Guide to the design, materials used and construction of wood pole lines in the UK
This technical study provides basic guidance about the design and construction of wood pole lines in the UK. It covers wayleaves, wood poles, stays, conductors, notices and anti-climbing devices, construction, and inspection/maintenance and refurbishment. Wood poles are used on low voltage, 11, 20, 33, 66 and 132 kV distribution systems, varying in lengths depending on location, weather environment and what may be attached to the pole due to operational or customer requirements.
Author(s): David Horsman
Overview of estates and wayleaves
Estates and wayleaves officers are responsible for the negotiation and acquisition of all consents and approvals necessary for the development and retention of all electricity distribution/communications apparatus on private land. This includes overhead lines, underground cables, ground mounted/pole mounted substations and ancillary equipment at voltages from low voltage (LV) to 400 kV. The estates and wayleaves section are also responsible for settling all claims for losses and damage resulting from the placing or presence of equipment on private land.
Types of consent
The main types of consent which are acquired by the estates and wayleaves section are as follows.
A licence giving the company permission to install and operate equipment on or under private land subject to an annual payment. Wayleaves can be terminated by the granter if the equipment prevents use or development of the land. Most of the overhead network, and some underground network, is secured this way. This is not legally binding on change of ownership of land, but equipment is protected by statute. The estates and wayleaves section manage thousands of different wayleave agreements for existing equipment and ensure that appropriate annual rent and compensation payments are made.
A permanent right to place equipment on or under private land. Places permanent restrictions on the owner's right to use and develop under the Easement. Subject to a single capital payment upon completion. Mostly used for underground cables, but some overhead lines are protected this way. Binding on all subsequent purchasers of land. Can be used for cables and overhead line routes.
A deed transferring freehold ownership of land, usually without restriction, for an agreed purchase price. Used mainly for substation sites.
A right for a set term of years to conditionally occupy land for purposes specified in the lease subject to an annual rent/single premium reflecting market value. Used mainly for substation sites. Rent reviews under existing leasehold site are dealt with by estates and wayleaves section.
Section 37 consent
Consent required under the 1989 Electricity Act from the Department for Business Enterprise and Regulatory Reform to the installation of overhead lines. Certain exemptions apply
Form B approval
Planning approval required from Local Planning Authority for overhead lines before Section 37 consent can be granted.
Other statutory and non-statutory consents
- River Crossing Consent – (Land Drainage Act)
- Special Roads Consent – (NRSWA)
- Department of National Heritage – (Ancient Monuments and Archaeological Areas Act)
- English Nature – (Wildlife and Countryside Act)
- British Waterways – (Canals)
- Railtrack – (Operational Railways)
- Forest Enterprise – (Commercial Forestry Plantations)
Town and country planning consent
Required under the 1990 Town and Country Planning Act for any substation over 29 m 3 capacity. Substations below this size are ‘Permitted Development’ under the Town and Country Planning (General Permitted Development) Order 1995. This order also permits alterations/additions to existing sites subject to some conditions/restrictions.
A compulsory wayleave granted by the Department for Business Enterprise and Regulatory Reform following a public hearing. Used as a last resort and only when absolutely necessary. Obtained in accordance with paragraph 6 of the Fourth Schedule to the Electricity Act, 1989.
Consents for overhead lines
The types of consent required before any overhead line can be erected on privately owned land and can be divided into two main areas.
Section 37(I) of the Electricity Act, 1989 states that an electric line shall not be installed or kept installed except in accordance with a consent granted by the Department for Business Enterprise and Regulatory Reform (Section 37 consent).
Before an application for Section 37 consent can be considered by the Department for Business Enterprise and Regulatory Reform, it is necessary to serve notice on and seek the approval of the relevant Local Planning Authority(ies). This is commonly known as the Form B procedure.
This is by far the most important process in the procedure for the acquisition of planning approval to the overhead line proposal as the Department for Business Enterprise and Regulatory Reform will not give consent to the installation of an overhead line to which a Local Planning Authority object unless a public inquiry is held.
When formulating a route for the proposed overhead line, it is necessary to balance the conflicting statutory responsibilities of developing and maintaining an efficient, economical system of electricity supply with the need to preserve amenity and mitigate any effect a proposal may have on the environment.
When submitting a Form B application to the Planning Authority, it is a requirement to give details of the steps taken to comply with this latter responsibility, which is contained in Paragraph 1 of the Ninth Schedule to the Electricity Act, 1989. Failure to do so may result in the application to the Department for Business Enterprise and Regulatory Reform for Section 37 consent being rejected. A Formal Environmental Impact Assessment may be required to be submitted with Section 37 applications for 132–400 kV overhead lines. If the Local Planning Authority approves the proposal following the Form B application, consent will normally be granted by the Department for Business Enterprise and Regulatory Reform. This consent carries with it a deemed planning consent under Section 90(2) of the Town and Country Planning Act 1990.
At this stage it is also necessary to consult with and, if necessary, gain approval from, other interested parties, e.g. English Nature (under the Wildlife and Countryside Act), English Heritage (Ancient Monuments), Wildlife Trust and so on. Some overhead lines are exempt from the need to obtain Section 37 consent. These are:
- (a) lines having a nominal voltage not exceeding 20 kV which are used or intended to be used to supply a single consumer;
- (b) lines within premises in the occupation or control of the person responsible for its installation
- (c) lines complying with the requirements of the overhead lines (Exemption) Regulations 1990
If a Local Planning Authority fails to give approval to an overhead line proposal, there are three options available.
- (a) Re-design the scheme and resubmit the Form B application.
- (b) Put the line underground.
- (c) Request the Department for Business Enterprise and Regulatory Reform to hold a public inquiry at which both parties will have the opportunity of putting their case before a Senior Engineering Inspector from the Department for Business Enterprise and Regulatory Reform and Inspector from the Department of the Environment whose decisions will be final.
Obviously, the latter option is very costly and time consuming and should be avoided unless absolutely necessary.
When planning an overhead line route it is extremely important that the requirements of Section 37 of the Electricity Act are taken into account especially if target dates for construction are to be set. This is equally as important on a diversion that requires Section 37 consent, where that diversion is being requested to facilitate a development, as it is on a major project, asset replacement or a new scheme. A minimum of 3 months should be allowed for Form B and Section 37 consent applications plus wayleaves, assuming that no objections will be raised.
It is illegal to erect an overhead line without Section 37 consent unless the relevant exceptions apply.
General description of types of supports and wood poles for overhead electricity distribution lines
Wood poles used in the Electricity Supply Industry for overhead lines are almost exclusively red fir imported from Northern Europe, e.g. Norway, Sweden or Finland. All poles must comply with BS 1990 which includes a creosoting treatment. By using red fir the industry are taking advantage of the long General Post Office (GPO) experience of some 100 years and also the plentiful supply of this type of pole. Poles should preferably be hard green (i.e. well hearted with close annual rings).
Felling and shipment
The poles should be felled between 1 November and 1 March. They are sawn off as close to the ground as possible to contain the natural butt of the tree. They are drawn out of the forest as quickly as possible to prevent attack by fungus and either stacked clear of the ground or stored in water. Water stored poles are generally considered to be superior as they are said to absorb the impregnation better. The common practice of floating poles down rivers to the coast generally provides sufficient immersion since the poles are left stored in the water until ready for shipment. This only applies to poles from Finland. Norwegian and Swedish poles are transported by road to the ports for shipment overseas.
When the poles are received in the supplier's yard they are stacked on creosoted timber dunnage so that air can freely circulate around and through the pole stacks. The poles are dressed and all barks are removed prior to delivery. The seasoning period varies from 6 to 12 months depending on the size of the pole. These seasoning periods are only approximate and should be taken as the minimum and the seasoning should continue until the moisture content of the wood has been reduced to 25% (ideally) and as a maximum not more than 28%. The samples are taken using a Mattson borer, the tests being made by the company's inspector. Moisture content is given by
The inspector also checks the poles for straightness, freedom from rot, deep shakes, all as specified in BS 1990, 1971. Checks on size using pole callipers are also made to ensure the pole measurements comply with the specification.
Fabrication of poles
All drilling, cutting, scarfing and slotting of poles should be done before creosoting and special poles such as A, H or Twin poles should be machined and marked at this stage.
A gouge mark is made on the pole at 3 m from the butt and includes the size and class, type of wood and year of impregnation.
Marking with the same information is also made on the pole butt for stores identification.
Treatment of wood poles
The method of creosoting used is known as the ‘empty cell’ or rueping process.
The poles are treated in batches in creosoting cylinders which are designed so that the creosote can be heated and the cylinder subjected to pressure or vacuum during the process.
Poles placed in the cylinders are first subjected to a preliminary air pressure of 45–60 psi followed by an injection of hot creosote (temperature 170–180 °F) with the pressure maintained. The pressure is then increased up to an average 160 psi (and not more than 200 psi). The quantity of creosote to penetrate all sapwood can be estimated from the timber cubic content and sufficient creosote to achieve this is pumped in. Creosote is thus forced into the cells of the sapwood for a period usually 2–3 h and pressing is continued until 15 lb of creosote has been absorbed per cubic foot of wood. The amount of creosote is measured from a calibrated tank.
The cylinder is then emptied and vacuum is applied to take away excess creosote until not less than 7 lbs/cubic ft of creosote remains. This is known as the net retention.
The company inspector finally makes a further sample boring to check the degree of creosote penetration.
The number of poles treated per charge depends on type and size.
Properties of creosote
In choosing creosote as the preservative agent for its wooden poles, the electricity industry has made use of the many years practical experience of the GPO, railways and civil engineering industry.
The first patent for the use of creosote oil in timber preservation was granted in 1938 to John Bethel who was the inventor of the full cell or bethel process, in which the hollow cells of the timber are left full of creosote, as distinct from the empty cell or rueping process, previously described in which the timber cells are left empty of creosote, which resides in the fibres and walls of the cells.
The full cell process is generally used for railway sleepers, piling and marine timbers such as docks and harbours.
The empty cell process gives a much cleaner finish and is used for both BT and Electricity Distribution Company poles.
Creosote has the following properties which make it particularly suitable as a preserving agent.
- Toxic to wood destroying agents which tend to affect the sapwood of the tree.
- Permanent – not liable to leach out.
- Chemically stable.
- Cost effective and easily obtainable.
- Safe to handle and easy to apply.
- Non-corrosive to metals.
- Gives good penetration.
- Does not cause dimensional changes in the wood.
- Does not affect the electrical resistance of the pole.
Creosote contains over a hundred different chemical compounds, 25–30 of which are toxic to the wood destroying strains of fungi. The large number of these toxic compounds prevents the fungi developing immune strains which can happen when only one preservative is used.
Other treatment for wood poles
Despite the almost universal treatment of poles with creosote in the UK other methods do exist make use of water borne chemical salts which are pressed into the poles using the full cell process. These methods are widely used abroad but the general availability of imported red fir poles which can easily be penetrated by creosote has not led to their adoption here, although some companies have poles in service which have been treated in this way. The chemicals used are generally highly toxic (mercuric and arsenic compounds are used).
Treatment of wood poles in service
After many years of life the preservative tends to deteriorate (although at a very slow rate), and if any untreated sapwood is present from the original treatment there is a possibility of internal decay especially near the ground line where the fungal wood destroying agents are most active.
To guard against this decay, many remedial treatment methods have been developed, in the past Cobra treatment was generally used throughout the UK. At present poles are treated with boron rods.
Cobra wood treatment
Basically the method employs the following stages:
- Inspection and testing of the poles for soundness – there is no point in treating poles in an advanced stage of decay.
- Injection of the preservative into the pole using a lever operated hollow syringe. The preservative is a mixture of arsenious acid, dinitrophenol and sodium fluoride. The injection is made just above and just below ground level and a measured quantity of preservative goes into each cubic foot of pole in this region.
- Fixing of a protective covering of light aluminium sheeting round the treated region of the pole to prevent poisoning of animals.
The Cobra treatment is claimed to lengthen the life of poles by 10 years.
Other methods employ the brushing of preservative paste onto the pole or the use of a preservative soaked bandage (Wolman process or Prinsote process).
Important notes: The toxic nature of the salts used by Cobra (wood treatment) Limited and the requirements and implications of the Health and Safety at Work Act mean that we take special care when disposing of wood poles. Any wood pole which is to be disposed of and has been Cobra treated should be cut 600 mm above and below the ground line (i.e. clear of the treated zone) and the treated section returned to a disposal agent to be disposed of in accordance with the requirements of the Control of Pollution Act 1974.
Usually, the Local Authority Environmental Health Department will dispose of the treated section of the pole. They may need to know the composition of the Cobra substance. This is as follows:
Total weight of substance used can vary between 250 and 350 g and comprises:
- 22% arsenious anhydride.
- 24% dinitrophenol.
- 50% sodium fluoride.
- 4% fillers.
Boron rod treatment
These rods are of a glass like nature and they dissolve over a period of ∼10 years to wick boric acid solution into the pole in the most likely areas of decay.
Boron rods should not be used close to water courses, ponds, rivers, canals or lakes due to the effect that the boric acid can have on aquatic life. However, boron rods can be used in all other places.
The 13 mm diameter holes are drilled around the baseline of the pole, usually three or four depending on pole diameter, and the boron rod is inserted into the pole and the hole plugged with a plastic bung.
The design of the wood pole supports for overhead lines has always been largely empirical due to the very nature of the material. The method of growth, seasoning and preservation all affect the strength of a pole and it is impossible to calculate with precision the resisting moment of the poles when set in the ground, or the ultimate strength of single or special poles. Various tests have been carried out from time to time to determine ultimate fibre stress and these tests have tended to confirm that the empirical figures generally used in calculations.
Note: Ultimate fibre stress of
The disadvantages of this lack or precision are more than offset by the many years of satisfactory service of supports designed using the empirical methods mentioned above.
Wood poles are extremely adaptable for use singly or as part of special arrangements and for mounting auxiliary gear, e.g. fuses and transformer and are used to carry overhead lines at voltages from 230 V to 132 kV.
Detail design of a single pole
A single wood pole loaded near the top acts as a cantilever beam and the stress in the fibres can be shown to be a maximum where the pole diameter is 11/2 times the diameter at the load point. BS 1990 specifies the pole diameter at ground level to be 11/2 times that at the load point. The ground line is assumed 1.5 m from the butt. If the pole top exceeds 2/3 of the diameter at ground level the maximum stress would occur above ground level and such a pole would be uneconomic since the longest section of pole would not carry the largest load
An intermediate pole is subject to various loads as follows:
- Direct downward load of the conductors and ice covering. Usually very small and can be completely ignored.
- The transverse load due to the specified wind load acting on the conductors with ice. This wind load exerts a simple bending moment on the support at ground level.
- The transverse load due to the wind load on the support itself. This load exerts a simple bending movement on the pole at ground level.
- If the pole is subject to downpull then the crippling load on the pole due to tension in the line must be taken into account.
When designing an overhead line it is important that each support is made to take its full share of the wind load if the design is to be an economic one.
Wind span is half the sum of the two span lengths adjacent to the support and this governs the normal span length of the line.
Calculation of wind span for most economical use of pole.
100 mm2 aluminium conductor steel reinforced (ACSR) conductor ENA 43-20 construction level ground
- Conductor diameter 14.2 mm
- Radial ice 9.5 mm
- Wind pressure 380 N/m2
- For a 9.5 m stout pole
- Diameter at ground level 280 mm
- Diameter at pole top 190 mm
- Factor of safety 2.5
From BS 1990
For imported fir
(see Fig 1)
where W1 is the wind load/metre run of conductor, W2 is the wind load/metre2, D1 is the diameter of pole at ground level, D2 is the diameter of pole at pole top, L is the wind span, N is the number of conductors, H1 is the height of conductors above ground level, H2 is the height of pole above ground level
Above example has not included wind load on cross-arm, insulators and fittings.
Fig 1: Bending moment about ground level
Resistance of earth to bending moment:
From Example 1 maximum allowable BM = 46378 N m. Assume average diameter of pole below ground level
=300mm projected area of pole planted 1.8m
=1.8 × 0.3= 0.54 m2
- Projected area of kicking block 0.32 m 2
- Earth bearing pressure = 7320 kg/m 2 = 7320 × 9.81 N/m 2.
- Pressure acting on pole = 7320 × 9.81 × 0.54 = 38,800 N.
- Pressure acting on kicking block = 7320 × 9.81 × 0.32 N = 22,980 N.
- Resisting moment of pole = 38800 × 0.9 m = 34920 N m.
- Resisting moment on kicking = 22980 × 0.5 m = 11490 N m block.
- Total resisting moment = 46410 N m which is greater than maximum bending moment.
Level ground: Crippling load on angle pole (see Fig 2).
Fig 2: Crippling load on angle pole
For 100 mm2 ACSR conductor from ENA 43-20, maximum permissible
(see Fig 3).
Fig 3: Crippling load due to tension
Note: 1800 kg figure is projection of 0° angle of deviation line and 137 m wind span on NOMOGRAM 1 page 40, ENA TS 43-20
For downpull conditions component acting down pole due to conductor tension must be considered.
Stay needed for 30° deviation, level ground: From Example 3, T R = 2267 kg wind load
Max permissible span 100 mm2 ACSR = 150 m (see Fig 4)
Fig 4: Max working load in stays
So that max working load in stays is not exceeded
Note: wind load on cross-arm and fittings not included.
Various grades and stranding have been used during the construction of overhead lines, most companies have now standardised on either one or two grades of stay wire.
7/4.00 mm strands
The strands are either 700 or 1150 grade galvanised steel. The 19 stranded 700 grade stay wire is no longer being used, but these types are already installed over a major part of the overhead line system.
Grade 1150 stay wire cannot be made off using hand splicing techniques; therefore ‘preformed’ helical fittings must be used for connections.
Helical fittings for stays
Preformed fittings used for installing stays are:
- Guy grip dead ends.
- Pole top make off.
- Stay wire splice.
- Insulator link assembly.
Up to 33 kV porcelain stay insulators to Energy Networks Association Technical Specification (ENATS) drawing 439107 are used as follows:
- Type 1 (single): For all stays on LV, 11 kV and all stays on EARTHED 33 kV structures.
- Type 2 (double): Linked in series for voltages up to 33 kV where steelwork is not bonded to earth.
Currently, there are three systems for installing stay rods in the UK.
- Excavated stay rod with wood or concrete holding block.
- Load-lock anchor stay anchoring system.
- Augered stay rod system.
Drilling and caulking the rod into position currently accomplish installation of a stay rod in rock.
Protection against climbing
A wrap of standard barbed wire may be fitted to stays at a position of at least 2.15 m above ground level. This shall be closely wrapped and extended for at least 1.5 m, it if is possible that the stay insulator will lie within the area to be wrapped, the barbed wire will be wholly installed above the insulator.
The Electricity Supply Quality and Continuity Regulations 2002 (Amended 2006) states
‘Every stay wire which forms part of or is attached to any support carrying a bare live electric line shall be fitted with an insulator no part of which shall be less than 3m above ground or above the normal height of any such line attached to that support.’
Stay insulators should be fitted to all overhead lines (even fully insulated) at a position where, should the stay break the section of stay wire below the insulator cannot make contact with any live portion of either line or support. Also should any part of the line become unattached (broken dropper or jumper etc.) and remain live, it cannot come into contact with the part of the stay below the insulator.
Ideally, stays should be installed to form an angle of 45° to the pole top.
Consideration should be given to settlement and the pole should initially be positioned leaning towards the stay rod between one half and one full pole diameter.
Manufacturer's instructions for installing ‘preforms’, load-lock anchors or augured stay rods should be complied with.
Multiple splayed stays should have a minimum separation at ground level of 2 m. Tandem stay arrangements should only be used on LV systems.
Stays supporting high-voltage (HV) lines should be bonded to steelwork at the top of the pole. The pole top make off provides a separate ‘king wire’ for bonding purposes.
All stays attached to metal fittings should be fitted with galvanised thimbles.
Conductor selection is a complex business. It is dependent not only on electrical, magnetic and economic considerations but also radio interference, stray fields and fault level capability. Mechanically, the conductor must support its own weight as well as any ice and wind loading and must maintain ground clearances under all predicted environmental conditions. The best choice electrically may not be the best choice mechanically or even environmentally.
The range of physical characteristics that are of interest in as far as overhead line conductors go are listed in Table 1.
to give adequate current carrying capacity and LV drop
to maintain ground clearance in long spans
avoid vibrational fatigue failure
to withstand a variety of loading conditions
to withstand environmental conditions (e.g. corrosion resistance)
to maintain the characteristics for 40–50 years
Table 1: Conductor characteristics
Nominal area, mm 2
Actual area aluminium
Stranding number/diameter, mm aluminium
Stranding number/diameter, mm steel
Table 2: Conductor specifications
Aluminium clad steel
Ω mm 2/km
temp coefficient of resistance
coefficient of linear expansion
×10 −6 per °C
modulus of elasticity
Table 3: Conductor characteristics
The characteristics required can be obtained by a combination of conductor geometry, material selection and control of the manufacturing process. To look at how these options can be manipulated so that an informed final choice can be made.
Although there are many different types of conductor geometry, including segmented and oval sections, generally wood pole lines will use conductors made from standard circular cross-section strands.
Conductors are generally made up of individual strands in order to improve flexibility and conductivity. In AC, the higher the frequency, the less the current actually penetrates into the surface of the strand. At MHz frequencies (as with a lightning surge) the penetration (or ‘skin’ as it is known) will only be a few tens of microns. Due to this skin effect most of the current at 50 Hz is carried in a surface layer around 1 mm thick. So for strands of, say, 3 mm diameter, almost all the conductor sections will be available for current carrying capacity. Extra-large conductor strands will thus add weight but give little increase in current carrying capacity. A solid conductor would carry even less current and would also suffer from vibration fatigue. A further gain in flexibility is obtained by reversing the twist in alternate layers. By convention the outer layer always has a right hand twist (Fig 5).
Fig 5: Twist directions
The standard geometry for strands of the same size means that there will be a central strand followed by a layer of 6 strands, then 12, then 18, 24 and so on. So conductors will normally have 7, 19, 37 or 61 strands and so on. Some conductors, especially those of dissimilar materials, may have a larger central strand or even a multi-stranded core.
There are other versions such as 32 mm2 hard drawn (HD) copper which has only three strands. In a manufacturer's conductor catalogue you may see the following descriptions for some ACSR conductors (see Table 2).
Horse, you can see, has 19 strands each of 2.79 mm diameter, the central seven forming the steel core. Lynx is similar but larger with 37 strands. However, caracal is the same size as lynx but is made up of only 19 strands. Finally, dog has a central steel core of seven small steel strands and just one layer of aluminium. Note also how some conductors have different amounts of greasing. All these variations give the line designer many options. By UK convention all ACSR conductors are given animal names. All aluminium conductors (AAC) are named after insects and all aluminium alloy conductors (AAAC) after trees and shrubs.
In the mechanical construction of the conductor there are two important parameters, the lay length and lay ratio. The lay length is the distance measured along the conductor axis in which any one strand makes one complete revolution. The lay ratio is the ratio of the lay length to the external diameter of that layer. In order to avoid ‘birdcaging’ where the inner layers bulge out of the conductor under stress, the lay ratio should be less than that of the layer beneath it. This rule is normally satisfied as this layer will have a smaller diameter anyway.
There are instances where the conductor diameter may need to be reduced. This can be achieved by compacting it during manufacture. The process deforms the strands and eliminates the interstices in between. This reduces wind loading but also decreases flexibility.
To obtain the desired conductivity and strength, the first port of call is the materials to be used. These may not be suitable for all our requirements (e.g. the best material for strength and conductivity may corrode too quickly) but we have to start somewhere!
The most common materials used for overhead line (OHL) conductors are:
- Copper – HD.
- Cadmium copper alloy.
- Aluminium–magnesium–silicon alloy.
- Galvanised steel.
- Aluminium clad steel.
Their characteristics are summarised in Table 3.
The treatment of the material during manufacture can introduce specific characteristics, e.g. the process or drawing down the feed rod introduces an increase in strength with a slight loss in conductivity. If this is not annealed ‘out’ then the material is known as HD.
Steel wire can be used as a strength member but must be protected against corrosion. This is done by coating with zinc (galvanising) or aluminium (by a continuous extrusion process). As the steel condition is important to the safety of the line, the integrity of the zinc coating is essential. This can be checked on line by the use of a high-frequency pulse. The pulse frequency is chosen so that the induced current will only penetrate the steel roughly equivalent to the thickness of the zinc layer due to the skin effect. The presence or not of the zinc will give a substantially different response.
So an OHL conductor can be made from a single material or a combination to suit. The choice is not always simple. For instance, copper has three times the density of aluminium and so will give a heavier conductor for the same conductivity. However, copper is far more corrosion resistant than aluminium and will have a longer life in coastal areas. It can also be erected at a higher tension – so reducing sags and increasing ground clearance. But yet again, aluminium is stronger than copper for the same weight – reducing pole loading. The options are wide and no conductor is perfect! However, a brief description of the more common conductor types is given next.
Common types of conductors
- AAC: As the name implies, this is made of electrical grade purity HD aluminium strands throughout. This is a low-cost conductor but with a limited strength-to-weight ratio. This can restrict the span lengths that can be used in areas where wind and ice loads can be high.
- AAAC: This is a heat-treated alloy to achieve a higher strength than AAC. It has a superior strength/weight ratio to ACSR and is harder than AAC and therefore less susceptible to surface damage. It can be used to reduce sags or increase span lengths compared with AAC. It also has a greater strength-to-weight ratio than ACSR for larger sizes and generally a lower AC resistance (lower losses) and no galvanic corrosion. Salt corrosion can still be a problem though in marine environments .
- ACSR: A method of significantly increasing the strength of AAC is to have a galvanised steel core. The mechanical performance is also improved by its higher modulus of elasticity and lower coefficient of expansion. Although more expensive per metre than AAC, its greater strength (and hence longer span lengths – fewer poles) means that the overall line cost can be lower. This makes it very popular for medium voltage lines. The steel core may be a single strand or be multi-stranded. If the steel is aluminium clad instead of galvanised then there is a conductivity gain but some loss of other mechanical properties. This type is known as ACSAR (aluminium conductor steel/aluminium reinforced) and is relatively expensive compared with ACSR.
- AACSR: Aluminium alloy conductor steel reinforced. Another ploy to increase strength in circumstances where conductivity is less important is to replace the aluminium in ACSR with aluminium alloy. Earth wire applications are one area where this could be used.
- ACAR: Aluminium conductor alloy reinforced. This conductor will not suffer from galvanic corrosion (no steel) but will have increased conductivity over AAC but lower strength than ACSR.
- HD copper: The most common conductor used within a few kilometres of the coast purely for its high corrosion resistance to salt.
- Cadmium copper: A copper alloy also used for its corrosion resistance and hardness but now no longer used due to environmental considerations.
- Compacted conductors: ACSR and AAAC conductors can be compacted by a final die drawing process to give an overall reduced diameter without reducing current capacity. This can reduce wind and ice loads but can reduce flexibility. Another advantage is the ability to increase the steel core size (e.g. by 200%), thereby substantially increasing strength-to-weight ratios without increasing wind loads.
- Covered conductors: Essentially covering the bare metal conductor with an insulating or insulated sheath reduces clashing problems, is wildlife friendly and can be far safer when touched accidentally in leisure activities or machinery. Corrosion is also reduced but particular attention has to be given to lightning protection and vibration.
There are many factors that will cause conductors to fail in service. The preponderance of one or other process depends on the way the conductor is used and the environment in which it is placed. The main causes of premature failure are:
This is caused by the continual movement (bending) of conductors at a fixed point where they are held. It is commonly due to the vibration that is caused by wind blowing across the conductor (aeolian vibration). Each conductor has its tension limit above which fatigue becomes a problem. This limits the span lengths unless anti-vibration methods are used. Typically aluminium conductors can be strung to 20% of their ultimate tensile stress (UTS) and copper conductors to 33% of their UTS. Vibration can cause conductor failure at connectors or clamps due to the dynamic bending stresses (the clamping pressure restricting strand movement) and any initiating surface damage. The vibration is caused mainly by wind speeds in the range 2–20 mph with most damage caused in light winds. Vibration is also dependent on the line rigidity, i.e. it increases with line tension and so is also temperature dependent as line tension increases as the temperature drops. Aeolian vibration occurs in the frequency range from 3 to 120 Hz.
A less frequent wind induced problem is galloping, where the conductor oscillates at low frequency (<3 Hz) and high amplitudes (up to the sag which can be several metres on transmission lines). This is more common on transmission tower lines. Galloping occurs under relatively high wind speeds (above 15 mph). Aerodynamic forces are coupled through the angle of attack (wind direction relative to the conductor). There may also be mechanical coupling when natural frequencies of different motions coincide. Typically this happens when eccentric ice weight and aerodynamic torsional moment reduce the torsional natural frequency until it is equal to the vertical natural frequency. The conductor becomes aerodynamically unstable – normally due to an asymmetric rime ice deposit. The conductor orbit is usually an ellipse whose major axis is vertical and minor axis horizontal.
Finally, sub-span oscillations can occur between the spacers of bundled conductors. They are a function of conductor diameter, spacing, wind angle and wind speed. Generally has the same frequency as galloping but with amplitudes up to the conductor spacing only.
Creep is the inelastic extension of a material under stress. As far as OHL conductors are concerned there are two types of creep – geometrical and metallurgical. The geometrical creep occurs as soon as a conductor is strung up for the first time. The tension causes the strands to ‘settle’ and the interstices to be reduced. There may also be partial elongation due to wire indentation by this process. This takes a matter of days or weeks depending on the tension. For the rest of the conductor life the creep is metallurgical and dependent on time, temperature and the stress level (e.g. line tension and snow/ice/wind loads). Eventually the conductor will stretch so far that clearances cannot be met. The creep can be calculated from laboratory experiments carried out under extremely strict conditions of temperature control – usually in long underground rooms where a 60 m span can be maintained under disturbance free conditions. Creep has to be allowed for in the initial erection stress applied to the conductor.
Corrosion is the normal life-limiting factor in the UK. This is in contrast to many other parts of the world. It appears that our environment is a particularly aggressive one with respect to aluminium conductors and the problems associated with corrosion can be particularly acute. Aluminium is generally considered to be a corrosion-resistant material. However, thermodynamically it is a very reactive material and relies on the formation of a very stable oxide layer on its surface to prevent corrosion in normal circumstances. However, as for any material that relies on a protective coating to prevent corrosion, situations which give rise to damage or prevent formation of this oxide coating can result in rapid corrosion. Two particular mechanisms are relevant to aluminium used in stranded conductors. First, crevice corrosion. In an area of restricted oxygen supply, such as the bottom of a crevice, the maintenance of a coherent oxide fill can be compromised and this leads to a ‘differential aeration cell’ set up between the base of the crevice and the mouth, where more oxygen will be available. The result of this will be pitting corrosion at the base of the crevice. The other issue of particular relevance is the action of aggressive ions such as chloride ions. As a result of their size and charge density such ions are particularly effective at infiltrating and disrupting the oxide film and therefore increase the risks of corrosion damage at susceptible points such as base of crevices.
Based on this brief description of corrosion processes, it is clear that the design of overhead conductors has created a very good environment for corrosion. If moisture and pollution penetrate into the interstices of a stranded conductor, corrosion will occur. If the pollution contains significant chloride ions, i.e. marine pollution, this can result in severe corrosion in relatively short periods of time.
The lifetime of bare aluminium conductors in the UK can vary from <5 years to >50 years. In extreme situations, with small conductors that were ungreased or poorly greased at the time of manufacture, failures have occurred in aggressive coastal environments in <2 years. Larger conductors with several layers of aluminium and with larger cross-section individual strands, which are well greased, will have lifetimes in excess of 50 years in relatively benign inland environments.
The damage which ultimately causes failures to conductors occurs as the result of internal corrosion. That is, corrosion due to the mechanisms described above when pollution and moisture penetrates into the interstices of the conductor. In order to accommodate the larger corrosion product the strands need to deform. Once they have significant pitting corrosion they are unable to deform and fracture will occur. Once strands fracture there is a transfer of current between strands which leads to accelerated corrosion due to the effects of AC which are particularly significant for aluminium. This process therefore is an accelerating process that ultimately causes the mechanical failure of the conductor.
Grease is used internally in conductors to prevent the ingress of moisture and pollutants in order to prevent the corrosion process occurring and therefore to increase the lifetime of conductors significantly. This grease is applied at the time of manufacture and if applied effectively can lead to extended lifetimes even in relatively aggressive conditions. It should be noted that the grease itself does deteriorate when exposed to severe marine pollution, particularly for small conductors and therefore even well-greased conductors do have a limited life in aggressive conditions. The level of greasing, the consistency of its distribution and its condition are therefore additional factors affecting conductor lifetime.
Greases fall into two main categories – hot applied and cold applied.
- Hot-applied greases are essentially oil within a wax matrix. The ‘drop point’ or melting point is set to be above the expected maximum temperature the conductor will reach.
- Cold-applied greases are essentially synthetic oils which may decompose before reaching their drop point. These are easier to apply but may not be suitable for high-temperature operations.
Greasing is also applied to different levels according to the corrosion risk – from category 4 (all strands greased) to the steel core only being greased. Fig 6 shows the categories schematically. They are detailed in EA Engineering Recommendation L38/1.
Fig 6: Greasing categories
Notices and anti-climbing devices (ACDs)
Protection against unauthorised climbing is a major concern when finishing off an overhead line. Few members of the public are aware of the dangers associated with wood pole overhead lines. Quite a few people refer to them as telegraph poles, believing that they have something to do with the telecommunication business rather than the distribution of HV electricity.
To make people aware of the danger, notices are attached to poles to indicate that they are dangerous. All HV wood poles have got to have at least one ‘danger of death’ notice. All supports are numbered to aid identification in the event of a fault or some other form of work. It also helps when doing routine patrols. Other signs on the pole would usually be fitted to transformer and switch poles. These would be ‘property’ and ‘nameplates’. These aid the identification of the system both to staff and to members of the public.
No other notices are allowed on the pole. However, in recreational areas crossed by overhead lines it is good practice to position additional notices beneath the conductors to warn of their presence.
In addition to the warning signs, ACDs need to be fitted to poles and stays if there is any foreseeable risk of anybody being able to gain unauthorised access to the pole. Depending on the location, these ACDs can consist of a simple wrap of 12 turns of barbed wire or an outrigger bracket, either pre-wrapped with barbed wire or wrapped on site.
In areas of high risk, enhanced ACDs are fitted. These consist of both of the above methods and constitute a considerable deterrent to any attempt at climbing the structure. All stays are wrapped with barbed wire to prevent someone trying to pull themselves up the stay wire and into danger.
Wood pole overhead line construction has changed significantly over the past few years. Most of the changes have been brought about by the need to mechanise a lot of the work previously achieved by manual techniques. Due to the downsizing of the workforce, previous methods are no longer practical. Line teams now need all the latest equipment in order to do the same amount of work previously undertaken by large teams of linesmen. The following section describes the construction of overhead lines in the order of work undertaken.
A great deal of damage can be caused by thoughtless access to private land. Compensation claims can sometimes amount to thousands of pounds and the goodwill for future access is completely lost.
A little planning prior to work commencing can alleviate any of the foreseeable problems likely to occur. Liaison with landowners is essential: they can give information about the particular access route that they would prefer to be used; they will also have first-hand knowledge of difficult terrain and areas where vehicles are likely to get bogged down. It is also valuable to discuss the type of vehicle access required. Overhead line construction can entail the use of various items of mechanical plant. Winches, drum trailers, excavators and equipment trailers are commonly used for construction.
Specialised vehicles will help to solve difficult land access problems, low ground pressure vehicles are used to prevent vehicles becoming bogged down. However, in particularly bad areas tracked vehicles, such as the muskeg or garron, are the best method of avoiding land damage.
All access routes need to be strictly adhered to and sometimes it is advisable to mark or fence the routes to be used as the shortest route is not always the best. Edges of fields adjacent to hedgerows tend to be drier and more able to sustain the weight of construction vehicles. Areas with a high concentration of peat soil are particularly difficult to access and require special consideration.
Excavations and foundations
In order for an overhead line to be successfully constructed, care must be taken to ensure that any excavation or foundation will be of sufficient strength and depth to support the structure and conductors. If the hole is too shallow the pole will not have sufficient stability; if it is too deep the clearance of conductors to ground level, roads, special crossings and so on will not be enough to comply with statutory regulations.
Poor conditions such as peat bogs and areas with a high water table need special foundations to be installed below ground level. These vary from a single wood baulk bolted directly to the pole side to a complicated arrangement of struts and timber baulks arranged to form a cantilever type platform from which the pole weight is evenly distributed.
Excavations for overhead line wood poles were traditionally achieved by either hand digging or by mechanical excavator. The shape of the excavation was such that it allowed the pole to be reared or tilted into the hole. If manual digging techniques were used the hole would be stepped so as to minimise the amount of soil to be excavated. However, for all manual excavations some form of shuttering is required, as the minimum depth of excavation for the smallest wood pole is 1.5 m.
Both the above methods of excavation are time consuming and cause considerable land damage. However, recent moves within the industry have been to use power augers wherever possible to drill the pole hole. These large diameter augers are usually quicker than other methods and have the advantage of leaving the pole, once installed, surrounded by virgin soil.
Another problem frequently encountered, particularly in the northern National Parks areas, is the need to excavate in rock. Explosives may be the only answer for really hard rock. Augers may be used for softer types.
Other types of excavation are needed for the installation of stay blocks and for erecting ‘H’ poles. Stays require the excavation to be underpinned to allow the stay block to be positioned at 90° to stay the rod. They also need a slot to allow the stay rod to lie in line with the pole top. This means that the linesman needs to enter the excavation to underpin the bank. Shuttering of such an excavation is not easy as the stay rod and stay block need to be installed together.
Pole dressing and erection
Dressing poles before erection is the easiest way to build an overhead line. Steelwork, stay tops, insulators and other fittings can be fitted easily while the pole is supported on a trestle at ground level. HV steelwork weighs up to 50 kg for a light duty terminal cross-arm. The equivalent cross-arm for heavy duty construction weighs up to 120 kg, so it is easy to imagine the difficulty of handling these weights at the pole top once it has been erected.
Poles are dressed in accordance with the specification requirement. M20 bolts are used for HV and M16 bolts for LV. Each bolt should have a washer behind the nut and, where possible, should be positioned so that the potential shear forces act on the shank of the bolt rather than the threaded part. Care is needed when fitting stay tops as these should be positioned so that the weight is taken on the pole and not on the tie straps of the cross-arm. One strand is taken out of the pole top stay fitting to be bonded to the steelwork (only on HV). This is known as the ‘king wire’
‘H’ poles require a large excavation to accommodate the underground bracing baulks and stability blocks. They can be up to 3 m long and usually 1.8 m deep. This also provides a challenge when shuttering as the linesman needs to access the bottom of the excavation to provide a firm and level base for the pole to rest on.
There are many ways of erecting poles and a lot depends on the site location and the access arrangements as to which method is used.
Although they are seldom used these days, they may be required to manoeuvre poles into place when the position of the pole means that other methods are impractical. LV lines running at the rear of housing could well be good examples of the sort of area where this method could be used. It relies on good co-ordination of team members and care must be taken when moving pikes. The pole is simply pushed into the upright position by means of the pikes.
Spar holm derrick
These are used to raise the pole partially to a position where it is possible to push or winch the pole to the vertical. Easily carried to site, they are useful where mechanical plant cannot gain access. A small hand winch on the side provides the lifting mechanism. Spar holm derricks can be used in tandem to lift large poles and side by side to lift H-poles.
This is used in conjunction with a winch to provide the initial lift of the pole from the vertical. Once the pole has started to rise the derrick falls forward and as soon as the fixing position of the winch rope rises above the top of the derrick it releases the derrick, which then has no further purpose.
This is a turntable attachment to the front loader of a JCB digger. It allows the operator to drive forward while raising and tilting the pole into the prepared hole. Once in the right position it can be lowered into place.
Works on the backactor of the digger. This device grips the pole and a turntable allows the pole to be rotated through 90°. The pole is then swung into position and lowered into the hole.
Massey ferguson jib
A simple device attached to the front forks of the digger, it allows the pole to be slung from the hook attachment with a sling. It is then lifted over the hole and lowered into place.
Marooka/lorry mounted crane
A pole manipulator on these machines is used to lift and position the pole.
This is an expensive way of erecting poles, but in some locations it can be the only practical way of doing the job within the time constraints. This method becomes more economical if used for erecting large numbers of poles. It has been used in this manner when the pole holes have been previously excavated. The poles are lifted with guy ropes attached, lowered into the holes and guyed off to ground anchors. Backfilling can then be accomplished when the helicopter has left site.
Distribution companies are now looking at alternative ways of pole erection using live techniques. Mechanical plant used for these operations must comply with safety rules and only skilled men are allowed to take part in these operations.
Backfilling of holes is an important phase of the work. All backfill materials should be regularly consolidated (every 150 mm) and it is good practice to replace backfill materials in reverse order of excavation. Topsoil must be used to finish the job and this is usually left proud to allow for settlement.
Site tidiness is worth mentioning at this point as this can cause trouble between landowners and distribution companies. Any materials left on site will inevitably cause problems either to livestock, machinery or to members of the public.
Conductor erection and tensioning
There are four main elements for conductor erection. These are:
- Work planning.
- Running out conductors.
- Pulling up sagging and tensioning.
- Making off and terminating.
Due to the different circumstances encountered on overhead line work, it is not possible to define a single method to cover all aspects. However, all jobs will fall under the general guidance notes associated with the above four elements.
Prior to running out conductors, a survey of site conditions needs to be thoroughly undertaken. Hazards need to be identified and plans made to safeguard against any problem arising during stringing of the conductors. Typical hazards could include:
- rough terrain,
- poor ground conditions,
- canals, rivers and reservoirs,
- overhead lines,
- school recreation areas.
This list is by no means complete, but it gives an indication of the type of problem that exists.
Wayleave constraints are another problem and these need careful attention. Agreements made with landowners need to be stressed to line teams in order to avoid misunderstanding.
Having assessed the physical obstacles that need action, the next stage would be to collect all the materials required. These should be assessed from the ‘materials schedule’ for that line. Depending on the length of line and type of conductors, the manpower requirements can then be resourced.
Before conductor running begins, any work involving tree clearance or crossing protection (scaffold) should be undertaken. Farmers should be informed so they can remove livestock from the fields.
Final preparation would be to position temporary backstays at the ends of the section to be strung (not required on terminal poles).
To pull the conductors through the length of the line, special winches are used. For three-phase HV lines, a three-drum rope winch is usually used. The ropes are pulled through the section by hand and at the same time they are positioned in rollers at the top of the intermediate poles. It is possible to run through section poles by installing heavy blocks at the pole top and, with careful planning, long sections can be pulled out.
At the opposite end of the line to the winch conductor drums are mounted in cradles with braking devices attached. This prevents overrun when pulling out and keeps the conductors under light tension during stringing, which helps to keep them clear of obstacles. The winch ropes are attached to the conductors using conductor stockings and swivels. Tension is taken within the winch, the drum brakes are adjusted and then the conductor is pulled through to the winch end of the line. During the pulling in operation and subsequent sagging and making off it is essential that good communication links are set up and that all areas with public access are adequately protected.
Pulling up, sagging and tensioning
The conductors are then backhung, that is to say they are terminated at one end of the section. Various methods of termination are used. These are as follows:
Sagging of the conductors is the next stage. Three things need to be known to work out the correct sag:
- conductor size,
- span length,
The sagging chart for the relevant conductor/construction is used to determine the sag with reference to the span length and temperature. Getting the sag right is important, as any variation would mean either that the ground clearance would be infringed or the structures and conductor are over-tensioned.
For new conductors an erection sag chart is used. This over-tensions the conductors to allow for stretching. Some conductors are tensioned for 1 h then made off.
A linesman can measure the sag of conductors in one of two ways: first, by use of sag boards nailed to the poles at a measured distance below the conductors the linesman sights through the boards while the conductor is pulled up. When the dip in the conductor is level with the top of the boards the sag is correct. The second method is to use a dynamometer to measure the tension in the conductor. This is usually positioned on the middle conductor and the rest are pulled up to match and sighted in by a linesman standing to the side of the line.
Once the conductors are sagged they can be terminated by one of two methods previously referred to. This needs careful attention to accuracy, especially on single span sections, as any discrepancy at the termination will be multiplied tenfold in mid-span. However, before finally terminating the conductors, the poles at either end of the section need to be checked for plumb. Temporary backstays and conductors all need to be adjusted until everything is satisfactory. Most terminations are easily achieved at the pole top. However, fittings for ACSR conductors are more easily accomplished at ground level.
Once the conductors have been terminated, the intermediate positions can then be bound in or made with helical fittings. Again, checks must be made at the pole position to judge whether the pole is plumb both longitudinally and transversely. The cross-arms on HV lines also need to be checked for squareness with the line.
Binding in of the intermediate poles completes the conductor stringing. Section poles can now be jumpered through and all construction equipment removed. Reinstatement of pole positions and access routes can now take place, where no further work is to be required.
Various items of plant are installed on overhead line wood poles. On HV lines, the most common item is the pole-mounted transformer. These vary in size and weight from 15 kVA weighing only 200 kg to a 200 kVA weighing ∼1.5 t. Other items of plant could range from any of the following:
- line switch,
- drop-out expulsion fuses,
- cable termination,
- surge arresters,
- fault indicators.
Recent developments in tele-control could mean that, on some poles, voltage transformer, solar panels and radio antennae may also be fixed to the pole.
Connections to the line and to the earthing system are made when all fixings are secure. Line connections are sometimes made to bails using live line taps. Earthing connections are made to all HV steelwork and tanks of transformers and so on.
On live LV lines, voltage regulators and static balancers are quite common, although the installation of such items is becoming less frequent as the HV system is reinforced.
The design of the poles must take account of the extra loading imposed by the fitting of plant, as some plant items might require an extra pole to support the additional weight. The general rule adhered to would be to install items up to 750 kg on a single pole and anything above to be installed on an H-pole structure.
Inspection\maintenance and refurbishment
Inspection of overhead lines is a statutory duty under the ESQCR 2002 which states in Part 1, Paragraph 5
A generator or distributor shall, so far as is reasonably practicable, inspect his network with sufficient frequency so that he is aware of what action he needs to take so as to ensure compliance with these Regulations and, in the case of his substations and overhead lines, shall maintain for a period of not less than 10 years a record of such an inspection including recommendations arising therefrom.
ESQCR gives no guidance as to the methods or frequency of inspections.
In addition, a generator or distributor has to determine a single measure of the risk assessment for every support which can be for example is high, medium or low.
The risk assessment shall follow the points below:
- Identify the assets.
- Assess the risks.
- Record the risks.
- Mitigate the risks.
The risk assessment is based on the test model below:
- The nature and situation of equipment test: This test addresses the principal characteristics of the equipment and its particular siting.
- The nature and situation of surrounding land test: This test takes view of the risk of danger from interference with equipment.
There are two aspects to this test:
- The geography of the land and its features (e.g. forests, rivers, flat fields, motorway, city streets).
- The use of the land (e.g. agricultural machinery, recreational areas, schools housing estate).
Once this risk assessment has been carried out and a risk category has been applied to a structure, i.e. high, medium or low risk then the operator must determine the inspection frequency.
The inspection can be carried out either by helicopter or on foot. Obviously during a foot patrol more information can be collated. It is also essential that periodic foot patrols are carried out to determine the exact condition of the overhead line asset which cannot always be done from the helicopter such as the condition of the pole which can only be determined using a hammer test or other suitable test.
Once all the information has been collated a tool such as condition based risk management which was developed by EA Technology can be used to determine where and how effective any maintenance or refurbishment program will be and where it is best to spend available money.
Maintenance and refurbishment
Once a new overhead line has been built it will require very little maintenance for a considerable time and the only visits to the overhead line will be the inspections detailed above. Once a line has aged however work will require doing to bring the safety and security of the line back-up to an acceptable high standard.
Overhead line maintenance is a statutory duty and ESQCR 2002 states in Part 3, Paragraph 3 - (1)
Generators, distributors and meter operators shall ensure that their equipment is:-
- (a) sufficient for the purposes for, and the circumstances in which it is used; and
- (b) so constructed, installed, protected (both electrically and mechanically), used and maintained as to prevent danger, interference with or interruption of supply, so far as is reasonably practicable.
The regulations give no guidance as to the methods or frequency of maintenance.
Operators of the overhead line would prefer to reduce the number of visits made to that overhead line so it is normal to carry out some form of refurbishment that will remedy all the defects on the overhead line or a section of overhead line.
There are two aspects to consider when assessing the performance of an overhead line:
- Mechanical design.
- Electrical design.
Any refurbishment policy should address the mechanical and electrical design of the overhead line and correct any deficiencies during the refurbishment. An example of this may be the mechanical strength of the conductor which may be under-strength based on the current weather loadings which may be applied to a new overhead line design compared with the old weather loadings applied to the existing design.