SIST EN 50341-2-9:2019

Overhead electrical lines exceeding AC 1 kV - Part 2-9: National Normative Aspects (NNA) for Great Britain and Northern Ireland (based on EN 50341-1:2012)

This NNA is only applicable to all new overhead lines above A.C. 1kV.
This Euronorm is only applicable to new overhead lines and shall not be applied to maintenance, reconductoring, tee-offs, extensions or diversions to existing overhead lines unless specifically required by the Project Specification.
For details of the application of this standard for overhead lines constructed with covered conductor refer to the Project Specification.
For details of the application of this standard to telecommunication systems involving optical fibres either incorporated in or wrapped around earthwires or conductors or suspended from overhead line supports, reference should be made to the Project Specification.

Nadzemni električni vodi za izmenične napetosti nad 1 kV - 2-9. del: Nacionalna normativna določila (NNA) za Veliko Britanijo in Severno Irsko (na podlagi EN 50341-1:2012)

To nacionalno normativno določilo velja le za vse nove nadzemne vode za izmenično napetost nad 1kV.
Ta normativ Euronorm velja le za nove nadzemne vode in ne velja za vzdrževanje, zamenjavo napeljave, odcepne prenosne tokokroge, podaljške ali preusmeritve v zvezi z obstoječimi nadzemnimi vodi, razen če tega izrecno ne zahteva projektna specifikacija.
Za podrobnejše informacije o uporabi tega standarda za nadzemne vode, izdelane z izoliranim vodnikom, glej projektno specifikacijo.
Za podrobnejše informacije o uporabi tega standarda za telekomunikacijska omrežja, ki vključujejo optična vlakna, vgrajena v zaščitne vrvi ali vodnike ali so ovita vanje oziroma visijo z nosilcev nadzemnih vodov, glej projektno specifikacijo.

General Information

• Status: Published
• Publication Date: 11-Mar-2019
• ICS: 29.240.20 - Power transmission and distribution lines
• Technical Committee: NVV - Overhead lines and overhead electrical conductors
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 22-Dec-2017
• Due Date: 26-Feb-2018
• Completion Date: 12-Mar-2019
• Ref Project: EN 50341-2-9:2017 - Overhead electrical lines exceeding AC 1 kV - Part 2-9: National Normative Aspects (NNA) for Great Britain and Northern Ireland (based on EN 50341-1:2012)

Relations

• Amended By: SIST EN 50341-2-9:2019/A1:2019 - Overhead electrical lines exceeding AC 1 kV - Part 2-9: National Normative Aspects (NNA) for Great Britain and Northern Ireland (based on EN 50341-1:2012)
• Effective Date: 06-Mar-2018
• Revised: SIST EN 50341-2-9:2016 - Overhead electrical lines exceeding AC 1 kV - Part 2-9: National Normative Aspects (NNA) for GREAT BRITAIN AND NORTHERN IRELAND (based on EN 50341-1:2012)
• Effective Date: 28-Mar-2017

Standards Content (Sample)

SLOVENSKI STANDARD
01-april-2019
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SIST EN 50341-2-9:2016
1DG]HPQLHOHNWULþQLYRGL]DL]PHQLþQHQDSHWRVWLQDGN9GHO1DFLRQDOQD
QRUPDWLYQDGRORþLOD11$]D9HOLNR%ULWDQLMRLQ6HYHUQR,UVNRQDSRGODJL(1

Overhead electrical lines exceeding AC 1 kV - Part 2-9: National Normative Aspects
(NNA) for Great Britain and Northern Ireland (based on EN 50341-1:2012)
Ta slovenski standard je istoveten z: EN 50341-2-9:2017
ICS:
29.240.20 Daljnovodi Power transmission and
distribution lines
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 50341-2-9
NORME EUROPÉENNE
EUROPÄISCHE NORM
May 2017
ICS 29.240.20 Supersedes EN 50341-2-9:2015
English Version
Overhead electrical lines exceeding AC 1 kV - Part 2-9: National
Normative Aspects (NNA) for Great Britain and Northern Ireland
(based on EN 50341-1:2012)
This European Standard was approved by CENELEC on 2017-04-26.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden,
Switzerland, Turkey and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN 50341-2-9:2017 E
CONTENTS
1 Scope . 6
1.1 General . 6
2 Normative References, definitions and symbols . 6
2.1 Normative references . 6
2.3 Symbols . 7
3 Basis of design . 7
3.2 Requirements of overhead lines . 7
3.2.2 Reliability requirements . 7
3.2.5 Strength coordination . 7
3.2.6 Additional considerations . 7
3.3 Limit states . 7
3.3.3 Serviceability limit states . 7
4 Actions on lines . 8
4.1 Introduction . 8
4.3 Wind loads . 8
4.3.1 Field of application and basic wind velocity . 8
4.3.2 Mean wind velocity . 9
4.3.3 Mean wind pressure . 9
4.4 Wind forces on overhead line components . 9
4.4.1 Wind forces on conductors . 10
4.4.3 Wind forces on lattice towers . 11
4.5 Ice loads . 11
4.6 Combined Wind and Ice loads . 12
4.7 Temperature effects . 13
4.8 Security loads . 13
4.9 Safety loads . 14
4.9.1 Construction and maintenance loads . 14
4.9.2 Loads related to the weight of linesmen . 14
4.10 Forces due to short-circuit currents . 14
4.11 Other special forces . 14
4.12 Load cases . 15
4.12.2 Standard load cases . 15
4.13 Partial factors for actions . 16
5 Electrical requirements . 19
5.2 Currents . 19

Great Britain and Northern Ireland -3/35- EN 50341-2-9:2017

5.2.1 Normal current . 19
5.2.2 Short circuit current . 19
5.5 Minimum air clearance distances to avoid flashover . 20
5.5.3 Empirical method based on European experience. 20
5.6 Load cases for calculation of clearances. 20
5.6.1 Load cases for calculation of clearances . 20
5.6.3 Wind load for determination of electric clearances . 20
5.6.4 Ice load for determination of electric clearances . 20
5.6.5 Combined wind and ice loads. 20
5.8 Internal clearances within the span and at the top of support . 20
5.9 External clearances . 21
5.9.1 General . 21
5.9.4 External clearances to crossing traffic routes . 21
5.9.6 External clearances to other power lines or overhead telecommunication lines . 22
5.9.7 External clearances to recreational areas (playgrounds, sports areas, etc.) . 22
5.10 Corona effect . 22
5.10.2 Audible noise . 22
5.10.3 Corona loss . 22
5.11 Electrical and magnetic field . 22
5.11.1 Electrical and magnetic fields under a line . 22
6 Earthing Systems . 22
6.2 Ratings with regard to corrosion and mechanical strength. 22
6.2.1 Earth electrodes . 22
6.4 Dimensioning with regard to human safety . 22
7 Supports . 23
7.3 Lattice steel towers . 23
7.3.6 Ultimate limit states . 23
7.4 Steel poles . 23
7.4.5 Structural analysis . 23
7.4.6 Ultimate limit states . 23
7.5 Wood Poles . 24
7.5.1 General . 24
7.5.5 Ultimate limit states . 24
7.7 Guyed structures . 25
8. Foundations. 25
8.2 Basis of geotechnical design . 25
8.2.2 Geotechnical design . 25
8.2.3 Design by prescriptive measures . 25
8.2.4 Load tests and tests on experimental models . 25
8.4 Supervision of construction, monitoring and maintenance . 25
9. Conductor and earth wires . 26

10. Insulators . 26
10.10 Characteristics and dimensions of insulators . 26
11 Line equipment – overhead line fittings . 27
11.9 Characteristics and dimensions of insulator fittings . 27
12 Quality assurance, checks and taking-over . 27
Annex J Angles in lattice steel towers . 27
J.4 Buckling resistance of angles in compression (see 7.3.6.4) . 27
J.4.2 Effective non-dimensional slenderness for flexural buckling . 27
J.4.3 Slenderness of members . 27
J.4.3.3 Primary bracing patterns. 27
J.5 Design resistance of bolted connections (see 7.3.8) . 28
J.5.1 General . 28
Figure NA.1 10-Minute mean wind speeds for GB, v in metres per second . 29
b, map
Figure NA.2 Ice thickness for GB, r and (r ) in millimetres . 30
o w
Figure NA.3 Weather Zones for site heights of 0 to 100m . 31
Figure NA.4 Weather Zones for site heights of 100 to 200m . 32
Figure NA.5 Weather Zones for site heights of 200 to 300m . 33
Figure NA.6 Weather Zones for site heights of 300 to 400m . 34
Figure NA.7 Weather Zones for site heights of 400 to 500m . 35

Great Britain and Northern Ireland -5/35- EN 50341-2-9:2017

European foreword
1. The British National Committee is identified by the following address:

British Standards Institution
389 Chiswick High Road
London W4 4AL
Tel:  + 44 20 8996 9000
Fax: + 44 20 8996 7799
email: [e-mail]
Attention: Secretary of PEL/11 Overhead lines – Standards Development

2. The British National Committee has prepared this NNA (part 2-9 of EN 50341) listing the GB National
Normative Aspects under its sole responsibility and duly passed this document through the CENELEC
and CLC/TC 11 procedures.
NOTE: The British National NC also takes sole responsibility for the technically correct co-ordination of this NNA
with EN 50341-1. It has performed the necessary checks in the frame of quality assurance / control. However, it is
noted that this quality control has been made in the framework of the general responsibility of a standards
committee under the national laws / regulations.

3. This Part 2-9 is normative in GB and informative for other countries.

4. This document shall be read in conjunction with Part 1 (EN 50341-1). All clause numbers used in this
NNA correspond to those in Part 1. Specific sub-clauses that are prefixed “GB” are to be read as
amendments to the relevant text in Part 1. Any necessary clarification regarding the application of this
NNA in conjunction with Part 1 shall be referred to the British NC who will, in co-operation with CLC/TC
11, clarify the requirements.
Where no reference is made in this NNA to a specific sub-clause, then Part 1 shall apply.

5. In the case of “boxed values” defined in Part 1, amended values (if any), which are defined in this NNA,
shall be taken into account in GB and Northern Ireland.

However any boxed value whether in Part 1 or in this NNA, shall not be amended in the direction of
greater risk in a Project Specification.

6. The GB and Northern Ireland standards/ regulations relating to overhead electrical lines exceeding
A.C. 1 kV are listed in subclause 2.1.

7. The British NC declares in accordance with clause 4.1 of Part 1 that this NNA follows both design
“Approach 1” and design “Approach 3”. The specific design Approach to be used shall be specified in the
Project Specification.
1 SCOPE
1.1 General
(ncpt) GB.1 General
This NNA is only applicable to all new overhead lines above A.C. 1kV.

This Euronorm is only applicable to new overhead lines and shall not be applied to maintenance,
reconductoring, tee-offs, extensions or diversions to existing overhead lines unless specifically
required by the Project Specification.

For details of the application of this standard for overhead lines constructed with covered
conductor refer to the Project Specification.

For details of the application of this standard to telecommunication systems involving optical
fibres either incorporated in or wrapped around earthwires or conductors or suspended from
overhead line supports, reference should be made to the Project Specification.
2 NORMATIVE REFERENCES, DEFINITIONS AND SYMBOLS
2.1 Normative references
(A-dev) GB.1 National statutes

Reference Name and Date of GB and NI Statute

Electricity Act 1989, Chapter 29,
Health and Safety at Work Act 1974 and subsequent amendments
SI 635 The Electricity at Work Regulations 1989 (Northern Ireland) 1991
SI 1355 The Electricity (Overhead Lines) Regulations 1970
SI 2035 The Overhead Lines (Exemption) Regulations 1990
SI 2665 The Electricity Safety, Quality and Continuity Regulations 2002
SI 381 The Electricity Safety, Quality and Continuity Regulations (Northern Ireland) 2012
SI 3074 The Overhead Lines (Exemption) Regulations 1992
SI 320 The Construction (Design & Management) Regulations 2007
SI 231(NI) Electricity (Northern Ireland) Order 1992
SR 142 The Construction (Design & Management) (Amendment) Regulations (Northern Ireland) 2001
SR 209 The Construction (Design & Management) Regulations (Northern Ireland) 1995
SR 536 Electricity Supply Industry Regulations (Northern Ireland) 1991
SR 21 Electricity Supply (Amendment) Regulations (Northern Ireland) 1993
SI 1039 (NI9) Health and Safety at Work (Northern Ireland) Order 1978
SI 2448 (S.165) The Electricity Act 1989 (Scotland)

(ncpt) GB.2 National normative standards

BSEN 1991-1-4:2005 Actions on Structures - Part 1-4: General Actions – Wind actions
BSEN 1995-1-1:2008 Design of Timber Structures – Part 1-1 General – Common rules and rules for buildings
BS 7354:1990 Design of high-voltage open-terminal stations
BSEN 10025  Hot rolled products of structural steels
BSEN 14229:2010 Structural timber – wood poles for overhead lines
BSEN 50182:2001 Conductors for overhead lines – round wire concentric lay stranded conductors

Electricity Association Technical Report (EATR) 111 - High Voltage Single Circuit Overhead Lines on Wood Poles
(1991)
Great Britain and Northern Ireland -7/35- EN 50341-2-9:2017

2.3 Symbols
(ncpt) GB.1 Additional symbols

A altitude of the site above mean sea level
SITE
a altitude in metres above sea level of the conductor
c altitude factor
alt
cdir wind direction factor
Dc diameter of the conductor, mm
f     yield strength for bolt
yb
Ki ice thickness coefficient
K shape factor
c
L length of conductor span, m
Nc number of phases and earthwires
q wind pressure on conductor, N/m²
c
qx wind pressure on structural element, N/m²
r basic radial thickness of ice, mm
B
ro radial ice thickness in mm in the absence of wind, mm
rr reference ice thickness, mm
r radial ice thickness in mm in the presence of wind
w
vb,0 fundamental basic wind velocity, m/sec
v 10-minute wind velocity at sea level taken from a GB map, m/sec
b,map
z height above ground, m
γ partial safety factor on wind speed and ice thickness (partial factors on actions)
v
γm partial factor on strength of structural materials
γ partial factors on permanent actions
dl
3 BASIS OF DESIGN
GB.1 Basic Requirements
3.2 Requirements of overhead lines
Design Approach 1
3.2.2 Reliability requirements

(ncpt) GB.1 Reliability levels
The partial coefficients to be used for the reliability levels are shown in Table 4.13.1/GB.1.
The required reliability level shall be stated in the Project Specification. For temporary loading
conditions reduced reliability levels may be specified.

3.2.5 Strength coordination
(ncpt) GB.1 Strength coordination
The required degree of strength coordination shall be stated in the Project Specification.

3.2.6 Additional considerations

(ncpt) GB.1 Additional considerations

Higher partial factors than those shown within this NNA may be specified in the Project
Specification. Any additional considerations shall also be stated in the Project Specification.

3.3 Limit states
3.3.3 Serviceability limit states

(ncpt)  GB.1 Specific requirement
These shall be defined in the Project Specification.

4 ACTIONS ON LINES
4.1 Introduction
Design Approach 1
(ncpt) GB.1 Peak factor equation
The formulation in the UK National Annex to BS EN 1991-1-4 modifies the parameter to define
peak pressures by adopting a “peak factor” of 3,0 with a quadratic equation, rather than 3,5 with
a linear equation, as used in BS EN 1991-1-4. The decision to change the formulation was due
to the use of the ten minute wind speed in BS EN 1991-1-4 and the greater accuracy in the
quadratic expression. As a consequence of this, equations included in clauses 4.3.4, 4.4.1.2,
4.4.3.2, 4.4.3.3 need to be amended as follows for use in the UK:

Replace the expression:    [1 + 7Iv(z)]
with:   [1 + 3,0Iv(z)]²
(ncpt) GB.2 Design approach definition
Approach 1 (as detailed in BS EN 50341-1:2012 Clause 4.1) shall be adopted for all new
overhead lines supported on steel poles or lattice steel towers.

Approach 2 (as detailed in BS EN 50341-1:2012 Clause 4.1) may be used subject to the
availability of suitable statistical data, and the agreement of the client. Before doing so, the
proposed methodology for design, and the quality of the statistical data, shall be subject to a
rigorous review by an independent expert to confirm that Reliability Levels equivalent to those
provided by Design Approach 1 would be achieved with the proposed Design Approach 2.

For overhead lines supported on timber poles, the project specification shall specify either design
Approach 3 or 1.
NOTE  Approaches 1 and 3 are based on different design philosophies and care should be taken not to combine
elements of the two when undertaking calculations. The parts of this standard which apply only to Approach 3 are shown
shaded grey in this document.
4.3 Wind loads
4.3.1 Field of application and basic wind velocity
Design Approach 1
(snc) GB.1 Calculation of basic wind velocity
Partial factor (γv) taken from Table 4.13.1/GB.1 for the specified Reliability Level shall be applied
to the basic wind velocity (v ) instead of applied to wind loading as given in Table 4.7 of
b,0
BSEN 50341-1. The partial factor Ψw shall not be used.

The fundamental basic wind velocity, v should be determined by the equation:
b,0
v = γ v c
b,0 v b,map alt
Where, vb,map is the fundamental velocity indicated in Figure NA.1 and calt is the altitude factor
calculated as follows:
c = 1 + 0,001 A
alt SITE
where: ASITE is the altitude of the site in metres above mean sea level

The above may be used for all site altitudes, but may be considered over-conservative at high
altitudes, in which case c may be calculated for each element greater than 10m above ground
alt
using the modified formula:
0.2
c = 1 + 0,001 A (10/h)
alt SITE
Where h is the height above ground level in metres at the point of application of the wind load.
For calculation of wind loading on conductors and insulators, h may be taken as the mean height
of the conductor attachment points. For calculation of wind on towers, structures may be divided

Great Britain and Northern Ireland -9/35- EN 50341-2-9:2017

in a number of panels of up to 10m in height, and h taken as the mean height of each panel.

4.3.2 Mean wind velocity
(ncpt) GB.1 Wind Direction
Wind direction factor C may conservatively be taken as 1,0 or from Table 4.3.2/GB.1 below.
dir
Table 4.3.2/GB.1 Wind direction factors
Direction 0 30° 60° 90° 120° 150° 180° 210° 240° 270° 300° 330°
C 0,78 0,73 0.73 0,74 0,73 0,80 0,85 0,93 1,00 0,99 0,91 0,82
dir
NOTE 1 Interpolation may be used
NOTE 2 The directions are defined by angles from due North in a clockwise direction, for wind from the
specified direction (eg. 0° means wind from due North)

NOTE. Unless stated otherwise in the project specification, wind from every direction from 0 to 345° shall
be considered for the design in 15° increments.

(ncpt) GB.2 Seasonal factor, c
season
Where a temporary loading condition will remain in place for less than 1 year, the appropriate
c factor may be applied in the calculation of mean wind speed as indicated in Table NA.2.7
season
in National Annex to BSEN 1991-1-4:2005. Note that the appropriate factor will not be applied in
conjunction with wind speeds of less return period than 50 years.

(ncpt) GB.3 Orography factor, co
The orography factor, c shall be taken as 1,0 where the average ground slope is not greater
o
than 5% (1:20), measured over a distance of 10 times the height of the supports from the line.
For greater slopes, reference shall be made to Figure NA.2 in National Annex to BSEN 1991-1-
4:2005+A1, “Definition of significant orography [definition of symbols given in A.3(3)]”. For sites
lying within the shaded area of that figure, the method given in BSEN 1991-1-4 A.3 for
calculation of c may be used. As an alternative, or if the topography is complex, calculation by
o
wind engineering specialists using digital terrain models may be less labour intensive and give
more accurate results.
(ncpt) GB.4 Loading on conductors
For calculation of the load on supports due to wind loading on conductors (excluding those
indirect effects due to conductor tension) the magnitude of the height above ground (h) adopted
for the calculation of V (h) shall generally be taken as the average height to the attachment of the
h
support considered except in the case of spans crossing deep valleys, river estuaries or hills
where the attachment heights would not be representative of the actual heights to the conductors
away from the supports. In these cases, the value of h adopted shall be adjusted to
approximately represent the mean height from the ground or water level to the attachment points
on the supports. Alternatively, advice from a wind engineering specialist may be sought.

4.3.3 Mean wind pressure
(snc) GB.1 Air density
Air density in Great Britain shall be taken as 1,226 kg/m³. Table 4.2 in BSEN 50341-1:2012 shall
not be used.
4.4 Wind forces on overhead line components

(snc) GB.1 Design Approach 3
Table 4.4.1/GB.1 details the design wind pressures and drag factors to be adopted for design
Approach 3.
shall be assumed to be 1,0 for wind span lengths up to 200m and
The span factor Gc
(0,75L + 30)/L metres for wind span lengths greater than 200m. Normal and High altitudes are
defined as follows:
Normal altitude: All of GB and Northern Ireland, except Scotland, site altitudes not exceeding
300m. For Scotland, site altitudes not exceeding 200m. More onerous requirements may be
detailed in the Project Specification.

High altitude: All of GB and Northern Ireland, except Scotland, site altitudes greater than 300m
but not exceeding 500m. For Scotland, site altitudes greater than 200m but not exceeding 500m.
For lines at altitudes greater than 500m, a special consideration should be made as detailed in
the Project Specification.
Table 4.4.1/GB.1 Wind pressures and aerodynamic drag factors
Load Condition Wind Pressure Aerodynamic drag
(N/m²) factors
q q C C
x c x c
High Wind (no ice) 1740 1740 0,8 1,0
Combined Wind and Ice (Normal altitude) 380 380 1,0 1,0
Combined Wind and Ice (High altitude) 570 570 1,0 1,0
Wind only (no ice) 0 760 - 1,0
Security (broken wire) 380 380 1,0 1,0
NOTE: for the leeward (shielded) pole, a shielding factor of 0,5 shall be assumed

4.4.1 Wind forces on conductors
Design Approach 1
4.4.1.1 General
(ncpt) GB.1 Calculation of G
c
The wind loading adopted for calculation of the mechanical tension in a section of line shall be
based on a value of conductor structural factor, Gc derived using a length value, Lm equal to the
section length or 800m whichever is the less, together with a height, h based on the mean height
of the conductor attachment point over length Lm. The mean height of the conductors shall be
adjusted for deep valleys, river estuaries and hills as described in 4.3.2/ GB.4 above.

4.4.1.3 Drag Factor
(ncpt) GB.1 Calculation of Reynold’s number
In the calculation of drag factor for conductors (c ) Method 3 shall be used for stranded
c
conductors, using an effective Reynold’s number (Re). The values given in 4.4.1.3 shall be used
for normal stranded conductors without ice. Other values are given in Table 4.4.1/GB.2 below.

Where: Re = (1,42 V cos Ø d)/ v
h c
-5
and where: v is the kinematic viscosity of air, taken as 1,46 x 10 m²/s

Øc is the angle between the wind direction and plane normal to the conductor

Table 4.4.1/GB.2 Typical Drag Factors for elements
Member type Effective Reynold’s Drag Factor (C )
c
number (Re)
Ice free Iced
Circular sections, smooth wire and smooth 1,2 1,2
≤ 2 x 10
bodied conductors
4 x 10 0,6 1,0
0,7 1,0
> 10 x 10
Normal stranded conductors with more than 1,2 -
≤ 6 x 10
seven strands 0,9 -
≥ 10
≤ 1 x 10 - 1,25
- 1,0
≥ 2 x10
Thick stranded cable, e.g small wire ropes, 1,3 -
≤ 4 x 10
round wire ropes, spiral steel strand with 1,1 -
>4 x 10
seven wires only 5
- 1,25
≤ 1 x 10
- 1,0
≥ 2 x10
Flat sided sections and plates All values 2,0 2,0
NOTE For intermediate values of Re, Cc should be obtained by linear interpolation

Great Britain and Northern Ireland -11/35- EN 50341-2-9:2017

4.4.3 Wind forces on lattice towers

4.4.3.1 General
(ncpt) GB.1 General
Method 1 shall be adopted when calculating wind loading on lattice towers.

4.5 Ice loads
(snc) GB.1 General
This clause replaces 4.5.1 to 4.5.2 in BSEN 50341-1:2012.

(ncpt) GB.2 Basic ice thickness
Modified design values for ice thickness, ice density, or the wind speed occurring simultaneously
with ice loading may be specified in the Project Specification, and may be either more or less
onerous than the values given below. Any such modified values shall be supported by local
experience gained over approximately 50 years or more of operation of overhead line systems in
the areas concerned.
(ncpt) GB.3 Ice thickness in the absence of wind
The basic ice thickness, in the absence of wind on conductors for the GB, should be
r
B
taken as:
 a − 200 
 
r = k r + but not less than
 
k r
B l  o  l o
 
 
where: k is a coefficient that is equal to:-
l
 2 4 
 
+  but not more than 1,2
 
D
 c 
where
D is the diameter of the conductor (in mm)
c
a is the altitude in metres above sea level of the conductor
r is the radial ice thickness in mm in the absence of wind shall be obtained from
o
Figure NA.2, appropriate to the position of the site.

Alternatively, shall be derived from a statistical analysis assuming an extreme distribution
ro
based on records of the annual maximum thickness of ice formation on components of form and
size similar to those to be used in the tower or its attachments at the latitude and altitude of the
site and having an annual probability of occurrence of 0,02.

NOTE : For the calculation of ice thickness on tower members, a similar procedure shall be adopted,
assuming kl =1,0 for flat sided members, and a = height of tower top above sea level.

(ncpt) GB.4 Reference ice thickness
The reference ice thickness, , to be considered for design shall be taken as:
r
r
= γ
r K r
r c B
v
where:
is the partial safety factor on ice thickness to be determined from table 4.13.1/GB.1.
γ
v
is the basic radial thickness of ice, determined as above.
r
B
is a shape factor, which should be taken as:
K
c
+ 0,3
N c
where is the number of phases and earthwires
N
c
1,3
N
c
K = 1,0 for tower members
c
(ncpt) GB.5 Ice weight
The weight of ice deposited shall be calculated, assuming a uniform coating of ice of
thickness, , and the unit weight of ice, in the absence of wind should be taken as 5 kN/m .
r
r
NOTE: Ice thickness and density may be subject to maximum and minimum values, which shall be
specified in the Project Specification.

4.6 Combined Wind and Ice loads

(snc) GB.1 General
This clause replaces 4.6.1 to 4.6.6 in BSEN 50341-1:2012

(ncpt) GB.2 Ice thickness in conjunction with wind
The basic ice thickness, , in conjunction with wind, on conductors for the GB, shall be taken
r
B
as:
 a − 200 
 
r = k r +  
B l  w 
 
 
but not less than , where:
k l r w
and a  are as defined in ice only case.
k
l
is the radial ice thickness in mm in conjunction with wind, to be obtained from
r
W
Figure NA.2, appropriate to the position of the site.

Alternatively, may be derived from records, having an annual probability of occurrence of
r
W
0,5.
(ncpt) GB.3 Reference ice thickness
The reference ice thickness, , to be considered for design should be taken as:
r
r
= γ
r K r
r c B
v
where:
γ is the partial safety factor on ice thickness to be determined from table 4.13.1/GB.1.
v
is the basic radial thickness of ice, determined as above
r
B
is a shape factor, which should be taken as 1,0 for tower members:
K
c
+ 0,3
N
c
where is the number of phases and earthwires
N c
1,3
N
c
K = 1,0 for tower members
c
NOTE: Ice thickness and density may be subject to maximum and minimum values, which shall be
specified in the Project Specification.

(ncpt) GB.4 Ice weight
The weight of ice deposited shall be calculated assuming a uniform coating of ice of
thickness, , and the unit weight of ice shall be based on knowledge of local environmental
r
r
factors, using the density values from BSEN 50341-1 Table 4.5. Unless otherwise specified
in the project specification, an ice density of 5kN/m shall be assumed for ice loading without

Great Britain and Northern Ireland -13/35- EN 50341-2-9:2017

wind, and 9kN/m for combined wind and ice cases.

Extreme ice thickness and corresponding ice density under still air conditions is not normally
considered for wood and steel pole lines following design Approach 3.

(ncpt) GB.5 Wind with ice
When ice is the leading variable action, VIH = 0 (ie. assumed still air conditions).
Ice thickness, r calculated from r using the formulae in Clause 4.5 above (considered to be
r 0
glazed ice).
When wind is the leading variable action, factor VIL = Vb,0 .B1 where factor B1 = 0,68

Ice thickness, r calculated from r using the formulae in Clause 4.6 above. Drag coefficient
r w
for iced conductors shall be calculated from Table 4.4.1/GB.2 above.

Design Approach 3
(ncpt) GB.6 Wood pole lines (design Approach 3)
For wood pole lines with conductors not exceeding 35mm² copper (or 60mm² for aluminium-
based conductors), the wind only loading case may be used for all altitudes, with no applied
ice loading. For all other lines, the following ice loadings are applicable when additionally
subjected to wind:
Normal altitudes 9,5mm radial thickness
High altitude 12,5mm radial thickness

A greater radial ice thickness may be defined in the Project Specification. The following unit
weight of ice/snow are considered depending on locality and altitude: glaze ice 9 kN/m³, wet
snow/ rime ice 5 kN/m³. The specific unit weight of ice to be used will be detailed in the
Project Specification.
4.7 Temperature effects
Design Approach 1
(snc) GB.1 Temperature effects
(a) Minimum temperature, with no other climatic load is not generally a critical loading
condition in the GB but for supports subject to uplift loading may generate maximum
uplift loads.
(b) The normal ambient temperature for extreme wind speed conditions in GB shall be
o
assumed to be 0 C for design Approach 1.
(c) A reduced wind speed combined with a minimum temperature condition is not a critical
loading condition in GB and need not be considered.
(d) The temperature to be considered for both icing in still air and combined wind and ice in
o
GB shall be assumed to be -10 C.
Design Approach 3
The temperature to be considered for both icing in still air and combined wind and ice in GB shall
be assumed to be -5,6°C
4.8 Security loads
(ncpt) GB.1 Security loadings (Failure containment or Broken Wire Conditions)

Towers shall be designed to resist the torsional or longitudinal loads, which would be generated
by combinations of broken conductor(s), and/or earthwires.

Full details of failure containment conditions shall be given in the Project Specification, which will
specify the following:
1) The combinations of conductors and earthwires, which shall be considered to be broken
simultaneously.
2) The basis for calculating the tensions in the conductors and earthwire or, alternatively,
nominal values to be assumed for the tensions.
3) Values to be assumed for the reduction factor, β, normally taken as 0,7 for conductors
on suspension towers and 1,0 for tension towers and earthwires on suspension towers.
4) The type of climatic loadings to be assumed to be acting simultaneously with the broken
conductors, and the return periods or partial load factors to be adopted.

NOTE: Security loadings need not be considered for timber poles and other types of support carrying
overhead lines at voltages above 1 kV. See Project Specification.

4.9 Safety loads
4.9.1 Construction and maintenance loads

(ncpt) GB.1 Construction and maintenance loads
Details of construction and maintenance loads shall be provided in the Project Specification.

4.9.2 Loads related to the weight of linesmen

(ncpt) GB.2 Loads related to weight of linesmen

All members on which it is possible to stand shall be designed to carry a vertical point load
applied in the most critical position of at least 1.5kN, inclusive of partial load factor.

Design loadings for walkways and/or working platforms when installed shall be defined in the
Project Specification.
4.10 Forces due to short-circuit currents

(ncpt) GB.1 Short circuit currents
The mechanical forces developed during a short circuit and applied to a structure, will be detailed
in the Project Specification.
4.11 Other special forces
(snc) GB.1 Other special forces
Where it is necessary to consider avalanches, creeping snow or earthquakes, the method of
calculation shall be defined in the Project Specification.

Great Britain and Northern Ireland -15/35- EN 50341-2-9:2017

4.12 Load cases
4.12.2 Standard load cases
(ncpt) GB.1 Standard load cases for design Approach 1
Table 4.6 in EN 50341-1 shall be replaced by Table 4.12.1/GB.1 and Table 4.12.2/GB.1.

Table 4.12.1/GB.1 Standard load cases for design Approach 1
Load Load as per Conditions Remarks
case sub clause
1a 4.4 Extreme wind load, all angles of wind incidence
which may be critical for particular elements, are to
be considered.
1b 4.4 Wind load at a minimum temperature May be critical for uplift
loading on crossarms.
2a 4.5 Uniform ice loads on all spans to be considered. Unit weight of ice
5 kN/m
2b 4.5 Uniform ice loads, transversal bending.
2c 4.5 Unbalanced ice loads, longitudinal bending. Not critical for GB
2d 4.5 Uniform ice loads, torsional bending. Not critical for GB
3 4.6 Combined wind and ice. Uniform ice loading on all Minimum unit weight of ice
spans should be considered. All angles of wind 5 kN/m
incidence, which may be critical for particular
elements to be considered.
4 4.2.6 Construction and maintenance loads. See Project Specification
5a 4.8.1 Security loads, torsional loads. See Project Specification
5b 4.8.2 Security loads, longitudinal loads. See Project Specification
6a 4.9.1 Safety loads, construction and maintenance loads See Project Specification
6b 4.9.2 Safety loads, loads due to the weight of linesmen See Project Specification

Design Approach 3
Table 4.12.2/GB.1 Standard load cases for design Approach 3
Load case Load condition Notes
1 High wind (no ice) As detailed in the Project Specification
2 Combined wind and ice Normal altitudes
3 Combined wind and ice High altitudes
4 Wind only (no ice) Conductor up to 35mm² copper equivalent area.
Wood pole lines - all altitudes
5 Security (broken wire) As detailed in the Project Specification
6 Construction and Maintenance As detailed in the Project Specification

For wood pole lines, Loading Cases 2 or 3 as appropriate shall be adopt
...