SIST EN 61472:2007
(Main)Live working - Minimum approach distances for a.c. systems in the voltage range 72,5 kV to 800 kV - A method of calculation
Live working - Minimum approach distances for a.c. systems in the voltage range 72,5 kV to 800 kV - A method of calculation
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.
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
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. Cette norme traite des surtensions de réseau et des distances de travail dans l'air entre des pièces et/ou des travailleurs à des potentiels différents. La tension de tenue requise et les distances minimales d'approche calculées suivant la méthode décrite dans cette norme sont évaluées en prenant en compte ce qui suit: - les travailleurs sont formés et qualifiés pour travailler dans la zone de travail sous tension; - les surtensions attendues ne dépassent pas la valeur choisie pour la détermination de la distance minimale d'approche requise; - les surtensions transitoires sont les surtensions déterminantes; - l'isolation des outils ne présente pas sur la surface un film continu d'humidité; - aucun éclair n'est vu ni entendu à moins de 10 km du lieu de travail; - l'influence des parties conductrices des outils est prise en compte; - l'effet de l'altitude 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.
Delo pod napetostjo - Najmanjše razdalje za dostop do sistemov z izmenično napetostjo v območju od 72,5 kV do 800 kV - Metoda izračuna (IEC 61472:2004)
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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
---------------------- Page: 2 ----------------------
EN 61472:2004 - 2 -
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).
__________
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NORME CEI
INTERNATIONALE IEC
61472
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
© IEC 2004 Droits de reproduction réservés ⎯ Copyright - all rights reserved
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Pour prix, voir catalogue en vigueur
For price, see current catalogue
---------------------- Page: 4 ----------------------
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
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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
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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
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 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.
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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.
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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]
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61472 © IEC:2004 – 13 –
2.1.3
fifty per cent disruptive discharge voltage
U
50
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
90
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
2
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]
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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
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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
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 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,
2
determine the required statistical impulse withstand voltage of the insulation in the work
location (U );
90
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).
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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
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
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
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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;
90
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.
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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 th
...
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