UK electricity pylons guide
This page provides some guidance for identifying and naming tower types. See the series page for an identification guide, and the pylon comparison page for additional assistance.
Each tower type has a specific designation that indicates the series, design, height etc. These designations follow a specific pattern, as follows:
series type(angle)(S|EC) height)
For example, “PL16 D2 STD” is a PL16 series double-circuit tower (the “D” in “D2”), allowing up to 2° of deviation (a change in the direction of the route) and of the standard height for a PL16 D2. “L12 DT E3” indicates an L12 double-circuit terminal tower (DT) with a three-metre height extension (E3).
The various aspects of the designation are explained in the sections below.
“PL” itself is now understood to denote “Primary Line” (132 kV), with “SL” denoting “Secondary Line” (33 and 66 kV).
The series name indicates the scheme or specification. For several decades, the individual tower suites had no formal identity. Tower suites ended up being named after the first scheme to use them (as with SWE PL16) or the designer’s contract number (e.g. Blaw Knox K9906). Starting with L2, each tower suite is named after its specification; this was to some extent also true with L132 and STL1 but not consistently. The same tower suite can be used by more than one scheme, thus acquiring multiple designations.
A few specifications involved separate designs from multiple contractors. These include STL1, L132 and L3 (with J L Eve and Blaw Knox implementations of each) and the four original implementations of L6 (Balfour Beatty, BICC, Blaw Knox and J L Eve).
Some series created prior to the adoption of metric measurements have been adapted for metric, resulting in metric sub-series. Series specifically designated as metric include:
- L2(c), from L2
- L3(c), from Blaw Knox L3
- L4(m) (unlikely that imperial L4 was ever constructed but the design work may have started as imperial)
- L6(c), from L6 (a combination of BICC and Eve imperial L6 towers converted to metric)
- L6(m) (metric redesign of L6)
- L7(c), from L7
- L8(c), from L8
Later series did not need a suffix as they were created as metric from the outset.
The metrication suffix of “m” or “c” is conventionally enclosed in brackets (parentheses) but the brackets are often omitted.
The specific meanings of “m” and “c” are disputed. One method of metrication is to simply replace all imperial components with the next size up in metric, in particular the angle (steel bar) thickness but likely also the bolts. Another approach is to apply modern calculations to the design and determine the optimum angle thickness regardless of the measurements specified originally. L6(m) is neither: it is a completely new design that does not match any of the four imperial L6 types.
L6(m) aside, neither the “m” process nor the “c” process inherently results in a change in the design of the tower.
This is the tower type; examples are listed below (not all are officially confirmed):
- Double circuit tower; typically six crossarms (three pairs) carrying two three-phase circuits
- Double circuit tower with double earthwires
- Single circuit tower, either three crossarms or flat formation
- Single circuit tower with double earthwires
- Double-circuit low-height tower. L12 LD is confirmed for the low-height double-circuit straight line tower; the L prefix apparently also applied to L9 towers, even though this series is exclusively low-height (as the low-height companion to L6).
- Double circuit terminal tower; a terminal tower is where the overhead line comes to an end
- Unexplained type, known only from L2; seems to be DT with staggered crossarm lengths (DTU shares the same tower body as DT)
- DTV 45°
- Unexplained type, known only from L3 (L3 and L3(c) DTV 45° shares the same tower body as DJ and DT)
- Double circuit transposition tower (obsolete concept in the UK but some such towers remain, now wired straight through)
- Single circuit terminal tower; for exclusively double-circuit series such as L4 and L6, this is a special tower that takes one of the two incoming circuits (three of the six phase conductors/bundles)
- Unexplained taller variant of ST (known from L2, L6 and L66)
- Single circuit terminal tower known from L2; appears to denote 380 kV (in practice, 400 kV) with L2 ST and STX being smaller 275 kV towers
- Double circuit transposition tower
- Double circuit junction tower
- Double circuit junction/terminal tower
- Double circuit junction tower with auxiliary crossarms (L2 and L3)
- Single circuit transposition tower
- Single circuit gantry, “X” suggested to denote extended width (known from L2)
- Single circuit 60° deviation gantry, used at crossings; the tower may have multiple configurations for different maximum angles up to 60° (there is no SF30 for example)
- Single circuit terminal gantry (known from L6)
- Unexplained gantry type
The “F” in the SF tower types supposedly stands for “flat” and indicates that the conductors are arranged side-by-side across the width of the structure. No double-circuit flat arrangement (DF) towers are known, although a drawing exists in [Transmission tower development] for such a tower in L12 series.
The angle indicates the maximum deviation permitted in the design. For example, an S30 (single circuit) or D30 (double circuit) tower allows the wires to change direction by up to 30 degrees. The 1956 plans in the Tower Bible include the degrees symbol in the designations, such as “D.2.°” or “D.D.T.90.°”, with an extraneous dot after the digits.
There are some special cases for terminal towers, in particular DT45 and DT90. DT90 has extra arms at the rear, seemingly to allow all of the downleads to exit to the same side of the tower instead of half on each side; this idea can be seen at Luton South Substation with a PL1a tower (satellite imagery helps illustrate this). DT45 remains to be explained.
L4, L6, L7 and L8 do not have separate D10 tower designs; a single D10–D30 design differs only in the design of the foundations.
The standard angle tower designations are given in the table below.
|D30 [0–10°] (L4, L6, L7, L8)|
|D30 [10–30°] (L4, L6, L7, L8)|
Earthwire changeover towers typically have their own angles such as D15 EWCO/D15EC and D40 EWCO/D40EC.
Some tower types use a suffix to indicate some kind of adaptation or characterstic. These include:
- With PL16 D2S and L3 DS it appears to denote “Scottish” but this is not yet confirmed. PL16 D2S is STL1 D2; the “S” suffix differentiates it from the standard PL16 D2 tower. L3 DS is the enhanced clearance line tower from T2175 (itself an L3 derivative) adopted into L3.
- This denotes “Earthwire Changeover”.
- “Earthwire Changeover”; less common variation of EC.
Each tower design has a nominal height; the standard height type is denoted with the suffix STD or SH. An E suffix followed by a height indicates a height extension. These extensions are pre-designed and come in increments of either feet or metres depending on the series. “E10” by itself could technically indicate either a 10 metre or a 10 foot height extension, according to the units in use, although the multiples tend to differ. In the Tower Bible, there are examples with the unit present, e.g. “D2° E20′” but the unit of height was often omitted as irrelevant. In modern practice you are more likely to see it on charts to differentiate feet from metres.
There are also height extensions with a “+” prefix instead of “E”. This indicates that the extension is added to the standard tower, where normal height extensions alter the lower portion of the tower. PL4 D10 has both E30 and E20+10′; the latter indicates a 10′ extension added to an E20 tower, instead of a dedicated 30′ extension.
An M suffix (seemingly denoting “minus”) followed by a height indicates a height reduction. A single reduction (e.g. “M.3”) can also indicate more than one latticework form, for reasons unknown (to be illustrated later based on incomplete plans on the RMweb forum).
Within the UK there are many tower series (families or suites) and at first it can be quite difficult to tell the various types apart from each other. There are a number of clues to watch out for, and the diagrams below help to illustrate some of them.
There are numerous bracing styles used for the body of the tower. These are illustrated in the following diagram, adapted from [Transmission tower development]:
It is not likely that a single tower will contain all of these bracing styles. Generally, the upper portion of the tower will be X-braced or zig-zag braced. L12 D and LD also make prominent use of diamond bracing lower down above the usual K bracing. The lowest bracing in many of the older types (PL1, PL4, PL7, PL16 etc) tends to take the form of reinforced X bracing, although K bracing can be found in some deviation angles. More recent designs (from L2 onwards, and presumably going back to the earlier L66), the lowest bracing of almost every tower model is K arrangement. Blaw Knox’s take on L6 is a known exception, where a more arch-like form was chosen.
Bracing style is one clue for differentiating L3c from L7, especially in photographs where it’s difficult to assess the tower height. Bracing style is also a clear difference between the numerous varieties of L6 such as Blaw Knox, Balfour Beatty and metric (L6m).
Crossarms can be “open” or braced in the vertical plane, i.e. the front and back sides of each crossarm (which tend to be angled inwards, especially crossarms with a triangular cross-section). This is one way to help differentiate PL7 from PL16 (especially the DD2 towers), as well as L4 from L12 when the huge height difference is not a clue. Crossarms with no vertical bracing still have lateral bracing along the bottom of each crossarm, visible when looking up at a tower from below.
Crossarm bracing can be thought of as “upwards” vs “downwards”, based on the direction it takes from the tower body. This is one way to distinguish L8 from L6, as well as PL7 DD2 from PL16 DD2. This trait is depicted in the following illustrations:
Most crossarms form an approximate right triangle on either side of the tower: horizontal below, and sloped above. Several tower types however have a nearly isosceles arm shape: L4, reduced-size L8, L9, L12 and SSE400. The two forms are illustrated in the diagrams below:
The comparisons page has a few more specific comparisons between easily-mistaken types.