Technical information on British pylons
- Tower types
- Tower characteristics
- Tower structure
- Tower construction
- Tower components
- Tower adaptation
- Tower design
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.
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.
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 all provided with several extended height variations, and some also provide height reductions. Generally, height increases are achieved by modifying the lower portion of the lattice structure, 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. Height extensions were originally in feet, which have now been replaced by metres. Metric extensions and reductions are simply numeric, e.g. E3 indicates a 3 metre extension. Older imperial extensions and reductions may also be found with the unit, e.g. “ E.10′ ” for a ten foot extension. Height extensions and reductions come in set increments, such as four feet, ten feet or three metres depending on the tower series. 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. The standard height tower is typically denoted by “STD” or “SH”.
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.
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:
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 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.)
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, 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.
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.
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.
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 are bolted. During assembly, portions of the tower are typically preassembled and then lifted into place with a crane. 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. 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:
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.
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.
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.
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.
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 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 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.
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 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 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 are sets of red, yellow and blue markers that indicate the order of the phases on the tower. These are rarely observed now.
The following items are features of the power line rather than the towers but are worth mentioning here also.
Stockbridge 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.
Spacers keep the conductors in bundles from touching.
Mobile phone masts
Thanks to their height, pylons are frequently used as mobile phone masts. For example, this L3 angle tower in Elstree:
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.
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 and L12. Quite how recent “recent” means is unclear, as L4 dates back to the 1970s.