Technical information on British pylons
Contents
- Overview
- Tower types
- Tower characteristics
- Tower structure
- Tower construction
- Tower components
- Tower adaptation
- Tower design
Overview
This page covers a few of the basic technical details on British high voltage transmission towers, commonly known as electricity pylons (to some extent even within the industry, although they pretend that this is not so). Many details are likely to apply equally well to the transmission networks in other countries, although this site focuses exclusively on British power infrastructure.
This page is not exhaustive and is not in any way authoritative. The information on this page is provided solely to offer a little bit of background knowledge to anyone curious about pylons. The information presented here is collated from a variety of sources and is not guaranteed to all be correct or accurate. Quite possibly the entire page is nonsense. Caveat lector!
As a note, the term “pylon” was chosen by British architect Sir Reginald Bloomfield, the man responsible for choosing the tower design to be used for the National Grid in 1927 (that came to be known as PL1). Within the industry, however, pylons are referred to as “towers”, despite the ambiguity with all other types of towers in the country.
Tower types
The most straightforward power line would be a direct (as the crow flies) line from start to finish, e.g. power station to substation or substation to town over uninterrupted level ground. In reality, power lines involve various changes in direction and elevation just as with roads and railway lines. Different tower types are required depending on whether the power line maintains a constant direction or changes angle or elevation. Special towers are needed at each end of the line to terminate the overhead cables (known as “conductors”), and anywhere that the line needs to tee off. Substations and crossings can also require special towers for various purposes.
The various tower types include:
- Towers that begin and end the power line
- Towers that carry the power line in a straight line
- Towers where the power line turns a corner
- Towers that take one power line down under another where they cross over
- Extra tall towers for spanning rivers
- Towers for junctions between power lines
For more details, see the tower types page.
Tower characteristics
Height
Broadly, the height of a pylon depends on the voltage that the line is carrying. The original 132 kV National Grid pylons were only around 24 m tall (and 20 m for the single circuit towers). More typically, 132 kV towers—now generally classified as distribution rather than transmission—are around 26–27 m tall. Supergrid towers, carrying 275 or 400 kV, stand between 40–50 m tall. These are the heights of the straight line towers; angle towers are generally shorter as the insulators are hung from a lower height.
Height extensions and reductions
Standard tower types are normally provided with a variety of height options. Early tower types only supported height extensions as standard, while from the 1950s onwards it was common to also offer standard height reductions. Generally, height increases are achieved by modifying the lower portion of the lattice structure or adding new steelwork, with the majority of the tower design left unchanged. The diagrams below show the five known height options for EE PL1 (Milliken) D2, with the modified portion highlighted in blue: standard height, 5-foot extension, 10-foot extension, 20-foot extension and 40-foot extension. The available extensions vary by tower suite and type.
Height extensions and reductions are denoted by a suffix followed by the change in height. “E” denotes a height extension and “M” denotes a height reduction. In some cases, height extensions use “+” instead. The standard height tower is typically denoted by “STD” or “SH”. Height extensions were originally in feet, which have now been replaced by metres. Typical height extensions in the 1930s and 1940s were +5 feet, +10 feet and then further 10-foot increments. However, extensions and reductions based on multiples of 4 feet were used in the 1950s for the Blaw Knox Supergrid types; Supergrid line and angle towers from the 1950s and 1960s tended to have a minimum height of M24 (minus 24 feet). L4(m) and L7(c) metric towers have heights in 3 metre increments, typically from M3 to E15 (D) or E12 (D30, D60, D90, DJT and DT); some of the remaining types have different limits.
Metric extensions and reductions are simply numeric, e.g. E3 indicates a 3 metre extension. However, such designations originally denoted feet, making designations ambiguous. Imperial types may also be found with the unit present, e.g. “ E.10′ ” for a ten foot extension. A value such as “M4.9” will be an imperial size converted to metric, here a 16-foot reduction “M16” converted to a metric size of 4.9 metres.
For L66, Blaw Knox designed a modular height extension system that was also used on L2, Blaw Knox L3, T2175 and L8. Separate 8 foot, 16 foot and 24 foot legs were provided. These legs could be added to the minimum height tower (M24′ ) for reduced heights of M16′ and M8′ and to produce the standard height tower. The same legs were used in conjuction with body extensions to provide taller towers. (From around E44′ upwards—towers with 32′ and 36′ body extensions—the leg design remained unchanged but the drawing numbers differed, suggesting the use of heavier steel angles to accommodate the extra tower weight.)
Callender’s S2 tower for SWE PL1(a)&(b) implemented height extensions a little differently. E20 simply added a new base section to the tower. E5 and E10 however used incomplete portions of the standard height tower onto which new steelwork was added:
Low height
In addition to height extension and reductions to full-size towers, there are also special dedicated low-height towers. L9 is a series of special low-height towers that complements L6. There is a very similar tower design as part of L12 series; here, an “L” prefix is added to the tower designations instead of using a separate series name.
Low-height runs can also be achieved with gantries. Where the 400 kV L2 line from Northfleet East Substation to Rowdown Substation passes to the north of London Biggin Hill Airport at Leaves Green (51.346° N, 0.043° W), a sequence of three SFX gantry pairs carries the line at low height. Earthwire changeover towers split the earthwire and both gantries in each pair has an earthwire peak. As all known gantries are single-circuit only, the gantries are installed in pairs.
Circuits
Towers in the UK can be either single circuit or double circuit. Each circuit is three-phase, with three conductors (wires) or conductor bundles. There is one crossarm per phase. Double-circuit towers typically have six crossarms: one crossarm per phase, three phases per circuit, two circuits. Single-circuit towers typically have three crossarms: one circuit with three phases. The three phases are referred to as “red”, “yellow” and “blue” and in some cases the order of phases is marked on the pole or tower with coloured tags.
Dedicated single circuit towers are shorter than double circuit towers: the staggered layout and reduced count of arms considerably lowers the height requirement. Example single and double circuit towers from the SEE PL1a family are shown below.
Towers with more than two circuits are extremely rare. Some towers have a fourth pair of crossams, for double earthwires.
True single circuit towers are no longer constructed except in the rare situation that a like-for-like replacement is required. Instead, double circuit towers are strung single circuit, sometimes with only half the crossarms fitted as needed.
Care must be taken to not misinterpret the wires, arms and insulators. Power lines with bundled conductors will have extra conductors and extra insulators (the latter often just on angle towers) but the circuit count remains unchanged.
Double circuit towers adapted to serve as single circuit towers remain the same height, but three of the crossarms are deleted:
Bracing patterns
There are numerous bracing styles used for the body of the tower. These are illustrated in the following diagram, adapted from [Transmission tower development]:
The tower bracing has to be effective under both tension and compression loads. For light loads, zig-zag bracing is sufficient and in the UK this is generally limited to the earthwire peak of angle towers, especially those designed by J L Eve. The top portion of the tower is generally X braced for additional strength under load compared to zig-zag bracing. X bracing by itself loses effectiveness under compression loads as the tower widens towards the bottom, as the bracing members could buckle. Inner triangles can be added for additional strength, giving rise to the diamond pattern. This is most prominent in L12; it can also been in some L6 variants including Blaw Knox and metric, and L7 D60.
The most strength comes from K bracing. A horizontal member is added to tie the legs together, below which is a half X with inner triangles for strength, in the general shape of a downwards-facing K.
In between diamond and K is “large diamond”, which is effectively back-to-back K bracing. Large diamond is most commonly seen in towers with height extensions.
Earthwire shade
Earthwire shade is the protection afforded by the earthwire to the conductors below. It is a measure of the angle between the earthwire and the top conductors. L7 for example is cited by SSEN to have 45° earthwire shade, which can be seen in the diagram below. A higher angle means that lightning has greater access to the conductors; a lower angle means that the earthwire has a greater chance of intercepting lighting strikes and keeping surges away from the power grid.
Older tower designs tended to have only a small distance in height between the top crossarm and the peak, so the earthwire protection was poor. One of the differences between the 1950s L2 series and its replacement L8 is a higher peak, to improve the level of earthwire shade. (Another significant difference is that L8 provided extended middle crossarms as standard to reduce conductor clashing.)
Entry angle
Terminal and junction towers are rated for entry angle. It is not always convenient to orientate the substation equipment to directly face the incoming line. One option is to build the terminal tower facing the substation and have the line come into the terminal tower at an angle. Most terminal tower types allow the incoming line to approach the tower from a 5° angle. If the line needs to come in from a wider angle, a different tower type is required. For L2, the DT45 tower can be used. For some other types including L4(m), L6 and L7, a junction tower (DJT) must be used instead; these typically allow a 45° entry angle when installed as a terminal tower. The CEB-L132 specification upon which PL16 and L16 were based allows for a 45° entry angle on standard terminal towers and is why this practice is commonly encountered.
Where the ground-level termination equipment needs to be all the way to the side of the tower, a DT90 tower used to be used (such as with PL1, PL4 and PL16). Such towers had auxiliary crossarms at the rear to allow the conductors on the distant side to be routed around the tower. This designation appears to be obsolete now, although newer terminal towers still permit the attachment of auxiliary crossarms for this purpose.
Double earthwires
Some towers take two earthwires instead of one. Normally the earthwire is attached to the top of each tower, but with this adaptation, each of the two earthwires is attached to ends of the top crossarm. PL4 towers use a fourth crossarm for this purpose, while PL7 and PL16 have special top crossarms that support both the phase conductors and the earthwires. Double-circuit tower designations (e.g. D2, D30) become“DD” prefix, giving DD2, DD10, DD30, DD60, DD90, DDT, DDT60 and DDT90. Single circuit towers designations change to “SS”: SS2, SS10, SS30, SS60 etc.
Some DD towers nonetheless have just the single earthwire in the centre, as can be seen on the PL16 line at Welham Green. This likely resulted from a reconfiguration of the line at some point in the past. The CEB L132 specification from 1940 indicates that the double earthwire is to extend a mile from the substation. This configuration can be seen at the substation at Picotts End, Hemel Hempstead, Hertfordshire where the 132 kV Elstree–Sundon line starts out on DD towers before changing to standard towers; here, you do have the proper dual earthwires as intended. (This can be seen on Google Street View; there are no photographs of that substation on this site.)
Photos of tower types from other countries suggests that double earthwires are a common feature on standard towers outside of the UK, with a dedicated top crossarm the standard approach.
Tower structure
Conventional pylons are steel lattice towers. These are towers built from strips of steel angle known as “bars”. These bars are all attached using nuts and bolts, rather than by welding. Outline diagrams of towers show only the bracing along the front edges of the crossarms and exterior faces of the tower body. For additional strength, there is typically bracing along the bottom of each crossarm, and diagonally within the tower itself.
Outline diagrams also show the steel bars all meeting at specific points. In reality, the bars are much thicker than the single lines shown and they often do not overlap where they meet. Gusset plates are frequently used at the junctions between bars. For strength it’s common to see two bars placed back to back.
Steelwork
Each of the plates and bars on a tower is cut and drilled from standard steel stock. Bars can be flat, L-section or U-channel. A mixture of mild steel and high-tensile steel is used as required.
When the UK steel industry adopted the metric system in the early 1970s, the cross-section dimensions of the steel stock changed from imperial increments to metric increments. New tower suites would be designed from the outset around metric stock; existing tower suites had to be adjusted to adapt to metric stock. The first all-metric type was L4(m); many existing types including Milliken PL1, L2, L3, L7 and L8 were adapted for metric stock. For example, L8(c) is metricated L8.
The individual bars that make up a tower can be simple straight sections (no bends) or they can have bends added for strength. The “hot rolled” process leaves the steel sections straight: the steel sections are limited to what can be achieved using only hot rolling at the steel mill. The alternative is the “cold formed” process where the steelwork fabricator adds bends to the steel bars for extra strength. The bends are formed with press or rolling tools on cold steel and involves additional labour and thus additional cost. The additional strength of cold-formed members allows for the tower to use fewer members, giving it a more “open” appearance, with simpler latticework, reducing its visual impact. The price difference is significant difference however; in their evaluation for the North–South Interconnector ([Meath-Tyrone]), ESB International noted:
Costs show approximately €2200/ton and €3500/ton for hot rolled steel and cold formed steel respectively. Therefore a cold formed tower would cost up to 60% greater than a hot rolled steel tower.
The specifications for both L4(m) and L7(c) each reference BS EN 10025 Hot rolled products of structural steels and make no mention to cold-formed steel. The dense bracing of L7(c) certainly suggests a hot-rolled steel design.
The technicalities of steelwork fabrication is beyond the scope of this site. Section 5 “Hot Rolled versus Cold Formed Steel” of [Meath-Tyrone] (pages 6 and 7) contains additional details on the differences between the two processes.
Tower construction
Traditionally, pylons are constructed in situ, starting with the foundations. For towers rated for 132 kV and above, each of the four corners is supported on a separate concrete foundation, referred to as a concrete pyramid or concrete frustum; the old 33 kV and 66 kV towers as well as PL1 could instead use earth grillage or “Malone” foundations (per [NSP/004/030]). Into each corner foundation is placed the first bar, or stub, onto which the next plates and bars will be bolted. During assembly, portions of the tower are typically preassembled and then lifted into place with a crane or similar apparatus. The entire process was documented on the website of one of the power companies (possibly SSEN).
An alternative approach is to assemble large portions of the tower off-site and then lower them into place by helicopter; this process is useful when towers need to be installed in extremely difficult-to-reach locations where constructing an access road to the site would be more trouble. What is claimed to be the first construction of this type in the UK took place in 2013 and is depicted extensively in YouTube video Pylon Erecting Bristol UK With Helirig Helicopter LN-0BX. This video also clearly shows the base of a tower being attached to the stubs.
The entire surface is painted for weatherproofing, a process that must be renewed periodically. The picture below shows paint flaking off an L2 D30 tower:
The four stubs are initially positioned using a template, a jig that ensures that all the stubs are set at the correct spacing and angle. The tower must be able to be bolted to the stubs without any steelwork needing to be bent. Once the foundations are poured, time must be allowed for the concrete to set before the template is removed and the remainder of the tower is bolted to the stubs. The CEB L132 specification from 1940 stipulates that the stub-setting template must not be removed until at least 24 hours after the foundations are completed; typically another two weeks must then pass before the tower is erected onto the stubs.
Tower components
Foundations and stubs
Larger pylons (132 kV and above) are typically secured in place using concrete foundations. Each of the four legs has its own independent foundation. Into each foundation is placed a stub: this is a length of steel angle set into the concrete. The remainder of the tower is then bolted onto the four stubs. In practice the foot of each corner can often be seen encased within concrete; for some towers with earth grillage foundations (rather than concrete frustum) these concrete muffs were added to protect the steelwork against ground-level moisture. The stub length below the ground varies according to the foundation depth: deeper foundations hold longer stubs. The concrete foundations initially protrude above the ground and may remain this way, but over time they can become covered over by the ground. Examples can be seen below, all from 132 kV towers:
Tower foundations are heavy despite their seemingly small size: steel lattice towers have a low weight for the height and strength requirements. Some comparisons are given in the table below, where the steelwork figures include the tower and the stubs together:
| Tower type | Steelwork weight | Concrete weight | Factor |
|---|---|---|---|
| L2 D STD | 11.2 tons | 17.9 tons | 1.6 |
| L2 D60 STD | 25.0 tons | 67.2 tons | 2.7 |
| L2 DT STD | 30.4 tons | 70.7 tons | 2.3 |
| L7(c) D STD | 4988 kg | 8636 kg | 1.7 |
| L7(c) D60 STD | 12111 kg | 52680 kg | 4.3 |
| L7(c) DT STD | 14739 kg | 51816 kg | 3.5 |
The foundations are specified according to whether each leg is subject to compression or uplift and by the deviation angle required. A straight line tower has four compression legs, as there is equal weight on all four legs. Angle towers however are subject to an overturning moment: the weight of the conductors is offset to one side and this weight attempts to pull the tower over. Thus, relative to the deviation, the inner angle legs undergo compression (they are being pushed downwards) and the outer angle legs undergo uplift (they are being pulled upwards). This is why the base area of angle towers gets progressively wider by deviation angle. The diagrams below show the difference between D and D90 foundations and stubs on L2 towers; the green line marks ground level. The stubs are shown in blue but the height is only a rough approximation.
Several tower series have D10 and D30 towers that are identical above ground, with symmetrical crossarms. The difference between the D10 (or D30[0–10°]) and D30 (or D30[10–30°]) towers is the size and shape of the concrete foundations on account of the greater imbalance exhibited by a 30° angle over a 10° angle.
Crossarms
Most towers support the conductors (overhead wires) on the ends of arms. Single circuit towers have three such arms, and double circuit towers have six. The terminology for these appears to vary. The term “crossarm” can either mean a complete arm assembly (from one side to the other) or an individual arm. Thus, a double circuit tower can be described as having three crossarms (three double-arm assemblies) or six crossarms.
The conductors themselves attach to insulators (explained below) that in turn connect to the crossarms.
Outriggers
The term “outrigger” evades precise definition. There seem to be at least two uses for the term. Various tower suites use small diagonal arms on the single-circuit terminal (ST) towers to route the wires around the tower and down to the ground-level equipment. These special arms are the main type of outrigger. Suspension insulators known as pilot insulator sets are attached to the outriggers to carry the conductors.
The L4(m) specification suggests that the term “outrigger” also indicates the short steel angles (bars) affixed to the crossarms to lengthen them, especially the top crossarms, increasing the crossarm length. The L4(m) DJT between Luton and Toddington has outward bars on the top crossarms to widen them, and longitudinal bars on the middle crossarms to deepen them.
Insulators
The phase conductors—the live wires that carry the high voltage—are connected to the tower by way of insulators: strings of non-conducting elements. The length of the insulator string depends on the voltage: the higher the voltage, the further the wires must be distanced from any part of the tower. If the insulator strings look unusually short then the line is configured for a lower voltage than the tower was designed for, e.g. a 132 kV line on L2 towers designed for 275/380/400 kV, or 33 kV insulators on 132 kV towers, a practice that is not uncommon.
Straight line towers carry the conductors on insulator strings suspended from the crossarms, which is why such towers are also called suspension towers. Angle towers, section towers and terminal towers have the insulator strings pulled almost horizontal under tension, hence the alternative name of tension towers. The two arrangements are illustrated below; note that all the dimensions are only approximate and are for illustrative purposes only.
Pilot insulator sets
A pilot insulator set (pilot set or unofficially a dolly) is a suspension insulator set used for routing conductors around the tower. They are used under several circumstances: on single-circuit terminal (ST) towers, on some junction towers and on the outer crossarms of an angle tower to maintain clearances.
Semi-tension sets
Certain situations call for an insulator set that is a cross between a tension set and a suspension set. Such an arrangement is called a semi-tension set or suspended (twin) tension set. A back-to-back pair of tension insulators are suspended a short distance below the crossarm:
More than one reason has been given for their existence. One of their functions is to increase clearance during line upgrades. Older ACSR conductors may be replaced with newer AAAC conductors with a new sag profile; modern computer modelling along with minor tower modifications allows existing towers to take heavier conductors than they were designed for. A line can also be uprated to carry more current, meaning that the conductors will run hotter. [Brechfa Forest Connection] describes uprating Upas (300 mm² AAAC conductor) lines from 50° to 75° operating temperature, which will increase the conductor sag. Greater sag will affect some of the clearances to ground, and one of the recommended options presented in the document (the implementation of which can be seen on Google Street View) was to replace existing suspension insulators with semi-tension sets, offering an increase in clearance of 1.5 to 2 metres depending on the tower. An alternative approach would be to raise the height of the tower.
This raises the question of why the tower cannot simply have regular tension insulators fitted. It has been claimed that it is not feasible to simply put tension insulators on a suspension tower, as suspension towers are not built to handle tension loads. Semi-tension sets relocate the new tension forces to a separate component below the crossarm, and then suspend the whole tension assembly from the tower, which still sees conventional suspension forces acting on it just as before.
Semi-tension sets seem to be fitted just to the bottom phases when increased clearance is required. A disadvantage of this approach is that the clearance (separation) between the bottom and middle phases is reduced, which could increase the risk of conductor clashing in high winds.
Semi-tension sets have also been said to be used when connecting solar farms to the power grid; presently no examples of such an arrangement are known.
Jumpers
Jumpers are the cables that span the tension insulators on angle towers. The incoming and outgoing conductors are at different angles and terminate on the tension insulators, with jumper cables connecting both conductors together. Where the cables go from the tower to substation or sealing end equipment instead (i.e. where the overhead line comes to end at a terminal tower), they are referred to as “downleads”.
Weights
Weights are used to maintain clearances between live components and steelwork. They may be suspended from insulators or added to jumpers. These weights stop the wind from blowing the wires far enough that they flash over onto the tower. With pilot insulators sets they can also be used to counteract the uplift forces of the jumpers.
The most curious example of jumper weights is the top tier of a modified L2 tower where the top crossarms are lowered and installed upside down; the jumper weights may be required because the crossarms intrude into the clearance space allocated to the jumpers.
Because L2 towers are operated at very tight tolerances (not least, running at 400 kV on towers originally intended for 380 kV), it’s fairly common to find jumper weights on the outer angles of D30 and D60 towers; the previous solution was solid bar jumpers but weighted jumpers are now used instead.
Jumper weights can also be used on 132 kV towers restrung with 200 mm² AAAC conductors: the new conductors are so lightweight that the wind can easily blow the jumpers too far.
Step bolts
Pylons are typically fitted with equally-spaced metal posts all the way up two opposing edges; these can give them a “prickly” appearance or a passing resemblance to Pinhead from Hellraiser. These step bolts are provided to allow workers to climb the tower. British Pathé short The Pylon Men from 1966, which depicts the construction of a BICC 400 kV L6 line, shows a worker scaling a tower at the start using these bolts as part of the tower’s assembly process; unsurprisingly this is performed without the assistance of safety gear of any kind!
Flickr photograph “Stromzilla” (“Strom” denoting “current”) depicts a German pylon with an almost identical step bolt arrangement (same design but opposite corners).
Anti-climbing devices
Anti-climbing devices are the barbed-wire assemblies placed around the tower to prevent people from climbing up it. Smaller towers have a single anti-climbing device that runs all the way around. Larger towers (including L2 and L6) more typically have a separate anti-climbing device on each corner for a total of four per tower.
Cattle guards
Cattle guards are flat metal strips placed horizontally across the bottom of the V formed between the corner leg and the first bracing bar on each side. These bars prevent livestock from getting caught in between the steel bars. They are briefly described in the L12 specification (ENA TS 43-11) which notes that they “avoid risks of livestock being caught between the tower members and injured”. It would appear that they stop animals from becoming wedged in the V.
Circuit colour plates
Each tower is fitted with circuit identification plates to show which circuit each conductor belongs to; the L2 and L3 drawings refer to them as circuit colour plates. Double circuit towers carry two three-phase circuits, one per side, and this allows line workers to identify which circuit is which, but even single-circuit towers are fitted with these identification plates. One such plate is fitted just below each crossarm; another may be fitted near the base of the tower for ease of access. National Grid Company Directive, Standard Technique: OH4N/4 (April 2015) shows the colour plates to be 100 × 300 mm in size with a 20.5 mm hole in the centre to fit onto a step bolt (page 17) and gives the intended positions on page 9.
Each plate is divided into multiple stripes, often three but also one, two and five. Commonly, the colour arrangements are symmetrical, but this is not always the case. For example, in Bushey the 400 kV circuit plates are black/yellow/green/yellow/black and green/white/blue/white/green (both symmetrical), while asymmetrical assignment of red/white/blue and yellow/red/green can be found on the PL7 132 kV towers in Bedford. A L7 D photo on Flickr shows two-colour plates (blue/pink or blue/red, and green/white).
The plates are always affixed by way of a central hole. The plates on the right side of either face are fitted onto a step bolt, and on the left side they have a dedicated bolt. This means that the plates are able to rotate and are sometimes seen at an angle. Consequently, a pair of circuits with exactly opposite colour assignments—such as red/green and green/red—cannot be used as any of the colour plates could be upside down!
Care must be taken to avoid confusion due to pigmentation fade; for example the Eve D2 in Highfield Lane in St Albans is tagged white/blue/white and yellow/green/yellow, but the latter is red/green/red at the DT, indicating that the red pigment on that D2 tower’s plates has faded to yellow.
The colour assignments may not be unique. For example, the line between Sundon Substation and Westoning Substation has circuit colours red/white/red and green/red/green. An SEE PL1a tower on the Elstree–Sundon line (as shown above) uses green/black/green and green/red/green, with the latter arrangement seemingly a duplicate of that used on a fairly nearby line. It’s possible of course that one of the colours is pink and the other is faded red!
Some examples are given below, all taken from photographs:
The article When is a pylon not a pylon? on the Science and Industry Museum blog depicts how the colours are shown on an Electricity North West circuit diagram. The colours indicated in their sample diagram are “O” (orange), “Bk” (black) and “W” (white).
National Grid document Guidance for UK Fire and Rescue Services also depicts a single example of a symbolic plate:
No further details or examples are given.
Phase plates
Phase plates are sets of red, yellow and blue markers that indicate the order of the phases on the tower. Towers were designed to carry the phase plates either on a corner leg or on horizontal bars. Phase plates are rare now.
Maintenance brackets
Details on maintenance brackets are scarce. Some tower types are fitted with transverse bars at the end of each of each crossarm; these are most commonly found on L6 line towers. The purpose seems to be to provide attachment points for stringing conductors and other maintenance.
Earthing arms
Earthing arms are wide arms placed below the bottom crossarm; their purpose seems to be to catch fallen conductors and earth them to the tower or pole. Earthing arms are typically seen at road crossings, and are generally found only on secondary line (33 and 66 kV) towers. The folded plate pole section of the 132 kV line in Clapham (just outside Bedford) uses them for one of the road crossings but not both. This practice seems to be largely abolished.
An example tower suite featuring earthing arms is Milliken E177.
Fall arrest systems
A fall arrest system (also known as a vertical lifeline system) takes the form of a cable that runs down the tower corner a short distance away from the step bolts. Line workers anchor themselves to this cable as they ascend and descend the tower. Should a line worker slip and begin falling from the tower, the fall arrest system will grip the cable and prevent the fall. Fall arrest systems are retrofitted to what appears to be a small percentage of existing towers in the UK. They may be found fitted to both corners with step bolts or only (for example as in Lydney) just one corner. Rapid Rail International’s Vertical Cable System lists a maximum fall distance of two feet (around 60 cm).
Line components
The following items are features of the power line rather than the towers but are worth mentioning here also.
Stockbridge dampers
Stockbridge dampers (also known as vibration dampers) are small devices attached to the conductors, typically adjacent to the insulators, to reduce damaging cable flutter caused by the wind. They are not part of the tower, although they are fitted to the conductors and earthwire at either side of the point of attachment.
Two Stockbridge dampers in a row are used at each end of a long span, i.e. where two adjacent towers are spaced further apart than the normal span length for that tower type. This is noted in the CEB L132 specification but can also be seen on a 33 kV Blaw Knox K9906 line in Lydney (on de-rated 66 kV towers) and on the 400 kV ZA route on L2 towers.
Spacers
Spacers keep the conductors in bundles from touching.
Tower adaptation
Mobile phone masts
Thanks to their height, pylons are frequently used as mobile phone masts. For example, this L3 angle tower in Elstree:
Optical fibres
Overhead power lines can be used to carry overhead fibre obtic cables. Optical fibres can be used to carry data for the distribution network operator (DNO), as spare fibres for lease, or for telecommunications. In UK practice, both Electricity Northwest and Northern Powergrid appear to use runs of 48 fibres as standard.
Optical conductors
Different methods exist for placing optical fibre cabling on power lines. One method uses special conductors that contain one or more embedded tubes carrying the fibres; these tubes can replace individual cable strands or form the core of the conductor. Depending on where the conductor is installed, this arrangement is known as Optical Ground Wire (OPGW) and Optical Phase Conductor (OPPC). The North American term “ground wire” substitutes for “earthwire” in order to use the standard initialism “OPGW” [Overhead Line Conductors Carrying Optical Fibres]. OPGW and OPPC runs can be observed where conductors terminate or splice, where the fibre cables pass down the tower body. Optical ground wire is said to have been patented by BICC in 1977.
Although the earthwire is not normally designated a conductor (on the basis that it normally carries no load), the term “conductor” can be applied to both OPGW and OPPC.
Optical wrap splices (see below) between cable runs take the form of “donut” splices suspended from conductors. Both optical conductor and optical wrap also use structure-mounted splice boxes. One such type is the “canister” assembly, such as the AFL (now part of Fujikura) type depicted below. These are fitted with a Tykoflex joint closure, the cylinder at the top. The fibres are fusion spliced at ground level within a suitable environment (splicing vehicle or “suitably covered work area”) rather than on the pole or the tower. Once splicing is performed, the canister can be mounted to the pole or tower.
Optical wrap
An alternative arrangement, suitable for adapting existing lines, is to wrap the optic fibre cable around the earthwire or one or more phase conductors. This arrangement is called Optical Attached Cable (OPAC), also known by the trademark SkyWrap®. OPAC is said to have been independently developed in the UK and Japan in the 1980s. The upper voltage limit when used with phase conductors is suggested to be 150 kV although in the UK the upper limit is 132 kV, as the next voltage up from that is 275 kV. OPAC is used on both steel tower and wood pole lines.
The same splice canisters can be used with wrapped cables as is used for optical conductors.
Tower design
The CEB 1928 annual report noted the following with regard the original design of the UK’s 132 kV transmission towers:
There is considerable scope for the structural designer in connection with the difficult question of determining the most suitable form of line supports, both with regard to the material to be used and the outline.
Ample proof of this is to be found by an inspection of the existing E.H.T. Transmission Lines throughout the world. Over-elaboration, for whatever reason, is to be deprecated, and true adherence to the strict engineering requirements will seldom result in a structure which will offend the eye. The economic factor has perforce to be fully weighed, and after exhaustive investigations of the claims of steel, reinforced concrete, narrow base, wide base and Eiffel towers, the Board, in consultation with Sir Reginald Blomfield, R.A., settled on a wide base straight line tower of the form depicted in Figs. 1 and 1A opposite.
Figures 1 and 1A depict CS PL1. The quotation above is reproduced from a scan depicted within the Science and Industry Museum article Pylons: controversial giants in the landscape; although it’s not known for certain, it’s likely that the various descriptions were not elaborated. “Narrow base” describes the German preference although such towers were also used in the UK. The meaning of “Eiffel towers” is uncertain, but it may refer simply to the tower outline rather than any involvement with Gustave Eiffel (who was already dead by this point). Transmission towers do exist in Ottawa that follow the same general outline as the Eiffel tower:
The section of line above runs along McRae Avenue, Laurentian View, Ottawa, Canada. This Canadian design simulates a continuous curve from the narrow peak to the wide ground-level outline, while 1930s and 1940s UK towers favoured a “bend line” or “kink line”, a sharp change in outline angle at a specific height, often at the level of the middle crossarm. More subtle outline shapes appeared in the 1950s in a manner more like the Eiffel tower, sometimes with a reverse curve where the tower tapers towards the top.
In any case, the Millken design used an excessively large base, a characteristic seldom repeated (and even then chiefly in Scotland where it may have been chosen for robustness against the elements). The South East England region used different towers (GEC and Callender’s) with considerably narrower profiles, and the various tower suites that followed continued with the narrower base dimensions.
Height requirement
The height of each tower is determined by the sag of the conductor spans: how close the cables come to the ground at their lowest point in between the towers. INMR article Implementing a Compact 400 kV Line demonstrates how the total height of a German 400 kV tower can be reduced from 58 m to 30 m by reducing the sag and switching to a single tier; the following diagram shows the degree to which the towers can be shortened as a result of the described invention:
Compare how the green cables sag less: this means that they can be suspended from a shorter tower. The CompactLine design also places all the phases side by side instead of the two tiers of the “Danube” lattice towers. The “Danube” towers also have a much taller earthwire peak than British towers: at only two tiers, it is not only taller than British two-tier designs (L9 and L12) but also taller than our three-tier L6 towers which are around 50 m tall.
Aesthetics
In the Scotsman article “Kinetic sculpture on the grid” (6th October 1994), Bruce Howie, chief engineer with ScottishPower’s Transmissions Division is quoted as saying:
The recent trend has been towards “swept up cross arms”, which are regarded as less obtrusive than the confident protagonists of Buchanan’s water colours.
Here he seems to be referring to the design of L4(m) and L12. Quite how recent “recent” means is unclear, as L4(m) dates back to the 1970s.