IEC 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
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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

– 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

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

– 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
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Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
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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.
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6) All users should ensure that they have the latest edition of this publication.
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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.
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;

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.
The contents of the corrigendum of October 2015 have been included in this copy.

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.
– 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]

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
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
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.]

– 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
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)
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
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 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,
determine the required statistical impulse withstand voltage of the insulation in the work
location (U );
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
– 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
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
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
U /(1 080K )
90 t
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
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).
4.3.2.2 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 that allows for a variety of configurations is k = 1,2 for
g
phase to earth and k = 1,45 for phase to phase.

g
NOTE 2 CIGRÉ Brochure 72 and IEC 60071-2 provide more information concerning the determination of k for
g
various gap configurations.
4.3.2.3 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 1 of average values or from Table E.1 or calculated for
a
a specific altitude and U by the method given in Annex E, for a reference altitude below
which most live work is done.
– 12 – 61472 © IEC:2013
Table 1 – Average k values
a
Altitude k
a
m average
0 1,000
100 0,995
300 0,983
500 0,972
1 000 0,941
1 500 0,909
2 000 0,875
2 500 0,841
3 000 0,805
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.
4.3.2.4 Floating conductive object factor k
f
Floating conductive objects can decrease, or increase, the electric strength of a gap by field
distortion.
A conductive object placed between two electrodes at different electric potentials, and not
connected to either one, is electrically floating and acquires an intermediate potential. The
extent of the influence these floating conductive objects have on the electric strength of the
gap varies depending on the number of floating conductive objects, their dimensions, shapes
and geometrical positions in the gap. Nevertheless, the presence of the floating conductive
object(s) reduces the net electrical length of the air gap.
When calculating the effects of floating conductive objects, all possible disruptive discharge
paths should be considered in determining the floating conductive object factor k . The sum of
f
all floating conductive objects in the direction of the gap axis constitutes the floating
conductive object length, F.
In the most common live line work situations on high voltage lines, the k factor depends on
f
the length of the remaining gap and on the lateral distance r of the conductive object from the
axis of the gap (see Figure 1). It has to be pointed out that D is obtained by subtracting the
length F from the original air gap L , i.e. D = L − F. Annex F provides evaluation criteria of the
f f
k factor as a function of F and D (P when phase to phase distances are considered), by
f
introducing the parameter
β= F(D+ F)
(or β= F(P+ F) when phase to phase distances are considered).
Experimental investigations (see Annex F) have shown that, in the more critical cases
representative of live line working configurations, the k coefficient may be as low as 0,75 for
f
phase to earth gap distances over 1,2 m.

61472 © IEC:2013 – 13 –
IEC  623/13
Figure 1 – Illustration of two floating conductive objects of different dimensions
and at different distances from the axis of the gap
Table 2 reports a simplified criterion for the k determination in dependence of β and L . The k
f f f
values are derived from the interpolation of the data shown in Annex F. Table 2 contains the
values of β in function of the original gap length L rather than in function of the remaining air
f
gap length D because the original gap length L is one of the important quantities that
f
characterise the constructed a.c. system.
For long or flat shaped conductive objects situated perpendicular to the air gap, for which no
specific experimental data exists, a conservative value k = 0,75 may be assumed.
f
– 14 – 61472 © IEC:2013
Table 2 – Floating conductive object factor k
f
Phase to earth gaps Phase to phase gaps
a b a
L k β L k
f f
f f
b
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
0,8 0,9 3,2 3,8 0,9
0,25
0,9 1,1 2,8 3,2 0,85
1,1 1,3 2,4 2,8 0,8
1,3 2,4 --- 0,75
NOTE  When β values over the ones tabulated are involved, tests or studies are needed in order to consider the
actual floating conductive object shape and dimension.
a
L = Original air gap length.
f
b
β = Ratio of the total length in the direction of the gap axis of the floating conductive objects (s) to the original
air gap length.
As far as the influence of the distance of the floating conductive 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 and conductive objects of
similar size, in the vicinity of the 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 conductive 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:2013 – 15 –
a) Worker not in the air gap
d >D
1 A
b) Worker using insulating stick
d > D
1 A
c) Worker at intermediate potential
The smallest distance between
d + d or d + d > D
1 3 2 3 A
d) Barehand work
The smallest distance between
d or d and d or d > D
1 2 3 4 A
IEC  624/13
Figure 2 – Typical live working tasks

– 16 – 61472 © IEC:2013
4.3.2.5 Damaged insulator factor k
i
When working next to insulators it is necessary to calculate the minimum residual insulation
length, D . This is done taking into account the effects of damaged insulators or portion of
Lins
damaged insulator on the minimum approach distance (refer to Clause 3 for the
methodology).
Care shall be 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
considered by ensuring that a minimum number of undamaged insulator units or insulator
length is present before commencing work. The minimum residual insulation length, D ,
Lins
shall be determined from equation (10) using K as given in equation (11) and a value of k
i
t
given in the empirically-derived formula (12). The residual insulation length may otherwise be
determined from test data or by other means.
U /(1 080K )
90 t
D = 2,17 (e – 1) + F (10)
Lins
where
K = k k k k k (11)
t s g a f i
k = 1 – 0,8 k (A /A) (12)
i d d o
where
A is the length of undamaged insulator or number of undamaged insulator units not shunted
o
by long arcing horn or grading ring;
is the length of damaged insulator or number of damaged units in an insulator of A
A
d o
length or units not shunted by long arcing horn or grading ring;
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
k = 1,25 for composite insulators affected by conductive or semiconductive damages
d
(see Annex G).
Refer to Annex G for consideration of arcing horn or grading ring spacing.
NOTE Portions of insulators 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 these portions can be shorted during work.
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;
– the stress of the actual transient overvoltage wavefront is less than the critical front;

61472 © IEC:2013 – 17 –
– 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, when no ergonomic distance is used, the value of 1,1 is recommended for K to reduce
s
the overall risk of breakdown of the 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 (when D is zero) with D calculated using K = 1,1 i.e.:
A E s
U
D =D + D > D
A 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
6 Calculation of minimum approach distance D
A
The following example is provided for demonstrating the use of equation (8) only and does not
suggest proper or typical selection of k values, u , or other factors.
e2
The electrical distance D is calculated (in metres) from:
U
U /(1 080K )
90 t
D = 2,17 (e – 1) + F (8)
U
where
F is the floating conductive object length (see 4.3.2.4);
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 Clause 5 and Annex
E
A), the minimum approach distance D can then be determined by equation (1):
A
D = D + D
A U E
()K =1,0
s
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 Clause 5 and Annex A).
In this example, electrical distance D is calculated for K = 1,0; k = 0,936; k = 1,2; k =
U s s g a
0,941 (from Table 1 for 1 000 m); k and k = 1,0, U = 525 kV, u = 2,2, F = 0 and D = 0,3 m.
f i E
s e2
D = 2,787 m (2,8 m)
U
D = 2,8 m + 0,3 m = 3,1 m
A
– 18 – 61472 © IEC:2013
Annex A
(informative)
Ergonomic distance
A.1 Overview
Two approaches, or a blend of both, can be used to establish an ergonomic distance:
– 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.2 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 need to be thoroughly trained in
live work and in the job at hand. During work, attention needs to be shared between the work
being performed and respecting the minimum approach distance. Adequate training and
practice in the work procedure will reduce the possibility of attention being diverted from
respecting the minimum approach distance by unexpected situations.
A.3 Protective barriers
A barrier is an obstacle that is in place to reduce or eliminate the ergonomic distance
requirement. It limits the distance workers can reach or approach to anything that is at a
different electric potential. A barrier could be metallic or non-metallic material.
A.4 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.5 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:2013 – 19 –
A.6 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. This includes equipotential bonding leads of conductive suits.
A.7 Environmental factors
Certain environmental conditions 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, supp
...