –
k insulator strings factor
i
k standard statistical deviation factor
s
L original air gap length
f
n number of damaged units in a string of n units
d o
n number of units in an insulator string that are not shunted by arcing horns or grading
o
rings
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 percent
e
U
two per cent statistical overvoltage
U fifty per cent disruptive discharge voltage
U
ninety per cent statistical impulse withstand voltage
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 the calculation of the minimum approach distances is based on three
considerations:
a) to determine the statistical overvoltage expected in the work location (U ) and from this,
determine the required statistical impulse withstand voltage of the insulation in the work
location (U );
b) to calculate the electrical distance D required for the impulse withstand voltage U ;
U 90
c) 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
D = D + D (1)
A U E
where
D is the electrical distance necessary to obtain U ;
U 90
D is the required ergonomic distance and is dependent on work procedures, level of
E
training, skill of the workers, type of construction, and such contingencies as
inadvertent movement, and errors in appraising distances (see Annex A for details).

61472 © IEC:2004 – 19 –
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
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
where
K is the statistical safety factor (1,1 for formula (5), (6) and (7)) (see Clause 5);
s
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 D
U
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 )
90 t
D = 2,17 (e – 1) + F (8)
U
61472 © IEC:2004 – 21 –
where
F is the floating object distance in metres (see 4.3.4);
U is the phase to earth or the phase to phase statistical impulse withstand voltage in kV;
K is given by:
t
K = k k k k k (9)
t s g a f i
4.3.1 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).
4.3.2 Gap factor k
g
The gap factor k takes into account the effect of the gap configuration on the dielectric
g
strength of air (see Annex D).
NOTE 1 Unless an appropriate gap factor can be selected for the structure configurations that exist at the system
voltage being considered, a generally conservative value of k = 1,2 for phase to earth and k = 1,45 for
g g
phase to phase are recommended, to allow for a variety of configurations.
NOTE 2 CIGRÉ Brochure 72 and IEC 60071-2 provide more information concerning the determination of k for
g
various gap configurations.
4.3.3 Atmospheric factor k
a
The atmospheric factor takes into account the effect of air density. Air density is influenced by
temperature, humidity and altitude. The effect of temperature and humidity is negligible in
comparison with the effect of altitude.
The electric strength of the air insulation in the work location is mainly affected by the altitude
above sea level. This effect, which varies to some extent with the gap length, or conversely
with the withstand voltage, is accounted for by the atmospheric factor k . The appropriate
a
value of k can be selected from Table E.1 or calculated for a specific altitude and U by the
a 90
method given in Annex E, for a reference altitude below which most live work is done.
The electrical distance D should be increased when live work is carried out in locations
U
higher than the reference altitude in order to account for the lower mean atmospheric
pressure. This can be done by multiplying D by an altitude correction factor, which can be
U
calculated using the equations given in Annex E.
4.3.4 Floating object factor k
f
Floating objects can decrease, or increase, the electric strength of a gap by field distortion.
A conductive object placed between two electrodes at different potentials, and not connected
to either one, is electrically floating and acquires an intermediate potential. The extent of the
influence these conductive floating objects have on the electric strength of the gap varies
depending on the number of floating objects, their dimensions, shapes and geometrical
positions in the gap. Nevertheless, the presence of the floating object(s) reduces the net
electrical length of the air gap.

61472 © IEC:2004 – 23 –
When calculating the effects of floating objects, all possible disruptive discharge paths should
be considered in determining the object factor k and a floating object distance F (sum of all
f
dimensions, in the direction of the gap axis, of the floating objects in the air gap).
The k factor depends on the dimension F of the conductive floating object in the direction of
f
the axis of the gap, on the length D of the remaining gap and on the lateral distance r of the
conductive object from the axis of the gap (see Figure 1). It must be pointed out that D is
obtained by subtracting the length F from the original air gap L , i.e. D = L − F. Papers of
f f
international level (see Annex F) provide evaluation criteria of the k factor as a function of F
f
and D (P when phase to phase distances are considered), by introducing the parameter
β = F()D + F
(or β = F()P + F when phase to phase distances are considered).
These studies and other experimental investigations have shown that, in the more critical
cases representative of live line working configurations, the k coefficient may be as low as
f
0,75 for phase to earth gap distances over 1,2 m.
Table 1 reports a simplified criterion for the k determination in dependence of β and L . The k
f
f f
values derive from the interpolation of the data shown in Annex F. Table 1 contains the values
of β in function of the original gap length L rather than in function of the remaining air gap
f
length D because the original gap length L is one of the important quantities that characterise
f
the constructed a.c. system.
61472  IEC:2004 – 25 –
Table 1 – Floating object factor k
f
Phase to earth gaps Phase to phase gaps
L *
k β **  k
L * f f f
f
β **
m
m
Over Up to Over Up to Over Up to Over Up to
--- 0,9 3,9 --- 1 --- 0,9 5,7 --- 1
0,1
0,9 3,9 --- 0,95 0,9 2,1 3,8 5,7 0,95
0,05
--- 0,5 4,7 --- 1 2,1 3,8 --- 0,9
0,5 1 3,3 4,7 0,95 --- 0,6 6 --- 1
0,15
1 1,2 2,7 3,3 0,9 0,6 1,6 4,6 6 0,95
0,1
1,2 2,7 --- 0,85 1,6 2,2 3,6 4,6 0,9
--- 0,4 4,9 --- 1 2,2 3,6 --- 0,85
0,4 0,9 3,7 4,9 0,95 --- 0,4 6,3 --- 1
0,2 0,9 1 3,1 3,7 0,9 0,4 1,4 5,1 6,3 0,95
1 1,2 2,6 3,1 0,85 0,2 1,4 1,8 4,4 5,1 0,9
1,2 2,6 --- 0,8 1,8 2,3 3,5 4,4 0,85
--- 0,3 5,1 --- 1 2,3 3,5 --- 0,8
0,3 0,8 3,8 5,1 0,95
* L = Original air gap length.

f
0,8 0,9 3,2 3,8 0,9
** β = Ratio of the total length of the floating object(s) to
0,25
the original air gap length.
0,9 1,1 2,8 3,2 0,85
1,1 1,3 2,4 2,8 0,8 NOTE  β values over the ones tabulated are not
practical.
1,3 2,4 --- 0,75
As far as the influence of the distance of the floating objects from the axis of the gap is
considered, it may be assumed that the reduction of the electric strength becomes negligible
when
r > 2,5 F
The influence of metallic caps and pins of suspension insulators is negligible and shall be
ignored.
The approach in Annex F gives general criteria for the determination of k . The real influence
f
of the floating objects requires a detailed analysis (see Annex F).
Figure 2 illustrates various distinct live working tasks and the configurations in which they can
occur. According to the considered configuration, a correct value of k and k should be
g f
determined.
See Annex F for more details.
61472 © IEC:2004 – 27 –
4.3.5 Insulator strings factor k
i
When there is no damaged insulator present, k is equal to 1,0. At all times, care shall be
i
taken that the electrical integrity of the insulator assembly is not impaired by tools in parallel,
moisture or contamination on the surface and damaged insulators (see Annex G).
The effect of damaged insulation on the withstand voltage in the work location shall be allowed for
by ensuring that a minimum number of undamaged insulators is always present while working live
near the insulation. The minimum system insulation length shall be determined from equation (8)
using a value of k given in the empirically-derived and conservative formula (10), unless the
i
minimum system insulation distance requirement is known from test data or by other means. The
calculated value of D is the minimum insulation length measured between live and earth
U
electrodes. In this case, it is not the electrical distance.
K
U /(1 080 )
90 t
D = 2,17 (e – 1) + F (8)
U
k = 1− 0,8k()n n (10)
i d d o
where
n is the number of damaged units in a string of n units;
d o
k is a coefficient characterizing the average state of the damaged units;
d
k = 1 for toughened glass insulators;
d
k = 0 to 1 for porcelain insulators, with k = 0,75 as an average value.
d d
Consideration of arcing horn or grading ring spacing shall also be taken into account,
(see Annex G).
The effect of arcing horns or grading rings is to shield to some extent the insulator units
between them. Consideration should be given to this when determining k . This can be done
i
by using equation (10), with n the number of units in an insulator string that are not shunted
o
by arcing horns or grading rings, and n the number of damaged units included in n .
d o
NOTE Units shielded by horns or rings do not significantly contribute to the dielectric strength of the string, hence
damage in this area is less important and the units can be shorted during work.
The situation with multiple string assemblies and vee-strings is more complex than for a
single string.
4.3.6 Composite insulator
For composite insulators, a method for determining the electrical and mechanical strength
while in service is under consideration.
5 Evaluation of risks
The overall risk of breakdown of the insulation at the work location is associated with a
number of situations described below. These situations, when combined, reduce the overall
risk of breakdown. They are as follows:
– the actual system voltage is not always at a maximum value;
– the location of the work is not likely to correspond to the place where a transient
overvoltage is at the maximum value;

61472 © IEC:2004 – 29 –
– the stress of the actual transient overvoltage wavefront is less than the critical front;
– approximately half of the transient overvoltages will be of negative polarity, and are less
severe;
– the frequency and amplitude of transient overvoltages are reduced by restricting reclosing
of circuit breakers.
Thus, the value of 1,1 is recommended for K to reduce the overall risk of breakdown of the
s
insulation to a level that correlates with other electrical work operations.
The overall risk of a breakdown occurring during live working, when an ergonomic distance D
E
is incorporated, will be lower because an overvoltage is unlikely to arise at the work location
at that instant where the ergonomic distance is entirely breached by inadvertent movement of
the worker or object. Because of this, a value of K = 1,0 can be used when a defined
s
ergonomic distance D is included and is great enough that the value of D is always greater
E A
than the value of D (equivalent to D when D is zero) calculated using K = 1,1 i.e.:
A U E s
D + D > D
U(K = 1,0) E U(K = 1,1)
s s
where D and D are D calculated using K = 1,0 and K = 1,1 respectively.
U(K = 1,0) U (K = 1,1) U s s
s s
But, in doing so, the effective ergonomic distance, D , is reduced and shall then be more
E
tightly controlled to retain the same overall risk of breakdown.
6 Calculation of minimum approach distance D
A
The electrical distance D is calculated (in metres) from:
U
K
U /(1 080 )
90 t
D = 2,17 (e – 1) + F (8)
U
where
F is the floating object length (see 4.3.4);
U = K U (from equation (5));
90 s 2
K is obtained in equation (9) K = k k k k k .
t t s g a f i
After selecting an appropriate value for the ergonomic distance D (see Annex A), the
E
minimum approach distance D can then be determined by equation (1):
A
D = D + D (1)
A U E
NOTE The value chosen for the ergonomic distance differs between users. It generally falls in the range of 0,2 m
to 1 m (see Annex A).
An example of the calculation results is shown in Table 2 for values of the various factors.
The electrical distance D is calculated against highest system voltage U for various levels
U s
of 2 % switching overvoltage u , and for K = 1,1; k = 0,936; k = 1,2; k taken from
e2 s s g a
Table E.1 for 1 000 m altitude; k and k = 1,0 and F = 0.
f i
61472 © IEC:2004 – 31 –
Table 2 – Example of calculation of electrical distance
for some switching overvoltage values
U u D u D u D u D
s e2 U e2 U e2 U e2 U
kV p.u. mm p.u. mm p.u. mm p.u. mm
72,50 2,20 304 2,30 319 2,70 379 3,50 495
82,50 2,20 350 2,30 367 2,70 429 3,50 572
100,00 2,20 423 2,30 444 2,70 530 3,50 711
123,00 2,20 532 2,30 559 2,70 669 3,50 905
145,00 2,20 639 2,30 673 2,70 809 3,50 1 081
170,00 2,20 767 2,30 808 2,70 958 3,50 1 315
245,00 2,20 1 163 2,30 1 229 2,70 1 505 3,50 2 085
300,00 2,20 1 501 2,30 1 561 2,70 1 930 3,50 2 733
420,00 2,20 2 276 2,30 2 423 2,70 3 011 3,50 4 457
525,00 2,20 3 095 2,30 3 311 2,70 4 198 3,50 6 589
765,00 2,20 5 620 2,30 6 085 2,70 8 243 3,50 14 398

61472 © IEC:2004 – 33 –
U(+) U(–)
L
f
U(–)
U(+)
r
d l d l d
1 1 2 2 3
IEC  884/04
Figure 1 – Illustration of two floating objects of different dimensions
and at different distances from the axis of the gap (see 4.3.4)

61472 © IEC:2004 – 35 –
a) Worker not in the air gap
d
d >D
1 A
IEC  885/04
b) Worker using hot stick
d
d > D
1 A
IEC  886/04
d
d
c) Worker at intermediate potential
The smallest distance between
d
d + d or d + d > D 2
1 3 2 3 A
IEC  887/04
d
d) Barehand work
The smallest distance between
d or d and d or d > D d
1 2 3 4 A
d
d
IEC  888/04
Figure 2 – Typical live working tasks (see Clause 2 and 4.3.4)

61472 © IEC:2004 – 37 –
Annex A
(informative)
Ergonomic distance
A.0 Introduction
Two approaches, or a blend of both, can be used to establish an ergonomic distance required:
– specify only an absolute minimum approach distance and let the skilled worker decide the
extra distance required for the particular job to be done;
– specify a complete minimum approach distance allowing a sufficient safety margin to
account for all possible contingencies.
A number of factors have to be considered before specifying the minimum approach distance,
or commencing work close to a live conductor. As it is impractical and inappropriate to
recommend an ergonomic distance here, the following points are provided as guidelines for
consideration by individual organisations.
A.1 Training, knowledge and skill
Basic to live working is knowledge of the hazards and means of personal protection, by
minimum approach distances and other methods. Workers shall have been thoroughly trained
in live work and in the job at hand. During work, attention shall be shared between the work
and observing the minimum approach distance. Adequate training and practice in the work
procedure will reduce the possibility of attention being diverted from observing the minimum
approach distance by unexpected situations.
A.2 Protective barriers
Barriers placed between the workers and the live conductors may provide the required
insulation level, or merely serve as a mechanical barrier. Only fully insulating barriers with
adequate mechanical strength can be placed closer to the live conductors than the electrical
distance.
A.3 Possibility of error
The possibility of errors being committed during the work depends on the work procedure
being used, personal factors, effects of the environment and the extent to which the
workers' actions are monitored by others.
A.4 Work procedure
Different work positions and methods will require different allowances for unintentional
movement. The stability of the worker's position can also vary from task to task, e.g. working
above the earth, compared with working on the earth. A complex or strenuous job is also
more likely to divert the worker’s attention away from observing the minimum approach
distance.
Because of these factors, consideration could be given to using a different ergonomic
distance for different work situations or procedures.

61472 © IEC:2004 – 39 –
A.5 Personal factors
A worker's physical, mental and emotional states are also possible causes for unintentional
movement. These factors are, in turn, influenced by the duration and strenuousness of the
job, for instance. Live working requires constant attention, both to the procedures and the
minimum approach distance, attention which can be readily distracted by personal factors.
For this reason, self control and safety awareness are essential skills to work at the minimum
approach distance.
A worker's ability to judge the minimum approach distance correctly is also important. For this
reason it may be beneficial to increase the ergonomic distance with the voltage. However, too
large a distance at high voltages will make small components on the live conductors difficult
to see, and tools heavier to handle.
Workers should not wear clothing with loose parts that could fall, blow or swing close to the
live conductors.
A.6 Environmental factors
Certain environmental influences are generally taken into account by prohibiting work at the
minimum approach distance under those conditions. For instance, work is normally not
permitted during nearby thunderstorms, or when there is a continuous film of water on the
surface of insulating tools.
Adverse conditions may also be created by other environmental conditions, either directly, or
by diverting attention away from the authorized work procedures. Strong winds move
conductors, supports or equipment (e.g. aerial devices) dangerously. Dust storms are an eye
hazard. Ice on structures will make footing insecure. Woodpecker holes present a climbing
hazard. Mist or sea spray may pose a hazard to system or tool insulation. Smooth surfaces
may be slippery. Darkness or glare may impair vision. Consideration should also be given to
the effect of high temperature and humidity to worker fatigue. These are some of the
environmental influences on work close to live conductors, some of which will need to be
taken into account when establishing minimum approach distances.
A.7 Monitoring
To warn workers of dangerous situations arising during the work, it can be sometimes
beneficial to require continuous monitoring by an observer. Failing that, the workers should be
encouraged to describe aloud to one another each step in the work procedure before taking it.
The procedure to be followed should also be detailed and discussed between the foreman and
workers before commencing work.

61472 © IEC:2004 – 41 –
Annex B
(informative)
Overvoltages
B.1 The different types of overvoltages
The overvoltage values are expressed in p.u., and 1 p.u. is equal to U 2/ 3 (see 2.7).
s
According to IEC 60071-1, voltages and overvoltages are divided in the classes shown in
Table B.1.
For live working applications, the overvoltages of major concern, in the voltage range of
interest, are the slow front overvoltages due to switching operations. Fast front transients,
such as lightning overvoltages, are not generally of importance, since live work is not
normally performed under conditions of inclement weather. Permanent and temporary low
frequency overvoltages are usually also not of concern, since they are not critical for the air
gaps which are required to withstand switching overvoltages during live work.
B.2 Transient slow-front overvoltages
Slow-front overvoltage amplitudes depend on many parameters such as the characteristics of
the system, the point of the system, the type of event considered. Moreover, even at the same
point and for the same family of events, the overvoltages have a statistical nature. So slow-
front overvoltage amplitudes are evaluated on the basis of a statistical approach: reference is
usually made to the overvoltage having the probability of 2 %
...