Guidelines for Overhead Line Design by hjkuiw354

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									NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


INDEX

Introduction

1. Selection of Insulators

    1.1   Introduction
    1.2   Pin Insulators
    1.3   Post Insulators
    1.4   Stay Insulators
    1.5   Cap and Pin Disc Insulators
    1.6   Insulator testing

2. Conductors

    2.1 Introduction
    2.2 Phase Conductors
    2.3 Corrosion Performance

3. Conductor Sag Tension Theory

    3.1 The Conductor profile Parabola vs Catenary
    3.2 Sag
    3.3 Slack
    3.4 Factors that affect conductor tension
    3.5 Multiple Span tension calculations – ruling Span
    3.6 Sag tension calculations
    3.7 Span ratios
    3.8 Wind Span
    3.9 Weight Span
    3.10 Examples

4 Crossarms

    4.1 Introduction
    4.2 Design loads
    4.3 Conductor spacing

5. Poles

    5.1 Introduction
    5.2 Wood pole Strength
    5.3 Pole Design loads

6. Pole Foundations

    6.1 Introduction
    6.2 Foundation strength

7. Ground Stays

    7.1 Introduction
    7.2 Stay Application
    7.3 Pole bending moment

APPENDIX 1 Conductor Loads
APPENDIX 2. Distribution Line Layout Steps

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                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
                                  Ergon Energy Corporation Limited ABN 50 087 646 062
                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


GENERAL INTRODUCTION
What are we designing for?

•   Compliance with Statutory Regulations

•   Safety of both our employees and general public

•   Economic utilisation of materials

•   To best meet the needs of customers with minimum environmental impact

•   To obtain a standard acceptable both from an engineering view and aesthetically (ie.
    have regard for the look of our construction from the public’s point of view).

What physical loadings do we have to allow for our design?

•   Weight of conductor and fittings

•   Conductor tension:-                       Terminal load
                                              Deviation load
                                              Differential conductor loads in adjacent spans
                                              Vertical loads
                                              Stay loads

•   Environmental Loads (eg. Wind) On Structures
                                   On Conductors

•   Construction and maintenance loads


LIMIT STATE DESIGN

Current practice for the design of Overhead Line Structural Components is to use a Limit State
design approach as set out in C (b) 1-1999 Guidelines for Design and Maintenance of
Overhead Distribution and Transmission Lines.

The Limit State design approach uses a reliability based (risk of failure) approach to match
component strengths (modified by a factor to reflect strength variability) to the effect of loads
calculated on the basis of an acceptably low probability of occurrence. This approach allows
component strengths to be more readily matched and optimised by economic comparison.

The corresponding Limit State wind pressures which correspond to the previously used working
stress values of 500pa and 660 pa and which result in equivalent failure rates based on typical
component strengths factored by strength factors which incorporate appropriate component
reliability factors are approx 900pa and 1200pa respectively. Limit State wind load pressures
are therefore greater than permissible stress loads by a factor of 1.8.

Conductor tension loads will increase in response to the higher design wind pressures by a
factor of depending on conductor everyday tension and conductor characteristics and generally
in the range 1.3 to 1.6.

Conductor weight loads will increase due to the effect of increased tension on structures with a
height profile above the average of neighbouring structures, however in general this factor is
fairly minimal in relatively flat terrain.



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                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


Design Component stresses is based on the ultimate stress at failure modified by a strength
factor, which takes into account the material strength variability.
Design component stresses are listed in the relevant sections of the Design Manual.

What physical conditions do we have to allow for in our design?

•   Conductor clearance:-                     To ground, roads etc
                                              To railway lines
                                              Over flood country
                                              To buildings etc
                                              Other lines

•   Topography:-                              Terrain
                                              Roads
                                              Railway lines
                                              Telecom
                                              Stays
                                              Special exclusion areas

•   Avoidance of obstructions:-               Airfields
                                              Roads
                                              Railway lines
                                              Telecom
                                              Stays
                                              Special exclusion areas

How do we allow for all these variable factors in our design?

In order to minimise the risk of failure of an overhead line it is necessary to ensure that each
component of an overhead line has been designed to meet all the electrical and mechanical
loads likely to be experienced in service as far as reasonably practical.

In order to achieve this, every line and every structure in that line could be individually
designed to meet the project requirements. This would be extremely time consuming and is
probably only justified for high value transmission lines. Another approach is to utilise a range
of standard structures with pre-designed electrical and mechanical capabilities and apply
them to a particular project. The advantage of this approach is that detailed structure design
is not required, and as long as the structures are used within specification, a line can be
constructed safely using standard building blocks. This approach allows for economics of
scale on material purchases and achieved a measure of uniformity of construction. It is also
worth noting that Ergon Energy has determined that, lines designed using the standard
structure drawings do require such approval.

Ergon Energy uses the standard structure approach for the majority of its lines and layout
staff need only select the appropriate structure to suit the specific application.

Where standard structures do not satisfy a line requirement, this should be referred to the
Lines Standards Department for a detailed design and approval.

To achieve a minimum cost line layout staff needs to apply the standard structures in the
most cost efficient way.

This requires:

       •   Clear understanding of structure capabilities
       •   Methodical approach using all available tools
       •   “feel” and “insight” which come with experience
       •   detailed checking of work undertaken
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                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
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                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


While there are a range of standard construction, layout and design staffs who apply these
standards to an overhead line need to be aware of some of the basic design principles so as
to apply the most appropriate structure to a defined requirement.

These notes provide an overview of the following factors in line design:

       •   Selection of Insulators
       •   Conductors
       •   Sag and tension theory
       •   Crossarms
       •   Poles
       •   Pole Foundations
       •   Ground Stays


1      SELECTION OF INSULATORS
1.1    Introduction

One of the most important and yet one of the most vulnerable links in transmission and
distribution is insulators. Porcelain and toughened glass are the materials principally used for
supporting conductors on overhead lines, and although these materials are relatively brittle
and inelastic, they have proven service experience and are still widely used. The design of
synthetic type insulators has improved both electrically and mechanically in recent times and
they are being used in urban areas to minimise radio interference and in areas where gunshot
or stone throwing is a problem. Insulator damage may occur due to such widely varying
causes as lighting (puncture), power arcs, stone throwing, corrosion, gunshot and pollution.

The following points must be considered in the selection of the appropriate insulation of an
overhead line:

       •   50Hz performance (usually a pollution requirement)
       •   Impulse capabilities
       •   Switching capabilities

1.2    Pin Insulators

This type was amongst the earliest designs, and although it has improved both electrically
and mechanically, it has altered little in appearance. It provides the most economic, simple
and efficient method of conductor support for voltage up to and including 33kV. Pin type
insulators for the lower voltages are designed so that the puncture voltage is higher than the
flashover voltage, however if the insulator glazing under the conductor is damaged (usually
caused by vibration) the insulator may puncture.

1.3    Post Insulators

These insulators are of one piece porcelain construction and have a cemented on a
galvanised malleable cast iron base provided with a taped hole for fixing stud. It will be
apparent that this type of construction renders it almost non-puncturable and a further
advantage is that if any expansion of the cemented base joint does occur the porcelain is put
into compression. If this occurs with the cemented joint of the screwed lead thimble of the pin
type insulator as discussed above, the porcelain is placed in tension, a type of load, which it
has little ability to withstand, and the porcelain will fail.




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                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


Where adequate additional insulation is not provided by the support (eg. Timber, fibreglass
etc.) then the insulators should be of two piece or non-puncturable construction to minimise
the risk of an electric hazard due to insulator failure.

1.4    Stay Insulators

The stay insulator inserted in the stay wire is usually of porcelain and is so designed that in
the event of failure of the stay insulator the stay wires will not fall to ground. All stays wires
attached to wooden poles supporting active conductors should be fitted with stay insulators.
The insulators should be mounted not less than 2.7 metres vertically above ground and have
a wet power frequency flashover voltage not less than one and a half times the highest
voltage conductor supported by the pole.

The selection and placement of stay insulators should be in accordance with ESAA C (b) 1.

1.5    Cap and Pin Type Disc Insulators

These insulators are used at tension positions (ie. Termination and suspension) in high
voltage lines and are available in 70kN and 160kV strengths to suit the various conductor
loadings. The cap and pin design ensures that the porcelain or glass of the high insulator is
always in compression. In areas of high pollution, particularly costal areas the pin of the
insulator should be fitted with zinc collar.

1.6    Insulator Testing

All porcelain insulators taken out of service must be tested before re-erection. Toughened
glass insulators however, need not be tested, since the smallest fault will cause disintegration
of the insulator.

Information on selection of insulators is contained in the Design manual section on
“Insulators”

2      CONDUCTORS
2.1 Introduction

Economically, conductors represent between 20 to 40% of the total cost of a line;
consequently their selection is of prime importance. In earlier days of electrical power
transmission, copper was mainly used as the material of overhead line conductors, however
with the expansion of electricity networks, several factors, such as price, weight, availability
and conductivity, have virtually compelled Overhead Line Design Engineers to concentrate on
aluminium based conductors, eg.

       AAC = All Aluminium Conductor
       ACRS = All Aluminium Conductor Steel Reinforcement
       AAAC = All Aluminium Alloy Conductor

Steel conductors are still widely used as overhead earth wires and also as phase conductors
on rural distribution lines, eg.

       SC/GZ = Galvanised Steel Conductor
       SC/AC = Aluminium Clad Steel Conductor

2.2    Phase Conductors

The conductors fulfil an electromechanical function; hence both the electrical and mechanical
aspects are to be considered.

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                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


Electrical parameters:
The most important parameter affecting the choice of conductor is its resistance, because it
influences voltage regulation, power loss and current rating.
For AC lines, the diameter of a conductor affects the inductance and the capacities. Up to a
voltage of 132kV, the above considerations are generally adequate, however at higher
voltages, the above gradient on the conductor surface may require the selection of a
conductor on the basis of its diameter, thus leading to the use of bundled conductor (ie. 2, 3
or 4 phase).

Mechanical parameters:
As already indicated, Aluminium based conductors represent the highest proportion of
conductor usage. The advantageous mechanical properties of aluminium alloys have also
been recognised for long time, but AAAC has always been more expensive than ACSR, for
equivalent conductivities. However there are cases where initial cost is not the governing
factor. One of these is the corrosion performance, since being monometallic, the risk of
bimetallic corrosion between the aluminium and the zinc on the steel core are nonexistent.
Consequently AAAC conductors are used on lines in coastal areas.

2.3     Corrosion Performance

Table 3.3.1 provides an indication of the relative corrosion performance of various conductor
types. The recommendations should be modified by local experience, for example, for salt
spray pollution the relative distances from the source depend upon the prevailing winds and
the terrain. Special circumstances such as crop dusting, which has been known to have
severe effects, should also be taken into account.

Table 2.1 Indication of relative corrosion performance of conductors

                          SALT SPRAY POLLUTION                             INDUSTRIAL POLLUTION
  CONDUCTOR
     TYPE                                      BAYS INLETS
                       OPEN OCEAN                                                 ACIDIC     ALKALINE
                                               SALT LAKES

        AAC                    1                           1                             1      3
      AAC/6201                 1                           1                             2      3
      AAAC/1120                1                           1                             1      3
      ACSR/GZ                  3                           2                             2      3
      ACSR/AZ                  2                           1                             2      3
      ACSR/AC                  1                           1                             2      3
       SC/GZ                   3                           2                             3      2
       SC/AC                   1                           1                             2      3
        HDCu                   1                           1                             2      1


        1 = Good performance
        2 = Average performance
        3 = Poor performance




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                                   Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN



3. CONDUCTOR SAG – TENSION THEORY
3.1     The Conductor Profile - Parabola versus Catenary

A parabola is the shape of a cable that supports a uniform horizontal load. An example of a
parabola is the cable of a suspension bridge that supports the deck below. Whereas a
catenary is the shape that is formed by a hanging cable whose weight is a constant per unit of
arc length. The word catenary comes from the Latin word catena, meaning chain.

Provided that the sag is less than 9% of the span length, there is less than 1% difference in
their shapes. So for most practical distribution applications the parabola will suffice and is the
assumption generally used for distribution design. The mathematical formulae, which are
derived for the parabola, are much simpler than the catenary formulae.

3.2    Sag
The following formula for the sag in a parabola can be used for level and non-level spans. A
level span is a span where the conductor supports are at the same elevation.

      w L2
S=
      8T
S = mid-span sag (m)
w = conductor weight (N/m)
L = horizontal span length (m)
T = conductor tension (N)

The conductor tension T is the tension at the low point of the cable, however the tension does
increase with conductor elevation. The tension at the supports will be no greater than an
additional 7% of the tension at the low point for a level span where the sag is less than 9% of
the span length.

Normally the conductor weight is given in kg/km, which must be converted into N/m to use in
the above equation.

      Wc × 9.81
w=
       1000
Wc = conductor weight (kg/km)

3.3 Slack
The difference in distance between the straight line between the supports and the distance
along the parabola arc (the stretched conductor length) is called the slack. For a level span
the slack is given by

      8 S2
K=
      3L
K = slack (m)
S = mid-span sag (m)
L = span length (m)




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                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN



3.4    Factors that Affect Conductor Tension

Temperature
As the temperature increases, the unstretched conductor length will increase by an amount
equal to: ∆ L = α T S

∆L = αTS

α = the coefficient of thermal expansion
T = the temperature increase in deg C
S = the span length in metres

This will result in a decrease in conductor tension and an increase in sag.

Wind
A wind load on the conductor will increase the apparent weight of the conductor resulting in
an in increase in tension.

The increase in tension will increase the cable length due to elastic stretch by an amount
given by given by:

∆L = (To − T ) / EA

To = the initial tension in newtons
T = the final tension
E = the coefficient of elasticity
A = the cross section of the conductor in metres.

This increase in resultant load will result in an effective sag in an inclined direction with both
horizontal and vertical components.

Ice
Ice build up on the conductor will increase the apparent diameter and weight of the conductor.
This is not an issue in Queensland however the same approach can be used for calculating
loads and sags if bird darverters are installed along a span.

Age
Conductor sag over time may increase due to the effects of strand settling in and
metallurgical creep. A higher tension may be used when the conductor is first erected to allow
for “settling in of conductor strands and for subsequent metallurgical creep of the conductor
material

Pole movement
Any movement of pole tops due to stay relaxation etc will have the effect of introducing
additional length into the span.


3.5    Multiple Span Tension Calculations - Ruling Span

The ruling span (or equivalent span) is defined as that span which behaves identically to the
tension in every span of a series of suspension spans under the same loading condition. In
general the flexibility of a wood pole is sufficient to ensure that an intermediate pin structure
can be considered as a suspension for the purposes of calculation of the ruling span
provided that the ratio of adjacent span lengths is not too extreme (eg less than 1:2).


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                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN




The ruling span can be calculated using,

             n
            Σ L3
               i
Lr =        i =1
              n
            Σ Li
            i =1

Lr = ruling span
Li = horizontal span length of span i
n = number of spans between strain structures.

This equation applies for lines in flat to undulating terrain. In very mountainous terrain with
large differences in elevation between structures, use of Equation (4) in Appendix E of
C(b)1-1999 Guidelines for Design and Maintenance of Overhead Distribution and
Transmission Lines may be required.

3.6       Sag and tension Calculations

These calculations are primarily used to calculate the conductor tension under one set of
conditions based on known tension under some other condition.

Conductor Tension Limitations
Conductor tension limitations are determined by the most onerous of the following
conditions:

•     Serviceability Condition or everyday condition (relates to vibration, construction and
      anchoring practicalities)- as specified in the table of “Standard Conductor Applications”
      following in this section at a temperature of 15°C.
•     Conductor Strength Limit State - Bare conductors – 70% of Conductor nominal breaking
      load at a temperature of 15°C.
•     Serviceability Condition – low temperature condition – 50% of conductor nominal breaking
      load. This relates to structural loadings at a temperature of 0°C. (This condition will
      generally never govern for the range of conditions proposed.)
•     Conductor Strength Limit State LV ABC conductors – 40% of Conductor nominal breaking
      load at a temperature of 15°C. (relates to insulation adhesion considerations).

In general the everyday or serviceability condition will govern and a tension change calculation
is used to calculate tensions and sags under other conditions. In some cases however the
maximum wind condition may govern at increased span lengths. The span at which the change
occurs is called the transition span.

Conductor stringing charts from which conductor tensions can be determined for differing
temperature and wind loading conditions are located in the “Stringing Charts” section of the
Design manual.

3.7 Span Ratios
Large differences in the lengths of adjacent spans can result in significant tension differences
across intermediate structures, which may not be able to be equalised by movement of the
pole top and may cause ties or pins to fail. In rural situations practice is therefore to limit
adjacent span ratios to 1:2. In short slack span urban situations, this practice is generally not
necessary.


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                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
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                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


3.8     Wind Span
The wind span at a particular structure is the length of span that determines the transverse
load on the structure due to wind action on the conductor and is defined as:

Lw = one half the sum of the adjacent spans.

3.9      Weight Span
The weight span at a structure is the length of span between the catenary low points on
either side of the particular structure and determines the vertical load due to the weight of
conductor at that structure.


3.10     Examples

Example 1.
Consider a span of Raisin (3/4/2.5 ACSR) conductor strung to a tension of 22% NBL at 15
deg C. with the following properties:

Tension        T = 5368 N (22 % NBL)
Weight         w = 1.893 N/m
Span Length    S = 250 m
Ruling Span Length is also 250 m

The sag under this condition is 1.893x250 2 /(8x 5368) = 2.76 metres
This sag can also be determined from the Conductor tension change program.

Example 2.
Now calculate the tension and sag under the maximum wind condition of 900 Pa
Using the conductor tension change program, the tension under this condition is 9895
newtons with vertical sag of 1.49 m and horizontal sag of 5.33 m.

Now calculate the tension and sag under the maximum operating temperature of 60 deg C
and no wind

Using the conductor tension change program, the tension under this condition is 3868 N with
vertical sag of 3.82 m.

Example 3.
Now consider what happens if the conductor is over tensioned by pulling an additional 100
mm out of the span during stringing.

This will cause the tension to increase however the resulting increase in elastic stretch will
partly reduce the effect.

We can treat the removal of this conductor length as being similar to a reduction in
temperature, which can be calculated using the formulae for thermal expansion - ∆ L = α T S.

Therefore T       = ∆ L/α S
                  = 0.1 / 13.9x10-6 x 250
                  = 28.8 deg C

By going to the Conductor tension Change Program enter option and using a final condition of
15-28.8 ie –13.8 deg C we can calculate the resulting tension as 6682 N and sag as 2.21 m.
This means that the conductor is over tensioned by a factor of 25%

Example 4.



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NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


Now consider what happens if the conductor is under tensioned because a stay foundation
relaxed to allow the pole head to move by 200 mm which, effectively puts an additional 200
mm of cable into the span.

This will cause the tension to decrease however the resulting decrease in elastic stretch will
partly reduce the effect.
We can treat the addition of this additional conductor length as being similar to an increase in
temperature which can be calculated using the formulae for thermal expansion - ∆ L = α T S.

Therefore T       = ∆ L/α S
                  = 0.2 / 13.9x10-6 x 250
                  = 57.6 deg C

By going to the Conductor tension Change Program enter option and using a final condition of
15 + 57.6 ie 72.6 deg C we can calculate the resulting tension as 3567 N and sag as 4.15 m.
This means that there is additional sag of 1.39 m, which will most likely to result in statutory
clearances not being maintained.

Of course if this span were one of a section, the effect of tension equalisation provided by
adjacent spans would tend to reduce these effects.

Example 5.
Now consider what happens if we raise one pole by 3 metres in a section with 250 m spans
either side on reasonably even ground.

The increase in chord length can be calculated by ∆ L = L- Sqrt( L2 + h2),

L = span length
H = increase in pole Height.

Therefore ∆ L = 250 – Sqrt(250 2+ 3 2) = 0.018 m

This will cause the tension to increase however the resulting increase in elastic stretch will
partly reduce the effect.

We can treat the reduction of this additional conductor length as being similar to a decrease in
temperature, which can be calculated using the formulae for thermal expansion - ∆ L = α T S.

Therefore T       = ∆ L/α S
                  = 0.018 /13.9x10-6 x 250
                  = 5.2 deg C

By going to the Conductor tension Change Program enter option and using a final condition of
15 – 5.2 ie 9.8 deg C we can calculate the resulting tension as 5586 N and sag as 2.65 m.
This means that there is an increase in tension of 4% which should be OK.

If we repeated the same exercise with a 100 m span (and 100 m ruling span), the tension
would increase to 6143 N which would be around 15 % overtension and may need correction
but then only if there are termination structures at each of the adjacent structures.


4      Crossarms
4.1    Introduction

Crossarms may be either wood or steel construction but the general design procedure is the
same. Wood crossarms do however have significant benefits with regard to electrical
performance associated with lightning outage performance. The mechanical loads to which
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                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


crossarms may be subjected should take into account the conditions likely to be experiences
in service so as to minimise the probability of failure, as far as reasonably practicable, these
mechanical loads should be determined in accordance with ESAA C(b)1.



4.2    Design Loads

In designing crossarms for single supports the crossarms can generally be treated as two
cantilevers fixed at the support. The crossarm at the support is therefore subjected to the
following bending moments:

       •   Bending moment due to weight span of conductor, this may be either positive or
           negative, depending upon whether the profile imposes a down pull or an uplift of
           the conductors. Refer to Appendix 1 for illustration of weight span.
       •   Bending moment due to transverse conductor loads - wind and deviation loads
           acting at top of insulator pin (intermediate structures only). Refer to Appendix 1 for
           details on wind span.(These loads are fairly minimal)
       •   Bending moment due to direct horizontal pull of conductors (termination or strain
           structure only).
       •   Maintenance loads resulting from additional conductor lowering or anchoring
           activities and loads due to pole top rescue.
       •   The self weight of the crossarm (This load is minimal)
       •   Kingbolts must also be checked for allowable bearing loads perpendicular and
           parallel to the timber grain.
       •   Crossarm brace bolts must be checked for allowable bearing loads at an angle to
           the timber grain.

Allowable stresses for timber are dependant on the duration of the application of the load
hence different allowable stresses are used for long duration, maintenance and short duration
or wind loads.

Table 4.1 gives the allowable long and short duration crossarm loads, for some of the more
commonly used crossarms the table makes no allowance for vertical load.

Table 4.1 Allowable Horizontal Crossarm Loads

                                                               CROSSARM LENGTH
           CROSSARM
                                                      2400                                  2700


                                       S.D.L.         Maint          L.D.L         S.D.L.   Maint   S.D.L.
      TYPE
                                        (kN)          (kN)           (kN)           (kN)    (kN)     (kN)

                      150x100           9.6            7.7            4.5           8.3     6.7     4.0
   Single Arm
                      175x125           17.5           14.2           8.2           15.4    12.4    7.4
                      150x100           19.2           15.4           9.0           16.6    13.4    8.0
  Double Arm
                      175x125           35.0           28.4          16.2           30.8    24.8    14.8

S.D.L. = Short Duration limit state Loads eg. Wind Loads
L.D.L. = Long Duration Loads eg. Conductor Weight
Maint = Maintenance loads




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NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


4.3    Conductor Spacing

Crossarms must also be selected to give the required separation at the support and at mid-
span. For information on conductor separation refer to the Design Manual section “Layout
Clearances”.



5      Poles
5.1    Introduction

Pole structures, and particularly single member pole supports are used to carry both high and
low voltage conductors. Good pole supports, properly chosen with regards to local conditions
and requirements, are a decisive factor in ensuring high continuity of service, long life of
equipment and low maintenance costs.

5.2    Wood Pole Strength Rating

Previously wood poles were classified into light, medium and heavy class, regardless of
strength group, and the Design Engineer had to determine the loading which each class and
strength group would carry.

All poles (wood or concrete) are now supplied with tip strength rating. The strength rating or
short duration loads (S.D.L.) is the strength corresponding to the maximum allowable working
pole tip load and must be multiplied by 1.8 to equate to limit state wind pressure loads on the
project areas of both the pole and the conductor.

Typical wind pressures:           Conductor 900 Pa or 1200 Pa in cyclonic areas
                                  Pole      1300 Pa or 1700 Pa in cyclonic areas

The pole long duration load (L.D.L.) is the continuous load that the pole has to withstand day
after day. It is assessed as equivalent to the load applied by conductor tension at 15° C no
wind and is half of the specified tip load (or approx 28% of the limit state load)

5.3    Pole design Loads

Unstayed poles may be subjected during service to the following horizontal loads:-

       •   Horizontal load due to wind acting on pole
       •   Horizontal load due to conductor wind span
       •   Horizontal load due to conductor tension on angle, unstayed termination and
           unbalanced strain poles due to differential conductor tension in adjacent spans.

A vertical load is only imposed on unstayed poles by the conductor weight span and weight of
fittings and very seldom becomes an important consideration, however for stayed poles a
vertical load is imposed by the stay as well as the weight and fittings and this becomes a
major consideration. Stayed poles are also subject to a bending moment, which is generally
greatest at the stay foundation.


6      Pole Foundations
6.1    Introduction

The design of support foundations is rather more difficult than the design of other overhead
line components, as the properties of soil are not as definite as those for other materials such


Check this is the latest version before use.       Page 13 of 20                 Reference P56M02R09 Ver 1
                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
                                  Ergon Energy Corporation Limited ABN 50 087 646 062
                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


as steel, aluminium, copper etc., and consequently for design purposes soil properties are
selected within very widely varying limits.

6.2    Foundation Strength

The allowable pole tip Pο (kN) due to foundation strength is given by the following equation:

       Tip Load Pο = KmaxD J3
                         12 (h + ¾ J)

       where Kmax = Passive Soil Reaction (kPa/m)
             D = Average below ground pole diameter (m)
             J    = Pole setting depth (m)
             H = Height of pole above ground (m)



7      Ground Stays
7.1    Introduction

It is necessary to stay overhead line supports at locations where the loads exceed the
capacity of the pole/foundations so that the stay wire, rod, bed log/screw anchor etc. take the
pull due to the conductors. Too much attention cannot be directed to the design, making off
and setting of stays, as the future safety of the line, particularly under adverse weather
conditions, depends equally as much on being correctly stayed as it does on the proper
erection of the conductors.

In the economic design of stays it is essential to match the strength of the component parts,
ie. The stay wire, rod, bedlog etc.

The maximum working strength of a particular stay type is determined by the least value of
the strength of:

       •        Stayrod
       •        Eyebolt
       •        Staywire
       •        Preformed Guy Grips
       •        Stay Insulators
       •        Foundations

7.2    Stay Application

It is important to note that when the stay attachment is at the load centre the horizontal
component of the stay load is equal to the horizontal load occurring at the load centre. This
occurs in the majority of cases as most stays on standard constructions are placed as close
as possible to the crossarm.

Where the stay attachment is not to close to the load centre, the horizontal load acting on the
stay, P, due to the conductor termination or deviation, must be calculated.




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                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
                                  Ergon Energy Corporation Limited ABN 50 087 646 062
                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


Select the appropriate stay whose allowable horizontal load, H, is greater than the calculated
load, P.

  L

                    x
                          P

         y                                                        P = L (1+3x/2y)




The calculation of the equivalent horizontal load, P, assumes that the bending moment
occurring at a point one third the height of the stay attachment above ground level is zero.

Table 7.2.1 lists the stay horizontal loads for a number of stay attachments for a conductor
load L1 = 30kN and a stay height above ground y = 9.0 m.

Table 7.2 Stay Horizontal Loads for 30 kN conductor load.


      STAY ATTACHMENT                           CONDUCTOR LOAD                              STAY HORIZONTAL LOAD
             (X)                                     (L1)                                            (P)


          At load centre                                     30kN                                  30.0kN

                0.5m                                         30kN                                  32.5kN

                1.0m                                         30kN                                  35.0kN

                1.5m                                         30kN                                  37.5kN

                2.0m                                         30kN                                  40.0kN



7.3    Pole Bending Moment

The pole must be designed to resist the maximum bending moment that will occur at the point
of stay attachment.

The allowable bending moment on wood a pole at the stay attachment points is given by the
following equation:

       B.M. = fZ ..... .... .........................where              B.M. = bending moment
                                                                        F    = design stress
                                                                        Z    = section modules

       fZ > L1 . x + wind on pole...........where                       L1        = conductor
                                                                        x         = height of L1 above stay



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                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
                                      Ergon Energy Corporation Limited ABN 50 087 646 062
                                      Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


Table 7.3 lists the maximum allowable conductor short duration load (S.D.L.) and long
duration load (L.D.L.) for a 14.0m x 8kN wood pole.

Table 7.3 Pole Bending Moment – Allowable Conductor Tension


                                       14.0m 8.0 kN Wood Pole

                              MAX. ALLOW                       MAX ALLOW
                                                                                         HORIZONTAL
      STAY                      S.D.L.                            L.D.L.
                                                                                        STAY TENSION
   ATTACHMENT                 CONDUCTOR                        CONDUCTOR
                               TENSION                          TENSION                     (kN)
                                      (kN)                               (kN)

  At load centre                                  Governed by allowable stay load

          0.5m                       101.4                               28.4               108.3

          1.0m                        50.7                               14.4               57.6

          1.5m                        33.8                                9.5               41.0

          2.0m                        25.3                                7.2               32.8




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                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
                                  Ergon Energy Corporation Limited ABN 50 087 646 062
                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN



APPENDIX 1
                                         CONDUCTOR LOADS


Transverse Wind Load(N)             = Cond. Dia (m) x Wind Pressure (Pa) x Wind Span (m)

Vertical Load (N                    = Cond. Weight (N/m) x Weight Span (m)



                   Wind Pressure                                  Wind Speed

           500Pa                                      100 km/hr

           900Pa                                      160 km/hr

           1200Pa                                     184 km/hr


                                                   2 x Wind Span

                                              Weight Span

                                 0°C
                                 50°C




                                        Weight Span @ 0°C

                                        Weight Span @ 50°C



                           0°C

                               50°C




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                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
                                  Ergon Energy Corporation Limited ABN 50 087 646 062
                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


                                       Weight Span @ 50°C


                           0°C
                                         Weight Span @ 0°C
                                             (negative)
                         50°C




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                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
                                  Ergon Energy Corporation Limited ABN 50 087 646 062
                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN



APPENDIX 2


DISTRIBUTION LINE LAYOUT STEPS

The following steps are suggested as the approach to be followed in designing a line from
scratch.
With experience or by reference to the tables of common applications in the Design manual
section “Pole Structures” many of these steps will not be required for jobs of a standard
nature.

1. Determine conductor size and type based on planning requirements and application.

2. Determine the proposed stringing tension based on the situation eg. Urban, semi urban or
   rural. Consideration in this decision should be given to the difficulty of staying and
   frequency of angles required by route restrictions.

3. Determine the Limit state design wind pressure on conductors appropriate to the location
   (eg 900 or 1200 pa).

4. Determine strain/angle pole locations taking into account the deviation angle limits on pin
   insulators as per the table in the Design Manual. If ratios of adjacent span lengths exceed
   2:1 in full tension rural situations, consider the use of a strain pole.

5. Determine expected span length on level ground from experience or by using suggested
   span and pole height / strength in the pole layout tables or the program Maximum span –
   ground clearance limitation. If poor soil foundations are anticipated, allowance should
   be made for additional pole setting depth at this stage. Consideration should also be
   given to any future requirement for subsidiary circuits.

6. If the terrain is not substantially flat, profile the line and determine pole locations and
   heights necessary to achieve ground clearances and likely strain/ angle positions.

7. Determine the ruling span using the Ruling span program for each section of line
   between strain structures.

8. Check any long spans to ensure that mid span phase to phase clearance requirements
   are met using the Maximum span - mid span clearance limitation program.

9. Use the Allowable pole tip load program to determine allowable (limit state) pole tip
   loads based on expected pole strengths and foundation conditions. These pole tip loads
   are after allowance has been made to take into account wind on the pole element.

10. Use the pole top loads from step 9 to input into the Allowable wind span program to
    determine the allowable wind span on unstayed intermediate poles. If these allowable
    spans are unrealistically low, return to step 9 using a greater pole or foundation strength.
    Consider the need for future subsidiary circuits in the selection of pole /foundation design.
    Use of bisect stays on small angles is an alternative option to increasing pole strengths.

11. Determine the weight span in particular on poles with a height which is significantly
    greater or less than their neighbours. This can be determined using the Weight span
    program which will output the weight span under the sustained load, maintenance and
    limit state conditions. If the weight span is negative, a strain structure should be selected.

12. Using the Crossarm design program, check that the proposed crossarm sizes are
    sufficient. Allowable weight spans for the selected crossarm sizes under the sustained
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                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
                                  Ergon Energy Corporation Limited ABN 50 087 646 062
                                  Ergon Energy Queensland Pty Ltd ABN 11 121 177 802
NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN


    load, maintenance and limit state conditions should exceed the weight spans determined
    from step 11.

13. Check that allowable horizontal stay loads from the Design Manual section “stays” exceed
    the limit state conductor wind and tension loads. Limit state conductor tensions can be
    determined using the Conductor tension change program.

14. For structures with multiple circuits or the stay attachment position away from the
    conductor attachment locations, use the Resultant stay load program to check that the
    stay horizontal load is not exceeded and that the bending moment in the pole at the stay
    attachment is not exceeded.

15. For any spans with different or unusual conductor configuration at one end and where mid
    span clearance may be an issue, use the Phase separation program to check
    clearances.

16. For any span where clearance to an adjacent structure may be an issue under conductor
    blowout, use the Conductor tension change program to calculate the horizontal swing
    under the 500 pa and 30 deg C condition. Add to this the relevant statutory clearance to
    check if clearance to the object from the line is sufficient. If not reduce span length or
    reposition poles and recalculate.

17. Conductor sagging information for listing on the construction plan for use by field staff in
    sagging the conductors can be determined using the Conductor sagging program.




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                                          Reference Approved by: Jim Brooks Network Lines Standards Manager
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