IEC 62264-3:2016

SLOVENSKI STANDARD
01-november-1999
Nadzemni vodi - Meteorološki podatki za ocenjevanje klimatskih obtežb
Overhead lines - Meteorological data for assessing climatic loads
Lignes aériennes - Données météorologiques pour calculer les charges climatiques
Ta slovenski standard je istoveten z: IEC/TS 61774
ICS:
19.040 Preskušanje v zvezi z Environmental testing
okoljem
29.240.20 Daljnovodi Power transmission and
distribution lines
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

RAPPORT
CEI
TECHNIQUE – TYPE 2
IEC
TECHNICAL
Première édition
REPORT – TYPE 2
First edition
1997-08
Lignes aériennes –
Données météorologiques pour calculer
les charges climatiques
Overhead lines –
Meteorological data for assessing
climatic loads
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61774 © IEC:1997 – 3 –
CONTENTS
Page
FOREWORD . 7
Clause
1 Scope. 11
2 Normative reference. 11
3 General. 11
3.1 Meteorological data. 11
3.2 Ice loads. 11
3.2.1 Icing processes. 11
3.2.2 Icing measurements. 15
3.2.3 Icing models. 17
3.3 Galloping (informative). 17
3.4 Strategy for employing data and models . 17
4 General meteorological data . 23
4.1 Introduction. 23
4.2 Weather parameters as required by IEC 60826 . 23
4.2.1 General. 23
4.2.2 Wind and thermal loads. 23
4.2.3 Weather elements required by icing models. 23
4.3 Availability of meteorological data for overhead line design. 25
4.4 Recommended procedures. 25
5 Ice load measurements. 27
5.1 Introduction. 27
5.2 Standard methods and recommended options for ice load measurements
associated with overhead line conductors. 29
5.2.1 General. 29
5.2.2 Consideration for ice load measurements . 29
5.2.3 Minimum recommendations. 31
5.2.4 Rods for additional investigations . 33
5.2.5 Characteristics of sites. 35
5.2.6 Standard procedures for measuring ice load on simple rods . 35
5.2.7 Training of observers . 37
5.3 Test spans. 37
5.4 Recommended procedures. 39
6 Icing models. 39
6.1 Introduction. 39
6.2 Types of icing models . 39
6.2.1 Empirical and deterministic icing models . 39
6.2.2 Climatological data used in icing models . 41
6.2.3 Application of icing models . 41

61774 © IEC:1997 – 5 –
Page
Figure 1 – Strategy flow chart . 21
Table 1 – Ice measurement parameters. 29
Annexes (informatives)
A Overview of weather parameters required by IEC 60826. 45
B Overview of meteorological terms, data handling programs and forecasting models . 49
B.1  Weather elements and weather parameters . 49
B.2  General observation procedures . 53
B.3  Meteorological forecasting models . 57
C Examples of construction of ice load measuring rods and applicability to various
types of icing . 59
C.1  Glaze caused by freezing rain. 59
C.2  In-cloud icing including hard rime and soft rime . 59
C.3  Wet snow accretion. 59
C.4  Dry snow accretion. 59
D Examples of test spans. 67
E Examples of icing models . 69
E.1  Glaze ice. 69
E.2  Rime ice. 69
E.3  Wet snow . 71
E.4  Glaze ice, rime ice and wet snow . 71
E.5  Rime ice and wet snow . 73
F Evaluation of icing models . 75
F.1  Introduction . 75
F.2  Glaze ice . 75
F.3  Rime ice . 77
F.4  Wet snow . 77
F.5  Relevance of local weather data . 77
F.6  Availability of information on icing models . 79
G Basic icing model concepts. 85
H Bibliography. 89
J Bibliography for further reading. 91

61774 © IEC:1997 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
_________
OVERHEAD LINES –
METEOROLOGICAL DATA FOR ASSESSING
CLIMATIC LOADS
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 co-operation 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.
2) The formal decisions or agreements of the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
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.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards.
In exceptional circumstances, a technical committee may propose the publication of a technical
report of one of the following types:
type 1, when the required support cannot be obtained for the publication of an
International Standard, despite repeated efforts;
type 2, when the subject is still under technical development or where for any other reason
there is the future but no immediate possibility of an agreement on an International
Standard;
type 3, when a technical committee has collected data of a different kind from that which is
normally published as an International Standard, for example "state of art".
Technical reports of types 1 and 2 are subject to review within three years of publication to
decide whether they can be transformed into International Standards.
Technical reports of type 3 do not necessarily have to be reviewed until the data they provide
are considered to be no longer valid or useful.

61774 © IEC:1997 – 9 –
IEC 61774, which is a technical report of type 2, has been prepared by IEC technical
committee 11: Overhead lines.
The text of this technical report is based on the following documents:
Committee draft Report on voting
11/115/CDV 11/125/RVC
Full information on the voting for the approval of this technical report can be found in the report
on voting indicated in the above table.
This document is being issued in the Technical Report (type 2) series of publication (according
to subclause G.3.2.2 of the ISO/IEC Directives) as a "prospective standard for provisional
application" in the field of climatic load databases because there is an urgent need for
guidance on how standards in this field should be used to meet an identified need.
This document is not to be regarded as an "International Standard". It is proposed for
provisional application so that information and experience of its use in practice may be
gathered. Comments on the content of this document should be sent to the IEC Central Office.
A review of this Technical Report (type 2) will be carried out not later than three years after its
publication with the options of: extension for another three years, conversion into an
International Standard, or withdrawal.
Annexes A to J are for information only.

61774 © IEC:1997 – 11 –
OVERHEAD LINES –
METEOROLOGICAL DATA FOR ASSESSING
CLIMATIC LOADS
1 Scope
This Technical Report (type 2) aims at providing advice on methods for developing climatic
load databases.This is necessary for the implementation of IEC 60826 which provides the
framework for National Standards on overhead line design. However, for its practical use, it is
required that design engineers acquire and utilise climatic data, since sufficient information is
often not available in existing building codes and standards. In particular there is a lack of
information on ice loads.
The objective of this report is met by:
a) reporting on the availability and proper use of climatic data
b) recommending simple standardized measurement techniques
c) reviewing icing models for computing ice loads.
The details of each of the foregoing aspects of overhead line design loads are presented in the
following clauses. Clause 3 describes the framework – or strategy – linking these separate
aspects together.
2 Normative reference
The following normative document contains provisions which, through reference in this text,
constitute provisions of this Technical Report. At the time of publication, the edition indicated
was valid. All normative documents are subject to revision, and and parties to agreements
based on this Technical Report are encouraged to investigate the possibility of applying the
most recent edition of the normative document indicated below. Members of IEC and ISO
maintain registers of currently valid International Standards.
IEC 60826: 1991, Loading and strength of overhead transmission lines
3 General
3.1 Meteorological data
The purpose of clause 4 is to provide an introduction to the meteorological data referred to
in IEC 60826, the weather elements and parameters from which they are derived, and an
explanation of some of the terminologies used by meteorologists. Information about the
availability of meteorological data for overhead line design obtained from questionnaires sent
to several utilities and national meteorological institutions is presented in 4.3. In 4.4
recommendations concerning procedures for obtaining the data most appropriate to IEC 60826
are given.
3.2 Ice loads
3.2.1 Icing processes
Atmospheric icing is a complex phenomenon which can take a number of forms. It is essential
both for the specification of measuring instruments and icing models that the distinguishing
features of these different forms are recognized.

61774 © IEC:1997 – 13 –
Atmospheric icing is a result of two main processes in the atmosphere which are named
accordingly:
a) in-cloud icing,
b) precipitation icing.
The latter one occurs in several forms among which the most important are:
1) freezing rain,
2) wet snow accretion,
3) dry snow accretion.
There is a third process resulting in the formation of so-called "hoar frost" but this does not
lead to significant ice loads on overhead lines and will not be considered further.
In-cloud icing is a process where suspended, supercooled droplets in a cloud (or fog) freeze
immediately upon impact on an object exposed to the airflow, for instance, a high level power
line above the cloud base.
The ice growth is said to be dry when the available heat transfer rate away from the object is
greater than the release of the latent heat of fusion. The density of the accretion is a function
of the flux of water to the surface and the temperature of the layer. The resulting accreted ice
is called soft or hard rime according to the density. A typical density for soft rime is 300 kg/m
and 700 kg/m for hard rime.
The ice growth is said to be wet when the heat transfer rate is less than the rate of latent heat
release. The growth then takes place at the melting point, resulting in a water film on the
surface. The accreted ice is called glaze with a density of 900 kg/m .
Precipitation icing can occur in several forms, including freezing rain, wet and dry snow.
Freezing rain comprises supercooled droplets which freeze immediately upon impact on
objects. The resulting accretion is also glaze. The ambient temperature is below the freezing
point.
When snowflakes fall through a layer of air with temperatures slightly above the freezing point,
the flakes may partly melt, become sticky and thus accrete on objects. This is called wet snow
accretion. The density and the adhesion may vary widely. If the ambient temperature drops
significantly below freezing after a wet layer of snow has accreted, the adhesive and
mechanical strength of the layer may become very high. In exceptional cases, wet snow
accretions are known to have occurred with ambient temperatures slightly below freezing.
Dry snow flakes may accrete at temperatures significantly below freezing and can, under very
low wind speed conditions, accumulate on objects to form a dry snow accretion.
It should be noted that the accretion on a conductor may be the result of more than one
process occurring during an icing event.

61774 © IEC:1997 – 15 –
3.2.2 Icing measurements
Clause 5 deals with ice load measurements. This is a complex problem requiring careful
interpretation of the results if they are to be applied reliably to overhead line design. Such
interpretations require an understanding of the different types of icing and their associated
meteorological conditions, the physical processes associated with ice accretion and their
interaction with the mechanical characteristics of the conductor system. There is a range of
conductor and overhead line parameters which should ideally be employed (conductor
diameter, stranding, torsional stiffness, height above ground, etc) and a range of icing and
meteorological parameters (ice weight, density, shape, precipitation, droplet size, wind speed,
air temperature, etc) that should ideally be measured at a test site. The cost of a test site is
often high and decisions have to be made between the merits of using basic measurement
systems or more expensive test spans.
3.2.2.1 Basic icing measurements
To obtain icing data covering a range of locations – comparable to meteorological stations –
basic, relatively inexpensive techniques that can be replicated have to be defined. Such
techniques should allow measurements to be made so that they can be related to the ice loads
which would be experienced by a real line at the measurement site. These techniques may
also provide a means for assessing the comparative exposure to icing at the site, relative to
other sites.
By defining certain minimum common requirements for the basic standard rigs, comparisons
can be made between measurements recorded at different sites in different areas or countries.
This will assist in validating the measurements, increase the confidence in the data and allow a
better understanding of conductor icing in relation to location, climate and meteorology. In
order to achieve this, minimum standard instrumentation and measurement requirements are
also specified. Both manual and automatic measurement systems are described because, in
some locations, manual data collection is either not practicable or may be more expensive.
In addition to the minimum requirements for the basic rigs, optional additional facilities and
measurements are described. These can improve the quality of the data and make it more
readily related to the specific lines under consideration.
3.2.2.2 Test spans
Test spans are an advantageous way of obtaining wind and ice load data as they more closely
represent real overhead lines. Because test spans are expensive to build and maintain they are
generally restricted to a few sites. Sometimes they are formed by building a few spans of
unenergised line on wood poles. They may also be provided by instrumenting or making
detailed observations of spans of in-service lines.
Organizations in a number of countries have operated test lines of varying complexity,
reflecting different objectives and subject to constraints imposed by cost, manpower and
location. It is not proposed to define a standard test span; a list of both "purpose-built" test
spans and "real-line" test spans is provided in annex D, giving some information on their
construction, location, instrumentation, etc.
However, it is strongly recommended that at each new or continuing test site, an ice rack
meeting at least the minimum requirements, as introduced in 5.2.3, should be erected. By
means of comparisons between the ice rack data and the test line data, transfer functions can
be constructed to make allowance for the differences in, for example, torsional stiffness or
diameter of the conductor and its height above ground.

61774 © IEC:1997 – 17 –
3.2.3 Icing models
The use of icing models to provide ice load data is obviously attractive. Some icing models use
relatively simple approaches and are restricted to a particular type of icing, while others start
from more fundamental equations and can address a range of icing types. A review of some
icing models is given in clause 6. Ice accretion involves complex processes in fluid mechanics
and thermodynamics. Empirical data are usually required for certain aspects of the theory, for
example the relationship between accretion density and windspeed, or the heat transfer
characteristics of a surface.
Icing models may be used both to estimate ice loads for defined conditions and to compute
icing statistics from a historical database of meteorological conditions. In most cases, the
precise information required as input to icing models is not available from such database. For
example, droplet size or snow intensity must be estimated from the available data.
Icing models enable transfer functions to be determined from ice measurements on rigs or test
spans. These transfer functions enable ice loads to be estimated for lines employing
conductors of different diameters, different mechanical characteristics and at sites with
different intensities of icing conditions. In this way, the models and measurements interact with
each other to improve the quality of such extrapolations and predictions.
3.3 Galloping (informative)
In addition to ice loads on conductors, ice accretion may also lead to a wind-induced oscillation
of the conductors known as galloping. This phenomenon, which is quite distinct from aeolian
vibration or sub-conductor oscillation, is a low frequency, mainly vertical oscillation of high
amplitude. Frequencies range up to about 1 Hz and amplitudes usually range from a few
metres to the sag of the span. The air temperatures are usually only a few degrees above or
below zero and the wind speed may range between 5 m/s and 25 m/s. The ice accretion
produces a conductor profile which generates both aerodynamic lift and moment. These
characteristics may be such that the conductor becomes aerodynamically unstable and gallops.
The start of the instability may often be observed in the middle of the span, and the intensity is
mainly influenced by the wind speed, the wind direction, the torsional stiffness of the conductor
and whether it is a single conductor or a bundle; in the case of bundles, other aspects such as
the tower and hardware characteristics may also be important.
This icing-related phenomenon is mentioned for completeness, but no further consideration is
given to it in this report.
3.4 Strategy for employing data and models
Figure 1 shows the general strategy for linking together the three aspects described in 3.2, 3.3
and 3.4 to provide the input data to IEC 60826. In the figure they are represented by the top
three boxes and are the resources ideally available to the overhead line engineer.
A simple case exists if the overhead line is not subject to icing and extensive data on extreme
wind speeds are available from the general meteorological database. Then the requirements of
IEC 60826 are met without modification, except for the transfer of data from their sites of origin
to the site of the overhead line.

61774 © IEC:1997 – 19 –
The situation is far more complicated as soon as icing occurs. However, a second level of
simplification would exist if an icing model or empirical equation of sufficient reliability for use
with the general meteorological database was available: ice rack data would not then be
necessary. On the other hand, if ice rack data were available to a sufficient extent, historically
and geographically, there would be no need for icing models. In general, however, data
acquired through the procedures indicated in figure 1 will be required.
The historical meteorological and icing data (top left and right boxes) need to be transferred
from sites of origin to the site of the overhead line (local data). Theoretical icing data are then
generated from the local meteorological data, using the icing model (top middle box). Since
droplet size and liquid water content are not directly available from the historical meteorological
data, they have to be evaluated from the other historical data before the calculation of
icing data.
A comparison between the theoretical and measured icing data is needed to ensure that the
experimental results can be correctly interpreted and that the theoretical models are realistic,
thereby improving the quality of the transfer functions derived from each. This is indicated by a
feedback from the "comparison box" to the icing model.
If the comparison turns out to be acceptable, the resulting icing data have to be further
modified by taking conductor and span data into account. The final icing data can then be
statistically processed to give the design data on ice load. In addition, together with the wind
data, the wind force on iced conductors is obtained and can be statistically processed.

61774 © IEC:1997 – 21 –
Measured
General Icing
icing data
meteorological data model
Use transfer functions
Use transfer
to convert to local
functions to convert
meteorological to local icing data
data
Evaluate
liquid water
content and
Calculate local
droplet size
icing data
Compare calculated and measured icing data.
If the difference is not acceptable this is used to adjust icing model
Calculate final
icing data taking
conductor and
span data into
account
Calculate wind force on
iced conductor
Statistical processing of the effect of wind and temperature wind on iced conductor and ice load
Design data
Figure 1 – Strategy flow chart

61774 © IEC:1997 – 23 –
4 General meteorological data
4.1 Introduction
In view of the need for general meteorological data in overhead power line design, it is the
purpose of this clause to clarify the present situation and to outline methods for a more
efficient way of accessing the data needed. Meteorology is a science strongly dependent on
evolution in computer capacity, and the users should play an active role in taking advantage
of this trend.
In 4.2 a review is given on the specifications of IEC 60826. Annex A gives a more detailed
survey.
Based on questionnaires to overhead line designers and meteorological institutions in different
countries, a survey is presented in 4.3 of the availability of meteorological data. A procedure is
recommended in 4.4 to assist in obtaining the needed data.
To improve the communication between overhead line engineers and meteorological insti-
tutions, annex B gives a brief introduction to terms used in meteorology, to general observation
and measuring procedures and to the format of data storage. Attention is also drawn to
new possibilities for obtaining better data at different sites by the Limited Area Model (LAM)
procedures.
4.2 Weather parameters as required by IEC 60826
4.2.1 General
The need for meteorological data is apparent from clauses 3.2, 3.3 and 3.4 of IEC 60826. For
convenience an overview of the parameters in question is given in annex A. A great part of the
need for meteorological data is associated with an evaluation of the ice load.
4.2.2 Wind and thermal loads
Wind data needed for overhead line design are generally found in national building codes.
When such codes are inadequate, additional data and relevant information should be
requested from the national meteorological institutions. In complex terrain, the selection of
design wind speeds should be performed by specialists.
Most relevant temperature data for overhead line design are generally available. Care should
be taken with remote areas having limited data coverage.
4.2.3 Weather elements required by icing models
The amount of ice is not among the weather elements generally measured by the
meteorological institutions. Clause 3.3 of IEC 60826 does not require any additional general
meteorological data. However, since icing models are introduced as a possibility to generate
ice load data, other meteorological elements are required. These are:
– wind,
– air temperature,
– precipitation,
– air pressure,
– clouds,
– relative humidity,
– visibility,
– liquid water content in the air,
– droplet size.
Each element is represented by its relevant parameters (see annex B).

61774 © IEC:1997 – 25 –
4.3 Availability of meteorological data for overhead line design
Questionnaires were distributed to overhead line engineers (OLEs) and meteorological
institutions (MIs).
The main objective of the questionnaires was to obtain an overview of the present state of the
use and need for meteorological information related to overhead line design and to learn the
potential for the availability of meteorological information in different countries.
A second objective of the questionnaires was to ascertain the availability of the meteorological
information needed for overhead line design as required by IEC 60826, in cooperation with the
respective meteorological institutions.
As a general conclusion, the need for meteorological data for the overhead line design varies
significantly from country to country. This variation reflects mainly the climatic and topographic
conditions in each country, as well as differences in national traditions and design
philosophies.
The answers from the meteorological institutions show that the systems for weather
measurements and observations are very similar. All countries follow the generally adopted
synoptic code for measurements and standard observation hours, but the number of stations
with more frequent observations (hourly and half-hourly) is more variable.
In general, temperature information required for overhead line design is readily available from
meteorological institutions.
Most countries provide wind maps for extreme winds covering the demands for designing
overhead lines. These maps are based on regular meteorological measurements. Directional
distributions of extremes may also be provided by meteorological institutions in several
countries. If needed, additional data may be provided.
The amount of data related to atmospheric icing is, however, limited at the meteorological
institutions. As this phenomenon is forecast only for aeronautical purposes, limited ground-
based data are available other than the generally observed weather parameters.
The level of cooperation between OLEs and MIs varies widely. In some countries there is a
long tradition of such cooperation. The meteorological institutions in such countries generally
cooperate with other branches of the building industry and have advanced to a high level of
knowledge about relevant parameters for engineering purposes.
The potential for further co-operation between OLEs and MIs seems to be high according to the
answers from both groups. As the MIs generally have no internal funds for special projects, the
majority have expressed their interest in serving the building industry further if all their costs
are covered.
4.4 Recommended procedures
In the implementation of the design principles as described in IEC 60826, the procedure to
follow will depend on the degree of climatic description existing for the country, as well as its
inherent climatic variations.
In many countries and regions climatic data are available at an acceptable level of accuracy,
and the parameters are found in building codes or meteorological publications.
In case such data are not published, the procedure to follow depends on the time horizon for
planning new overhead lines.
61774 © IEC:1997 – 27 –
As a minimum, it is necessary to undertake the following two actions:
– Contact the relevant meteorological authorities to check what information is readily
available.
– Seek meteorological assistance to evaluate parameters in areas not covered by available
data.
In addition, as part of system planning, including rebuilding old lines, data acquisition
programmes should be carried out in cooperation with the meteorological authority. It is
therefore recommended to:
– investigate cooperation on special observations and measurements to be performed within
the regular meteorological network,
– investigate special measurement programmes that may be undertaken, totally or partly, by
the meteorological institution,
– determine the need for separate measurement programmes to be undertaken by the
utilities.
In the case of special measurements, data should be correlated with regular meteorological
data or other reference data with time series long enough for extreme value analysis. The
confidence of design values, based on the special measurements, will depend on the length of
the measurement series and how quickly satisfactory correlation with the reference data is
obtained.
5 Ice load measurements
5.1 Introduction
As mentioned in 3.2.2, ice load measurement on overhead line conductors is a complex
problem. In addition, the environments are most often unfavourable for sensitive instruments.
Therefore unconventional measuring techniques are needed to obtain reliable data.
The design ice load employed for a certain overhead line should ideally be based on
measurements on the same conductor type as the one to be used on the line with the same
span lengths, line configuration, height above ground, etc. As the costs of such measurement
systems generally are prohibitive, simpler and cheaper devices have to be used. Data from the
latter must then be scaled (transformed) to meet the requirements set for the input data in
question.
The purpose of this clause is first to describe a few recommended measuring devices for the
various types of icing. Such devices may be applied in greater numbers to quantify the
geographical variations of ice loads (see 5.2).
At this stage, it is inappropriate to recommend standard test spans. Instead, in 5.3 some
general rules for such test spans are given. In annex D examples of test spans used in a
number of countries are briefly summarized.
However, if an extensive programme for ice load measurements is to be carried out on the
basis of some of the devices described in 5.2, the programme may be supplemented by a
smaller number of test spans in order to increase the confidence level when converting the
measured ice loads into conductor ice loads.

61774 © IEC:1997 – 29 –
5.2 Standard methods and recommended options for ice load measurements
associated with overhead line conductors
5.2.1 General
These recommendations apply to simple devices used to gather information about icing
conditions for overhead line design purposes. Their simplicity should allow greater numbers of
them to be employed and, where appropriate, they may supplement the more costly and
accurate ice measurement rigs or test spans which can be used only in limited numbers.
The purpose of the recommendation is to specify rig design according to the various expected
types of icing, to point out considerations for selecting the locations for rigs and to specify
measurement methods.
5.2.2 Consideration for ice load measurements
As the physical ice accretion processes vary from soft and hard rime icing (dry and wet) and
supercooled rain to wet snow and dry snow, the measuring devices are specified accordingly.
The selection of device must therefore take into account the icing process most likely to occur.
When selecting a rig configuration, method of measurement (manual or automated) and
programme of measurements, the main aim of the measurement must be considered. A
summary is given in table 1.
Table 1 – Ice measurement parameters
Purpose Ice load data required Time scale
Calibration or development of icing Time series of all icing events Minutes to hours
models
Statistical evaluation of icing Maximum value of each event Hours to days
models
Design values Annual extremes Years
In addition, the need for any supplementary information has to be taken into account, such as:
a) ice density measurements,
b) geometry of the accretion,
c) variation with conductor diameter,
d) variation with conductor stranding,
e) variation with torsional stiffness.
Because of the possible variations in icing rig design, it is important to establish certain
minimum common features regarding both the rig construction and the data acquisition
routines to be implemented at every site. Additional features should be included in the rig
design and measurement programme as supplements where they are considered advisable.

61774 © IEC:1997 – 31 –
5.2.3 Minimum recommendations
In this subclause, the construction and installation methods of simple, standardized ice load
measurement rods are described. Measurements can be done manually or automatically. The
basic types of construction and some guidelines for installing rigs are described. The
dimensions of some special instruments could be different from the recommended values in
this report. Some examples of constructions for automatic measuring devices are shown in
annex A.
5.2.3.1 Diameter of rod
The diameter of the the rod shall be 30 mm (as specified in IEC 60826).
5.2.3.2 Length of rod
The recommended length of the rod is 1,0 m. If, however, the accretion is expected to exceed
15 cm in diameter, as for example in the case of wet snow, the rod length shall be 2,0 m.
A greater rod length will not significantly improve measurement accuracy.
5.2.3.3 Characteristics of rod
The rod shall be rigid in respect of torsion and bending. When measuring the load caused by
glaze, in-cloud icing and dry snow accretion, the rod may be a smooth cylinder. For wet snow
accretions it is preferable that the rod is stranded because the growth of a cylindrical snow
sleeve can be significantly different for smooth cylinders as compared with stranded
conductors.
5.2.3.4 Rod orientation
A pair of rods shall be installed horizontally, one rod perpendicular to and the other parallel to
the expected prevailing wind during icing events.
The requirement will also be met by one rod mounted on a rotatable rack, capable of
orientating itself to face the wind direction.
Where icing events are limited to those caused by the dry growth of in-cloud icing in which the
wind direction is relatively constant, a single rod installed vertically may be sufficient.
5.2.3.5 Height of rods
The height of rods shall be 5 m above the expected highest snow surface at the sites. Although
IEC 60826 refers to the basic ice load measured at 10 m above ground, 5 m is used in this
report for practical reasons. The height of a passive ice meter can be at 1.5 m above ground
level when measuring freezing rain (see annex C). For freezing rain, the measured values can
be used directly. For in-cloud icing and wet snow, the measured values shall be multiplied by a
factor to give the basic ice load. If no other information is available, refer to IEC 60826.
In complicated terrain, the ice load is significantly influenced by the sheltering effects of the
local micro-terrain. In some locations, no ice accretes on rods installed at heights of 5 m,
whereas a large iceload may be observed on rods installed at heights of 10 m. For this reason
it is useful to consult experts on icing problems when selecting the height of rods.

61774 © IEC:1997 – 33 –
5.2.4 Rods for additional investigations
Recommendations are given below for carrying out additional investigations. It should be noted
that, where additional rods are employed, they shall be installed such that their ice accretions
do not interfere with each other or with the support system. Similarly, ice shed from one rod
must not affect other rods.
5.2.4.1 Additional rods of the same diameter
Where ice load or ice density measurements are made by techniques which lead to the
destruction of the ice accretion, additional rods offer the opportunity to make a number of such
measurements during continuing icing events (see 5.2.6.1.1).
5.2.4.2 Rods of various diameters
In IEC 60826, a standard rod diameter of 30 mm is specified and a correction coefficient, K , to
d
allow for different conductor sizes is defined. If no other information is available, refer
to IEC 60826 where values for K covering diameters from 10 mm to 60 mm are provided in
d
figure 14.
Ice load measurements on rods covering a range of diameters are valuable for verification and
refinement of these
K data.
d
5.2.4.3 Rods at various heights above snow surface
Ice loads for in-cloud icing in mountainous regions are strongly dependent on exposure height
because not only the wind speed but also the liquid water content in the air varies with height.
In contrast to figure 15 of IEC 60826, some forms of precipitation icing can also be dependent
on wind speed and therefore of height above ground.
In this report an exposure height of 5 m above the snow surface is specified. In addition, care
should be taken with regard to the sheltering effect of the local micro-terrain. In normal cases,
this expos
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