Tower (pylon) types
- Suspension towers
- Angle towers
- Terminal towers
- Junction towers
- Earthwire changeover towers
- Transposition towers
- River crossings
For the different families/suites/series, see the tower series page.
Straight sections of the power line use suspension towers (also called straight line towers or line towers): the conductors are attached to vertical strings of insulators suspended from the crossarms.
Suspension towers are designated “D” for double circuit and “S” for single circuit. Many older tower series (all the PL types and L66) allow a small change of angle even on suspension towers. For this reason, the suspension towers are designated “D2” and “S2” instead of “D” and “S” to indicate that a change in angle of up to 2° is permitted. From L2 series onwards, this practice was ended and suspension towers are simply “D” (and possibly “S” for single-circuit adaptions to double-circuit-only series). Towers that take advantage of the 2° deviation option have the insulators slanted to one side from the off-centre pull of the conductors, in the same manner as what is shown in the photos below.
Where the line changes direction, stronger towers are required to withstand the horizontal pull of the conductors. Lattice towers do not use guy wires, so the tower and its foundations must support all of the horizontal load directly. Towers that allow the line to change direction are known as angle towers or deviation towers; “deviation” refers to the change of angle itself.
Angle towers have separate insulator strings for the incoming and outgoing conductors. These towers are tension towers: the insulator strings are pulled almost horizontal by the weight of the conductors, instead of hanging vertically. Jumpers span the insulators to connect the incoming conductors to the outgoing conductors. These towers can also be referred to as section towers when they simply connect two sections of line without any change of angle.
Angle towers are indicated with a designation suffix stating the maximum number of degrees of change in angle. Typically these limits are 10°, 30°, 60° and 90° although L12 has 25° and 55°. Earthwire changeover towers are considered separately from angle towers but they too have deviation angles, typically 15°, 20° and 40° depending on the tower series. The angle suffix follows “S” (single circuit) or “D” (double circuit, not “deviation”), giving single circuit towers of S10, S30, S60 and S90 and double circuit towers of D10, D30, D60 and D90. (The degrees symbol was historically included, e.g. “S.10°” or “D30°”. Thus, the L4 D30 tower above has a maximum deviation (change of angle of the power line) of 30°. For a greater change of angle, a D60 tower is needed, then D90. The “D” in “D30” means “double circuit” rather than “deviate up to”; a single-circuit tower that allows deviation to 30° would be S30. As noted earlier, D2 refers to a suspension (straight line) tower that allows 2° deviation.
L4, L6, L7 and L8 don’t have separate 10° tower designs. In each series a single D30 design covers both angle limits, with the D30 type having stronger foundations. The two types are thus D30[0°–10°] and D30[10°–30°].
Double circuit suspension towers are symmetrical: the arms on either side are the same length and use the same bracing pattern. Double circuit angle towers can be symmetrical or asymmetrical depending on the series and angle. The outer angle arms are often extended in length to maintain clearance between the conductors and the tower. The inner angle arms are then shortened accordingly. D60 and D90 towers are always asymmetrical. With D30 and D10 towers this seems to depend more on age: the older PL series tended to have a symmetrical D10 and asymmetrical D30, while the newer L series (e.g. L2, L4, L7, L8 etc) use symmetrical D30 towers. L16 (L55/L132) appears to be a newer-style designation to an older design from the PL era and consequently L16 D30 is asymmetrical. (The same principles apply to single-circuit towers, but those are already asymmetrical on account of using an odd number of arms.)
There are also 10° angle towers with suspension insulators. Thus far, this approach—which is more commonly found abroad—is only known in Britain from PL1.
This technique is more common abroad:
- Example from Auckland, New Zealand (Flickr)
- Example in Kista, Stockholm; note the insulators on the inner angle don’t start at the ends of the crossarms (Flickr)
- Quad circuit tower in a German-speaking country (Flickr)
- Close-up of the insulators from the previous example (Flickr)
Angle towers can also be used to accommodate changes in elevation, to prevent upwards forces acting on suspended insulator strings. It has been reported, albeit not verified, that 10° angle towers were invented to accommodate changes in elevation rather than angle, although they are also used for lesser changes in angle; this seems unlikely considering that 10° angle towers date back to the original Milliken PL1 schemes. With the change in design from D2 to D for suspension towers, L2 D10 will also stand in for what a D2 would have covered in older generations.
Special towers are used at each end of a run of overhead lines. Where the line comes to a complete end, a terminal tower (also termination tower) is used. Terminal towers are designed to withstand having all the cable weight on one face, instead of the equal front and rear pull of an intermediate tower. Terminal towers are designated “ST” (single-circuit terminal) for single circuit and “DT” (double-circuit terminal) for double circuit. Double circuit lines may also split onto two single-circuit terminal towers at a substation. Downleads connect the terminal tower to the ground-level equipment. To prevent the downleads from touching, the top crossarms on a terminal tower are often wider to provide additional clearance. The downleads may also pass through a gantry structure before coming to an end.
Some tower series included a DT90 tower. As seen with the SEE PL1a example at Luton depicted below, DT90 towers allow the downleads to extend entirely to one side of the tower. More generally, the ground equipment faces away from the overhead lines by 90°. Typically the line must approach a terminal tower almost perpendicular, although PL16 and L16 allow the line to approach at an angle up to 45°. L2 has a DT45 tower separate from the DT to allow for this, and more recent tower suites require a DJT to be used for entry angles from 5–45°.
Known terminal tower types include:
- Single circuit terminal tower; for exclusively double-circuit series, this is a special tower that takes one of the two incoming circuits (three of the six main wires)
- Double circuit terminal tower; a terminal tower is where the overhead wires come to an end
- Double circuit terminal tower; allows the incoming line to approach the tower at up to 45°
- Double circuit terminal tower with auxiliary crossarms placed on the rear, to allow the line to terminate to one side of the tower instead of behind the tower. With 275 and 400 kV towers this designation appears to be obsolete.
- The “U” is suggested to mean “underground”, i.e. designed for vertical downleads into a sealing end platform. This type is known only from L2 and has extended top crossarms to allow for vertical downleads.
- DTV 45°
- This is the L3 equivalent to DTU where “V” is suggested to denote “vertical [downleads]”. Again, the top crossarms are of extended length.
- The new designation by National Grid for DVT 45°.
- Unexplained taller variant of ST (known from L2, L6 and L66); for L2 there is an extra crossarm for the earthwire
- Single circuit “flat” terminal gantry (known from L6)
- L2 single circuit terminal tower; appears to denote 380 kV as opposed to the 275 kV ST and STX towers
Junction towers allow a line to be teed off. These towers have cables on three sides. Supposedly, a DJ tower is a tee-off only while a DJT also has downleads. Actual usage seems to vary and DJT appears to be the standard designation.
The L7(c) DJT at Redbourn is non-standard because the contractors dug the foundations 45° out. The crossarms on a DJT tower are all supposed to be at 45° to the conductors, not perpendicular to them; this is why the auxiliary crossarms all face away from the tee. This mistake consumed a weekend to redesign the tower to accommodate the error.
Known junction tower types include:
- Double circuit junction tower
- Double circuit junction tower
Gantries have all the phases arranged in a horizontal line from left to right. Although [Transmission tower development] depicts a double-circuit gantry possibly related to L12, observed and documented gantries are single-circuit. The standard designation in various series is “SF60”, denoting a deviation limit of 60°. SF60 towers are documented to exist in L4(m), L6, L7(c) and L8(c) series.
Gantries have multiple purposes, including:
- Carrying one line under another at a diamond crossing, with one gantry per phase
- Single circuit termination, equivalent to an SX tower
- Low-height runs, such as near airport runways
At diamond crossings one of the two gantries takes the earth conductor, and this can be carried on a special peak that is only present on one of the two gantries. The gantry below is possibly the PL16 type; it is used in a substation in between the terminal tower and the termination equipment.
Known gantry types include:
- Standard single circuit 60° deviation gantry, used at crossings
- Single circuit gantry, “X” suggested to denote extended width (known from L2 and L3)
- Single circuit terminal gantry (known from L6)
Earthwire changeover towers
Earthwire changeover towers (abbreviated and suffixed as “E/C”, “E.W.C.O.” or “EC”) have an uncertain origin. They may have originated with the Blaw Knox tower type now known as PL16. Per the CEB L132 specification the first mile outwards from each substation has double earthwires for greater lightning protection. Following this, there is a tower with three earthwire peaks: two peaks to terminate the outer double earthwires and a centre peak to begin the single earthwire. This is possibly where the term “earthwire changeover” originated.
In modern usage, earthwire changeover towers provide for splitting a double-circuit line into two single-circuit spans. Each single-circuit span typically runs to a termination point (e.g. an ST single-circuit terminal tower at a substation) or to (or from) a gantry at a crossing. The single earthwire for the incoming double-circuit line splits into two, one for each circuit:
At Chalton there are two PL16 DD10 earthwire changeover towers used for the latter function: they split the double circuit line onto a pair of miniature S30 towers that pass around both an L2 and an L6 tower. The earthwire for the line runs between the centre peaks of the two DD10 towers via one of the S30 towers; a second, short earthwire runs from one DD10 to the other using one of the outer peaks, via the second S30.
Earthwire changeover towers are rated for the maximum angle of deviation, typically 15°, 20° or 40° depending on the type. Since the two circuits—one per site—split out in different directions, the outgoing angle is taken to be the mean (centre) of the two outgoing circuits. There is also a limit placed on the angle of separation between the outgoing lines. For L4(m) and L7(c), according to the respective specifications, “[the] single-circuit lines shall not diverge from each other by more than 20° for mean deviations between 0° to 20°, or 40° for mean deviations between 20° and 40°.” For BICC L6, the maximum angle of separation (divergence) is 40° for both D20EC and D40EC, according to an unspecified L6 towers chart (badly cropped, losing the number and date).
Most earthwire changeover towers are rated as deviation towers and are derived from the next angle up of deviation tower: D0 EWCO from D10, D15 and D20 EWCO/D20EC from D30, and D40 EWCO/D40EC from D60. D0 EWCO is an oddity from L3 not found in other series and seemingly omitted from L3(c).
The full designation of a tower includes the relevant suffix, e.g. D15 E.W.C.O. or D40EC. National Grid’s GIS data omits the suffix, referring to such towers simply as “D15”, “D20” or “D40”.
Transposition towers exchange the vertical positions of the phases on the line. For example, moving the top phase to the middle crossarms, middle phase to the bottom crossarms and bottom phase to the top crossarms. Single-circuit transposition towers are designated SX and double-circuit transposition towers are designated DX.
The first photo below depicts an unidentified transposition tower thought to have been on what remained of the Rannoch-Tummel-Abernethy line in Scotland. This tower is said to date back to the 1920s. The second photo is of a type previously identified as PL1b, although in this instance it’s part of a SEE PL1a line from Piccotts End in Hemel Hempstead to Elstree Substation, a line which also features PL16, PL4, L4(m) and L7(c) towers! This tower is found on a SEE PL1a section and is therefore most likely SEE PL1a. In both cases, you can see the extended top and bottom crossarms necessary for the transposition wiring that has since been removed. Another (and slightly different) SEE PL1a DX tower used to exist in Crofton Avenue in Bexley, London; that line is currently a mixture of L4(m) and PL1a in that area and the SEE PL1a DX tower seems to have been replaced with an L4(m) D.
Special towers are not required for single-circuit transposition, as simply installing one tower facing the opposite way is enough to achieve transposition across two successive spans.
River crossings frequently use extremely tall towers. Some can be based on regular towers but typically they have a special design. The steep angle from the ground up to the tower requires heavy duty anchoring using special gantries.