UK electricity pylons guide

Contents

• Overview
• Designations
  • Series
  • Type
  • Angle
  • Suffixes
  • Height
  • Non-standard nomenclature
• Recognition
  • Tower bracing
  • Crossarm style
• Designations: Part II

Overview

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.

For additional terms, see the glossary page.

Designations

Each tower type has a specific designation that indicates the series, design, height etc. In most cases 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).

See the Designations: Part II section below for a more detailed overview of tower series designations.

Series

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.

Type

This is the tower type; examples are listed below (not all are officially confirmed):

D

Double circuit tower; typically six crossarms (three pairs) carrying two three-phase circuits

DD

Double circuit tower with double earthwires

S

Single circuit tower, either three crossarms or flat formation

SS

Single circuit tower with double earthwires

LD

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).

DT

Double circuit terminal tower; a terminal tower is where the overhead line comes to an end

DTU

Unexplained type, known only from L2; seems to be DT with staggered crossarm lengths (DTU shares the same tower body as DT)

DTV

As DTU but for Blaw Knox K9906 (terminal tower supporting external sealing ends or sealing end platform)

DTV 45°

Unexplained type, known only from L3 (L3 and L3(c) DTV 45° shares the same tower body as DJ and DT)

DX

Double circuit transposition tower (obsolete concept in the UK but some such towers remain, now wired straight through)

ST

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)

STX

Known from L2 and L6 and unexplained; L2 STX is the same as ST but with a fourth crossarm

STJ

Unexplained (mentioned in manual T14)

STL

Unexplained (mentioned in manual T14)

380ST

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

DX

Double circuit transposition tower

DJ

Double circuit junction tower

DJT

Double circuit junction/terminal tower (used either as a junction tower or for termination at up to 45° incoming line angle)

DJX

Double circuit junction tower with auxiliary crossarms (L2 and L3)

SX

Single circuit transposition tower

SFX

Single circuit gantry, “X” suggested to denote extended width (known from L2)

SF60

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)

SFT

Single circuit terminal gantry (known from L6)

SCT

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.

Angle

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 SEE PL1a tower (satellite imagery helps illustrate this). DT45 remains to be explained.

L4(m), 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.

Earthwire changeover towers typically have their own angles such as D15 EWCO/D15EC and D40 EWCO/D40EC.

Suffixes

Some tower types use a suffix to indicate some kind of adaptation or characterstic. These include:

S

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.

EC

This denotes “Earthwire Changeover”.

EWCO

“Earthwire Changeover”; less common variation of EC.

Height

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 (denoting “minus”, or “minor” in National Grid document T238) 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).

Non-standard nomenclature

Some tower suites use non-standard nomenclature. Milliken Brothers and Blaw Knox standard practice was to designate only the tower body (or lower portion of the tower body in the case of K1373/K4611) rather than the crossarm arrangement or upper portion of the tower. For example, tower type D1 could be either 60° angle or terminal depending on which crossarms were fitted (and presumably which foundations were dug and what stubs were installed).

This has led to at least one conflict: “D2” can mean a double circuit suspension tower with 2° deviation (historically written “D.2°” but commonly reduced to simply “D2”) or a tower body suitable for 60° deviation or full line termination.

† unconfirmed at present

Recognition

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.

Tower bracing

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 (Milliken 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 (L6(m)).

Crossarm style

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(m) 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(m), 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.

Designations: Part II

In order to give each tower series a page, each series needs to be given a name of some form. Unfortunately, this is not very straightforward.

Starting with L2 in the 1950s, tower suites have been assigned official designations that are typically taken from their respective specifications (in this case BES L2). Prior to L2 it appears that tower suites never received official designations. The drawings were labelled either with the identity of the power line construction scheme (e.g. SEE PL1(b) or SWE PL16) or with the designer’s contract number (e.g. T2175, C534). The same general design could have multiple designer contract numbers, for example “revised Milliken” (SS-PL1) contract numbers included E112 (NWE PL1), E141 (SWE PL1), E142 (EE PL1) and T161 (CE PL1).

Sometimes a drawing can be in two or more contracts at once. J L Eve L3 Stourport–Ludlow was contract C673, while Beauly–Blackhillock–Kintore was contract C864. The latter contract involved a brand new design of line tower but the angle towers remained unchanged; thus, the C673 drawings were re-used and bear both contract numbers (C673 and C864). Where the same set of tower designs exists in more than one drawing range, they are treated here as a single type unless there are discernible above-ground differences. Note that it’s difficult if not impossible to know whether a particular drawing set is ever the first set to depict the tower type in question.

The UK power industry was thus left without any way to refer to tower suites. Repairs and alterations to a line over time mean that individual towers or a portion of a line may be replaced with whatever tower type is current at the time that new towers are constructed. Since these new towers don’t match those used for the rest of the line, some means is needed to refer to their type.

Broadly the industry approach is to refer to a tower suite by the name of its original project where one is known. The CEB/CEGB prefix should be included in this designation but is often omitted. For example, the towers used for the SWE PL16 Andover–Bournemouth–Salisbury line are referred to as “PL16” (formerly “SWE PL16”). Sometimes the designations are ambiguous; the G route tower schedule (Braehead–Greenock) refers to certain S2 towers as type “S.W.E.” referring here not to SWE PL16 but to the SWE PL1(a)&(b) Callender’s type. Likewise, a CE PL3 single circuit tower added to a line in Scotland was recorded simply as “NWE” (even though this type is not known from any NWE schemes).

There are however a variety of types with no specific name. The 132 kV Eve types in particular resist naming. A batch of UKPN/SPN tower data uses “C534” for Eve’s Zebra conductor type, a suite that is also described as “L16” and in some cases incorrectly as “L55”. “L132” (see the CEB L132 page) gets used in an confusing manner for types based on that specification and may be used chiefly for Eve towers. Sometimes Eve towers are just described as “J.L.EVE” for want of a proper designation.

Even with L2 onwards there can be confusion. While it was widely understood that L6 existed in multiple versions, the actual set of versions was not correctly known (there are six basic types: the four imperial originals and the two metric types). L3 was more troublesome as it was taken to be Blaw Knox only, with the Eve version largely unknown. There is also the Blaw Knox T2175 derivative in Scotland that originated the L3 DS tower.

In essence, one must be very careful about how one refers to a tower suite. Where possible, include the scheme name in full, e.g. SEE PL7 rather than PL7, to avoid confusion. PL16 is the major exception as it’s a ubiquitous type universally referred to by that name … except that “PL16” is used by some organisations to refer to multiple incompatible types, for reasons unknown.

The tower nomenclature used on this site is consequently ad hoc in nature.

A few notes:

• There is no such tower type as “PL1”. Tower suites used on PL1 schemes include CS-PL1 (CS PL1, EE PL2), SS-PL1 (SS PL1, SWE PL1, NWE PL1 etc), GEC (SEE PL1(a) and the unconfirmed MEE PL1) and the full Callender’s suite used with SEE PL1(b). PL1b is not synonymous with or the same tower type as PL1.
• Although “PL7” conventionally refers to the Watshams tower suite used for SEE PL7, EE PL3, NWE PL6, NWE PL12, MEE PL9 and so forth, at least one industry employee understood the term “PL7” to refer to a completely different suite, a precursor to PL16 (designed by Blaw Knox), covered here as Blaw Knox K5735. The CEB/CEGB region containing this PL7 scheme is not known nor is the location of any such towers.

Various details below are on the individual series pages remain incorrect and out of date. Corrections will be made over time.