SIST EN 60099-5:1998
(Main)Surge arresters -- Part 5: Selection and application recommendations
Surge arresters -- Part 5: Selection and application recommendations
Provides recommendations for the selection and application of surge arresters to be used in three-phase systems with nominal voltages above 1kV. It applies to non-linear resistor type gapped surge arresters as defined in EN 60099-1 and to gapless metal-oxide surge arresters as defined in EN 60099-4.
Überspannungsableiter -- Teil 5: Anleitung für die Auswahl und die Anwendung
Parafoudres -- Partie 5: Recommandations pour le choix et l'utilisation
Comporte des recommandations pour le choix et l'emploi des parafoudres à utiliser sur des réseaux triphasés de tension nominale supérieure à 1kV. Elle concerne les parafoudres à résistance variable avec éclateurs conformes à la EN 60099-1, ainsi que les parafoudres à oxyde métallique sans éclateur conformes à la EN 60099-4.
Prenapetostni odvodniki - 5. del: Izbira in priporočila za uporabo (IEC 60099-5:1996, spremenjen)
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN 60099-5:1998
01-april-1998
3UHQDSHWRVWQLRGYRGQLNLGHO ,]ELUDLQSULSRURþLOD]DXSRUDER ,(&
VSUHPHQMHQ
Surge arresters -- Part 5: Selection and application recommendations
Überspannungsableiter -- Teil 5: Anleitung für die Auswahl und die Anwendung
Parafoudres -- Partie 5: Recommandations pour le choix et l'utilisation
Ta slovenski standard je istoveten z: EN 60099-5:1996
ICS:
29.240.10 Transformatorske postaje. Substations. Surge arresters
Prenapetostni odvodniki
SIST EN 60099-5:1998 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN 60099-5:1998
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SIST EN 60099-5:1998
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SIST EN 60099-5:1998
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SIST EN 60099-5:1998
CEI
NORME
INTERNATIONALE IEC
99-5
INTERNATIONAL
Première édition
STANDARD
First edition
1996-02
Parafoudres -
Partie 5:
Recommandations pour le choix et l'utilisation
Surge arresters -
Part 5:
Selection and application recommendations
© CEI 1996 Droits de reproduction réservés — Copyright — all rights reserved
Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in
utilisée sous quelque forme que ce soit et par aucun pro- any form or by any means, electronic or mechanical,
cédé, électronique ou mécanique, y compris la photocopie et including photocopying and microfilm, without permission
les microfilms, saro l'accord écrit de l'éditeur. in writing from the publisher.
rue de Varembé Genève, Suisse
Bureau Central de la Commission Electrotechnique Inte rnationale 3,
Commission Electrotechnique Internationale CODE PRIX
X
International Electrotechnical Commission
PRICE CODE
IEC MemutywapoANaa 3nestporexwwecnaa Houwccwa
Pour prix, voir catalogue en vigueur
• •
For price, see current catalogue
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SIST EN 60099-5:1998
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SIST EN 60099-5:1998
IEC Publication 99-5
Publication 99-5 de la CEI
(First edition - 1996)
(Première édition - 1996)
Surge arresters
Parafoudres
Part 5: Selection and application
Partie 5: Recommandations pour le choix
recommendations
et l’utilisation
C O R R I G E N D U M 1
Page 5
Page 4
FOREWORD
AVANT-PROPOS
Insert the following text between the first and
Ajouter le texte suivant entre le premier et le
the second paragraphs after note 6):
deuxième alinéa après la note 6):
The text of this standard cancels and replaces
Le texte de la présente norme annule et
IEC 99-1A, published in 1965.
remplace la CEI 99-1A, publiée en 1965.
April 1996
Avril 1996
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SIST EN 60099-5:1998
99-5 ©IEC:1996 – 3 –
CONTENTS
Page
FOREWORD 5
7
INTRODUCTION
Clause
SECTION 1: GENERAL
9
1.1 Scope
1.2 Normative references 9
1.3 General principles for the application of surge arresters 9
1.4 General procedure for the selection of surge arresters 11
1.5 Polluted housing arrester withstand 15
SECTION 2: NON-LINEAR RESISTOR TYPE GAPPED SURGE ARRESTERS
ACCORDING TO IEC 99-1
17
2.1 Characteristic data of gapped surge arresters
2.2 Selection of gapped arresters phase-to-earth 21
SECTION 3: GAPLESS METAL-OXIDE SURGE ARRESTERS
ACCORDING TO IEC 99-4
3.1 Characteristic data of gapless metal-oxide surge arresters 31
3.2 Selection of gapless metal-oxide surge arresters phase-to-earth 35
SECTION 4: APPLICATION OF ARRESTERS
4.1 Principle of insulation co-ordination 47
4.2 Protection from slow-front overvoltages 47
4.3 Protection from lightning overvoltages 51
SECTION 5: SURGE ARRESTERS FOR SPECIAL APPLICATION
5.1 Surge arresters for transformer neutrals 65
5.2 Surge arresters between phases 67
5.3 Surge arresters for rotating machines 69
5.4 Further special applications of surge arresters 71
5.5 Surge arresters for abnormal service conditions 71
SECTION 6: MONITORING (SUPERVISION)
6.1 General 73
6.2 Discharge counters 73
73
6.3 Monitoring spark gaps
73
6.4 Device for monitoring the continuous current
Annexes
A Determination of temporary overvoltages due to earth faults 75
B Current practice 87
Bibliography 89
C
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SIST EN 60099-5:1998
99-5 ©IEC:1996 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
SURGE ARRESTERS —
Part 5: Selection and application recommendations
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization
comprising all national electrotechnical committees (IEC National Committees). The object of the IEC is to
promote international cooperation on all questions concerning standardization in the electrical and
electronic fields. To this end and in addition to other activities, the IEC publishes International Standards.
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. The IEC
collaborates closely with the International Organization for Standardization (ISO) in accordance with
conditions determined by agreement between the two organizations.
The formal decisions or agreements of the IEC on technical matters, express as nearly as possible an
2)
international consensus of opinion on the relevant subjects since each technical committee has
representation from all interested National Committees.
The documents produced have the form of recommendations for international use and are published in the
3)
form of standards, technical reports or guides and they are accepted by the National Committees in that
sense.
In order to promote international unification, IEC National Committees undertake to apply IEC International
4)
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
5)
equipment declared to be in conformity with one of its standards.
Attention is drawn to the possibility that some of the elements of this International Standard may be the
6)
subject of patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 99-5 has been prepared by IEC technical committee 37: Surge
arresters.
The text of this standard is based on the following documents:
FDIS Report on voting
37/144/RVD
37/123/FDIS
Full information on the voting for the approval of this standard can be found in the report
on voting indicated in the above table.
Annexes A, B and C are for information only.
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SIST EN 60099-5:1998
99-5 © IEC:1996 –7–
INTRODUCTION
This part of IEC 99 is intended to allow optimal selection and application of surge
arresters specified according to IEC 99-1 and IEC 99-4.
More complex calculation methods than those indicated here may be used in order to
obtain more precise determination of the requirement for the system concerned; however,
these calculations should be performed in accordance with the principles given in this
standard.
The characteristics of surge arresters derived from the application of this standard are
different for different systems. No particular numerical value may be favoured. It is likely,
however, that for some systems, or in some countries, the system reliability requirements
and design are sufficiently uniform that the recommendations of the present standard may
lead to the definition of narrow ranges of arresters. The user of surge arresters will, in that
case, not be required to apply the whole process introduced here, to any new installation
and he may reproduce the selection of characteristics resulting from prior practice.
Corresponding numerals may be introduced by national authorities in annex B.
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SIST EN 60099-5:1998
99-5©IEC:1996 - 9 -
SURGE ARRESTERS —
Part 5: Selection and application recommendations —
Section 1: General
1.1 Scope
This part of IEC 99 provides recommendations for the selection and application of surge
arresters to be used in three-phase systems with nominal voltages above 1 kV. It applies
to non-linear resistor type gapped surge arresters as defined in IEC 99-1 and to gapless
metal-oxide surge arresters as defined in IEC 99-4.
1.2 Normative references
The following normative documents contain provisions which, through reference in this
text, constitute provisions of this pa rt of IEC 99. At the time of publication, the editions
indicated were valid. All normative documents are subject to revision, and pa
rties to
agreements based on this pa rt
of IEC 99 are encouraged to investigate the possibility of
applying the most recent editions of the normative documents indicated below. Members
of IEC and ISO maintain registers of currently valid International Standards.
IEC 71-1: 1993, Insulation co-ordination - Part 1: Definitions, principles and rules
IEC 71-2: 1976, Insulation co-ordination - Part 2: Application guide
NOTE – The third edition of this standard is presently under revision.
IEC 99-1: 1991, Surge arresters - Part 1: Non-linear resistor type gapped surge arresters
for a.c. systems
IEC 99-3: 1990, Surge arresters - Pa rt
3: Artificial pollution testing of surge arresters
NOTE – This Technical Report applies to gapped surge arresters according to IEC 99-1.
IEC 99-4: 1991, Surge arresters - Part 4: Metal-oxide surge arresters without gaps for a.c.
systems
IEC 507: 1991,
Artificial pollution tests on high-voltage insulators to be used on a.c.
systems
IEC 815: 1986, Guide for the selection of insulators in respect of polluted conditions
1.3 General principles for the application of surge arresters
IEC 71-1 specifies withstand voltages for two ranges of highest voltages for equipment:
- range I: above 1 kV to 245 kV included
- range II: above 245 kV
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SIST EN 60099-5:1998
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99-5 ©IEC:1996
For range I systems containing overhead lines, the main risk to equipment arises from
induced and direct lightning strokes to the connected overhead lines. In cable systems not
connected to overhead lines, overvoltages due to faults or switching operations are most
likely to occur. In rare cases, however, lightning induced overvoltages may also be
generated. In systems of range II, in addition to range I factors, switching overvoltages
become impo rtant, increasing with higher system voltages. Overvoltages may cause
flashovers and serious damage to the equipment and thereby jeopardize the supply
of power to users. It is essential to prevent this by the proper co-ordination of surge
arresters with the insulation. It is, therefore, recommended to use surge arresters if there
are possibilities of lightning overvoltages or high switching overvoltages which may be
dangerous to the equipment.
These surge arresters should constitute a reliable pa rt of the system. They are designed
to withstand the voltages and the resulting currents through them with a sufficiently high
reliability taking into account pollution and other site matters. In each system such voltage
stresses are (see IEC 71-1):
- operating voltage;
- temporary overvoltages;
slow-front overvoltages;
- fast-front overvoltages;
where the slow-front overvoltages due to switching are of particular importance for ar-
resters protecting range II equipment.
As a general principle, the best protection of equipment and high surge arrester rated
voltages are contradicting requirements. Thus the selection of an adequate arrester
constitutes an optimization process, which has to consider a great number of system and
equipment parameters.
Gapless metal-oxide surge arresters are of particular advantage for earthed neutral
systems, because they offer better protection against slow-front overvoltages. This
arrester type is today widely installed in these systems and the application of arresters for
such systems tends to concentrate on metal-oxide surge arresters. In some isolated or
resonant earthed neutral systems, where earth fault temporary overvoltages may have
long durations, gapped surge arresters may offer advantages, if protective levels are
required to be low. While being the surge arrester traditionally used in all voltage ranges,
the consideration of gapped arresters may be adequate for systems of range I, especially
in the lower voltage range.
1.4 General procedure for the selection of surge arresters
The following iterative procedure, shown in the flow diagram of figure 1, is recommended
for the selection of surge arresters:
- determine the continuous operating voltage of the arrester with respect to the
highest system operating voltage;
- determine the rated voltage of the arrester with respect to the temporary
overvoltages;
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SIST EN 60099-5:1998
13 –
99-5 © IEC:1996 –
Continuous
Highest
operating voltage
operating voltage
I
Temporary
4
Rated voltage
overvoltages
I
Nominal
Lightning
4
•
discharge current
discharge current
I
+
Line discharge
Discharge
41
class
energy
Pressure relief
Fault current
class
Surge arrester
I
i i
Lightning impulse Switching impulse
Station
protection level
protection level
layout
+!
Prospective slow-
front overvoltage
Representative impinging
lightning overvoltage
+
Station
layout
•
Conductor length
+
between arrester
and protected object
A
Co-ordination
Co-ordination
switching impulse
lightning impulse
withstand voltage
withstand voltage
I •
Rated insulation level
No
Yes
1
Arrester selected
Figure 1 — Flow diagram for the selection of surge arresters
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SIST EN 60099-5:1998
99-5 ©IEC:1996 - 15 -
- estimate the magnitude and probability of the expected lightning discharge currents
through the arrester, determine the transmission line discharge requirements and select
the nominal discharge current, the high current impulse value and the line discharge
class of the arrester;
NOTE — If high current impulse values different from the standardized values are necessary (see IEC 99-4,
table 6, note), these values should be selected according to the lightning discharge current.
- select the pressure relief class of the arrester with respect to the expected fault
current;
select a surge arrester that fulfils the above requirement;
determine the lightning and switching impulse protection characteristics of the arrester;
-
locate the arrester as close as possible to the apparatus to be protected;
- determine the co-ordination switching impulse withstand voltage of the protected
equipment taking into account the representative slow-front overvoltages and system
layout;
determine the co-ordination lightning impulse withstand voltage considering:
• the representative impinging lightning overvoltage surge as determined by
the lightning pe rformance of the overhead line connected to the arrester and the
acceptable failure rate of the protected equipment;
• the substation layout;
• the distance between surge arrester and protected equipment;
- determine the rated insulation level of the equipment from IEC 71-1;
if a lower rated insulation level of the equipment is desired, then a lower continuous
operating voltage, a lower rated voltage, a higher nominal discharge current, a higher
line discharge class, a different arrester design or a reduced distance between arrester
and protected object should be investigated.
NOTE — A lower continuous operating voltage or a lower rated voltage may reduce the service reliability of
the arresters.
Details of this iterative procedure are given in sections 2, 3 and 4 of this document.
1.5 Polluted housing arrester withstand
Pollution on the arrester housing may cause sparkover or temperature increase of grading
components in gapped arresters and high temperature increase of the varistors in
metal-oxide arresters. To prevent arrester failures in polluted areas, arresters able to with-
stand the relevant polluted conditions have to be chosen. Although not explicitly specified
in IEC 99-1 and IEC 99-4, arresters used in normal operating conditions should withstand
the medium pollution stresses according to pollution level II of IEC 71-2. If the arrester
installation area is subjected to a higher pollution, the surge arrester pe rformance may be
adversely affected. If arresters of inadequate design are used in heavy (pollution level Ill)
or very heavy (pollution level IV) polluted areas, periodic cleaning or greasing may be
effective in preventing the events stated above.
When live washing of arresters is intended, arresters designed for such service conditions
are required.
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SIST EN 60099-5:1998
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99-5 ©IEC:1996
Section 2: Non-linear resistor type gapped surge arresters
according to IEC 99-1
2.1 Characteristic data of gapped surge arresters
2.1.1
General
Basic characteristics of surge arresters with series spark gaps are their rated voltage,
their sparkover voltages, their nominal discharge currents and their residual voltages at
these currents.
The protective performance is characterized by the sparkover voltages for front-of-wave,
lightning impulse and, when applicable, switching impulses; and is also characterized by
the residual voltages at nominal discharge current and, when applicable, at switching
impulse currents. For a given rated voltage, different types of arresters and, therefore,
different protection levels exist.
Additional characteristics of an arrester to be considered are continuous operating
voltage, long duration discharge class, pressure relief class, pollution withstand capability,
live washing capability and special mechanical properties.
2.1.2 Rated voltage
The maximum permissible r.m.s. value of the power frequency voltage between the
arrester terminals, at which it is designed to operate correctly as established in the operat-
ing duty test. The rated voltage is used as a reference parameter for the specification of
operating characteristics.
NOTE – Some types of arresters to be used in range II are designed to reseal at power frequency voltages
higher than the rated voltage. This voltage is generally called the "temporary overvoltage reseal voltage".
Since IEC 99-1 does not specify tests to assure the correct operation of such arresters, test details and
application should be agreed between user and manufacturer.
In some cases, e.g. for the pollution test according to IEC 99-3, the maximum r.m.s. value
of power frequency voltage which can be applied continuously between the arrester
terminals should be known. For arresters to be used in range I according to IEC 71-1,
this voltage may be equal to the rated voltage of the arrester. For arresters to be used in
range II it is usually lower. As IEC 99-1 does not specify tests to assure this voltage, the
applicable value should be obtained from the manufacturer.
2.1.3 Protective levels
The lightning impulse protective level of the surge arrester is the maximum of the following values:
- the standard lightning impulse sparkover voltage;
- the residual voltage at nominal discharge current.
NOTE – When considering the protection of equipment from fast front overvoltages it is assumed that the
withstand strength of the oil-immersed insulation in the transformers is at least 15 % above its full lightning
impulse withstand strength for voltage durations shorter than 3 µs. Therefore, the maximum voltages
specified in IEC 99-1, table 8, for the front-of-wave sparkover are 15 % higher than those for the standard
lightning impulse.
Other types of insulation as in instrument transformers, cables or gas insulated substations (GIS) may have
different withstand characteristics and the front-of-wave sparkover voltage may need special consideration.
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SIST EN 60099-5:1998
99-5 © IEC:1996 – 19 –
The switching impulse protective level is applicable to the protection of equipment from
slow-front overvoltages. It is the maximum of the switching impulse sparkover voltage and
the switching impulse residual voltage.
NOTE – When the switching impulse sparkover characteristic of an arrester type is not known, only approxi-
mate information about this is obtained from the power frequency sparkover voltage.
2.1.4 Nominal discharge current
The peak value of a discharge current having a 8/20 shape, which is used to classify an
arrester. It is also the discharge current which is used to initiate the follow current in the
operating duty test and to establish the protective level of the arrester for lightning
overvoltages.
2.1.5 Long duration discharge class
A number related to the energy absorption capability of an arrester to discharge long lines.
Increasing class numbers (see IEC 99-1, table 5) indicate increasing system voltages and
line length and decreasing surge impedance and overvoltage factors.
2.1.6 Pressure relief class
A number related to the capability of an arrester to withstand internal fault currents after
a failure without violent shattering of the housing. Reference is made to clause 8.7 of
IEC 99-1.
2.1.7 Pollution withstand characteristics
For arresters to be used in polluted areas according to IEC 71-2, pollution levels Ill and
IV, a pollution test according to IEC 99-3 is necessary. From this test, information about
the sparkover performance is obtained. The flashover performance of the housing can be
checked in accordance with IEC 507.
2.1.8 Live washing characteristics
Application of live washing may require a special arrester design and suitable tests have
to be defined.
In the design of the washing equipment, care should be taken on the following points:
– water with an adequate resistivity must be used;
– the pressure and the nozzle configuration should be arranged so that the arrester
in its whole length and circumference is wetted as uniformly and simultaneously as
possible. For this it is necessary to consider the maximum permissible wind speed.
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SIST EN 60099-5:1998
99-5 ©IEC:1996 - 21 -
2.2 Selection of gapped surge arresters phase-to-earth
2.2.1
Rated voltage
It has been common practice to select an arrester to withstand the stresses due to the
temporary overvoltages resulting from an earth fault on one phase causing a voltage rise
on unfaulted phases at a time when an arrester operation occurs on one of these phases.
Other causes of temporary overvoltages have to be considered and the voltage rating of
the arrester should be chosen on the basis of the highest of these overvoltage conditions.
In some cases it may be necessary to consider temporary overvoltages arising from the
simultaneous occurrence of different phenomena such as sudden loss of load together
with an earth fault, taking into account their probability of occurrence.
The following causes for temporary overvoltages should always be considered:
- Earth faults:
These overvoltages occur in a large pa
rt of the system. Guidance for the determination
of temporary overvoltage amplitudes is given in annex A. The duration of the over-
voltage corresponds to the duration of the fault (until fault clearing). In earthed neutral
systems it is generally less than 1 s. In resonant earthed neutral systems with fault
clearing it is generally less than 10 s. In systems without earth fault clearing the
duration may be several hours.
- Load rejections:
After disconnection of loads, the voltage rises at the source side of the operating circuit
breaker. The amplitude of the overvoltage depends on the disconnected load character-
istics and on the short-circuit power of the feeding substation. The temporary
overvoltages have particularly high amplitudes after full load rejection at generator
transformers depending on magnetizing and overspeed conditions. The amplitude of
load rejection overvoltages is usually not constant during its duration. Accurate calcu-
lations have to consider many parameters.
As a guidance, the following typical values may be used:
-
In moderately extended systems, a full load rejection can give rise to phase-to-earth
overvoltages with amplitude usually below 1,2 p.u. The overvoltage duration depends
on the operation of voltage-control equipment and may be up to several minutes.
- In extended systems, after a full load rejection, the phase-to-earth overvoltages
may reach 1,5 p.u. or even more when Ferranti or resonance effects occur. Their
duration may be in the order of some seconds.
- For load rejection of generator transformers the temporary overvoltages may reach
amplitudes up to 1,4 p.u. for turbo generators and up to 1,5 p.u. for hydro generators.
The duration is approximately 3 s.
When the time dependence of the amplitudes is known, a suitable representation of
the overvoltage is the maximum amplitude, with a duration equal to the time while the
amplitudes exceed 90 % of this value.
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SIST EN 60099-5:1998
99-5 ©IEC:1996 - 23 -
In some systems the following causes of temporary overvoltages need consideration:
- resonance effects, e.g. when charging long unloaded lines or resonances between
systems;
- voltage rise along long lines (Ferranti effect);
- harmonic overvoltages, e.g. when switching transformers;
backfeed through interconnected transformer windings, e.g. dual transformer station
-
with common secondary bus during fault clearing or single-phase switched three-phase
transformer with an unbalanced secondary load.
Temporary overvoltages due to ferro-resonances should not form the basis for the surge
arrester selection, but should be eliminated.
Combination of causes such as earth faults and load rejection may result in higher tem-
porary overvoltage values than those from the single events. When such combinations are
considered sufficiently probable, the overvoltages for each cause have to be compounded,
taking into account the actual system configuration.
NOTES
1 The selection of the arrester rated voltage corresponding to the highest system temporary
overvoltages is based on the assumption that the highest system voltage is not exceeded under normal
operating conditions. If abnormal system voltages are likely to occur, thereby increasing the probability of
arrester operations during such conditions, it may be necessary to use an arrester with a higher rated
voltage than that recommended above.
2 Operating voltages with frequencies other than 50 Hz or 60 Hz may require special consideration
in the manufacture or application of surge arresters and should be a subject of discussion between
manufacturer and user.
Arresters for isolated or resonant earthed neutral systems without automatic earth fault
clearing should be able to withstand the rated voltage continuously due to the possible
long duration of the temporary overvoltage. Arresters for systems with automatic earth
fault clearing need only withstand the maximum phase-to-earth system voltage. This
reduced value can be obtained from the manufacturer.
2.2.2 Nominal discharge current
2.2.2.1 Factors influencing the lightning discharge currents
As a general rule, arrester currents due to lightning strokes are less than the stroke
current. In the case of direct strokes to lines, travelling waves propagate in opposite
directions from the point of impact. Flashover of line insulation provides a parallel path
to ground, which diverts a portion of the stroke current. In the case of strokes to more
than one conductor, or flashovers between conductors, two or more surge arresters
may operate and share the current. Only in the case of a direct stroke very near to the
terminal of the arrester, where no flashover occurs before arrester operation, is the
arrester called upon to discharge most of the lightning stroke current. The probability of
such an occurrence can be significantly reduced by proper shielding. Information concern-
ing lightning surge parameters can be obtained from general statistical data or from local
statistical data. The relationship between lightning surges and surge arresters discharge
currents may be obtained from travelling wave calculations.
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SIST EN 60099-5:1998
99-5@ IEC:1996 – 25 –
Overhead lines may be protected against direct lightning strokes to the conductors by the
use of shield (overhead ground) wires, which are positioned to intercept lightning strokes
and to direct the stroke current to ground via metallic tower or pole structures. Where
wood-pole structures are used, low-impedance conductors are used to connect the shield
wires to ground.
Almost
...
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