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UK electricity pylons

Contents

Overview

Although there has been considerable interest over the years in high voltage electricity infrastructure, there is little in the way of publicly-available collated data on the subject. The details on the various pylon types is scattered about and difficult to find. These pages are an attempt to address this deficiency by bringing information about UK pylon types into a single location.

The term “pylon” is found in everyday usage, although the industry term is “tower” (transmission tower, lattice tower, steel lattice tower). Lattice refers to the open metal framework, compared to the poles more commonly used with the lowest voltages (especially 11 kV, 33 kV and the “trident” 132 kV poles).

Note that there are significant gaps in the coverage of these pages while they are under construction. More detail will then be added in future as it is discovered.

Series

See the Series page for an overview to UK pylon series, intended to aid in recognition.

Photographs of each series are provided on the individual series pages where such photographs can be taken (chiefly in Hertfordshire) or have been provided by others. All photographs are original material and are released to the public domain, unless otherwise stated.

Designations

Tower designations follow a specific pattern, as follows:

<series> (D)(L)<type>(<angle>)(S) ([E,M,+]<height>)

The series name indicates the family, series or specification. Within a series, the same tower type (e.g. D30, DD2) may take on different forms according to the contractor responsible. There may also be a sub-series suffix, such as with L4m or L7c.

The remaining details are summarised below, then explained in more detail.

The tower type indicates the basic form; examples are listed below:

S
Single circuit tower
D
Double circuit tower
ST
Single circuit termination tower
DT
Double circuit termination tower
DJT
Double circuit junction tower

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. In the Tower Bible, the degrees symbol is also used, e.g. “D.2.°”, “D.D.T.90.°” (with an extraneous dot after the digits.)

Each tower design has a nominal height. An E suffix followed by a height indicates a height extension. These extensions are pre-dedesigned 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′”. The Tower Bible also has examples with a “+” prefix instead of “E”, without explanation. An M suffix followed by a height indicates a height reduction (unconfirmed).

The D prefix indicates twin earth wires located on the top crossarms, instead of a single earth wire strung from the top. The L prefix indicates a low-height design (unconfirmed).

The S suffix indicates a deviation tower with suspension insulators (unconfirmed).

Characteristics

Circuit count

Most power is transmitted and distributed in three phases. Three-phase power transmission does not require a return conductor: the current from each wire is returned via the other two. This is why power lines are found in multiples of three. Each three-phase circuit requires three cables. The 11 kV feeds to farms and villages on wood poles are generally single circuit. The higher voltage 33 kV feeds, also on wood poles, are more commonly double-circuit, either two parallel sets of three-wire poles, or double poles with six wires.

Steel lattice transmission towers can be either single circuit or double circuit, although double circuit is by far the most common. Each of the three phases per circuit can be a single cable or a bundle of two, three or four cables separated by spacers.

Typical double-circuit towers have six crossarms, three per side, with one phase per crossarm, although other arrangments exist. Typical single-circuit towers have three crossarms, two on one side and one on the other, but there are also single-circuit routes using double-circuit towers with one side of each tower unused.

Straight line and deviation

Circuit termination

Sealing end compound

A sealing end compound is a fenced-off enclosure where the route changes from overhead lines to underground cables. The overhead lines terminate on a special tower, with downleads connecting the overhead cables to the underground cables. See Sealing-end compounds on EMFS.info for more details. Illustrated below is the St Albans sealing end compound near the entrance to Nightingale Lane off Highfield Park Drive (51.737° N 0.303° W). This is primarily a PL16 route, connecting Cell Barnes substation in St Albans to the Elstree–Rye House 132 kV route at Coursers Farm Anaerobic Digestion Plant between Tyttenhanger and Colney Heath, but the termination tower here is not a PL16. The route reaches St Albans on towers, and changes to underground cables until the substation. The photographs were taken on 1st May 2021.

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Sealing end compound; note the downleads between the tower and the ground-level equipment
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Tower detail
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Downlead termination
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Termination detail

Sealing end platform

These are the “baskets”, the enclosed raised platforms placed alongside towers where the downleads from the tower connect to underground cables. This approach is possibly intended to reduce the land usage requirement compared to a sealing end compound.

Voltage and conductors

Tower adaptation

Double earth conductors

Some towers take two earth conductors (wires/cables) instead of one. Normally the earth conductor is attached to the top of each tower, but with this adaptation, each of the two earth conductors is attached to ends of the top crossarms. PL4 towers use a fourth pair of crossarms for this purpose, while PL7 and PL16 have special top crossarms that support both the phase conductors and the earth conductors. The normal tower designation (e.g. D2, D30) gains a “D” prefix, giving DD2, DD30 etc. This adaptation is only known to exist with double circuit towers: DD2, DD10, DD30, DD60, DD90, DDT, DD60 and DD90. Only a limited number of drawings exist in the Tower Bible.

SWE PL16 D2 (Blaw Knox)
SWE PL16 DD2 (Blaw Knox)

Some DD towers nonetheless have just the single earth conductor in the centre, as can be seen on the PL16 route at Welham Green. This likely resulted from a reconfiguration of the route at some point in the past. Ian McAulay notes that (in Scotland at least) these towers “seem to have been used for the first half dozen or so spans coming out of the substation”. This configuration can be seen at the substation at Picotts End, Hemel Hempstead, Hertfordshire where the 132 kV Elstree–Sundon route starts out on DD towers before changing to standard towers; here, you do have the proper dual earth conductors as intended. (This can be seen on Google Street View; there are no photographs of that substation on this site.)

Low height

Glossary

Abbreviations

downlead
A cable connecting one of the phase cables on the tower to ground-level equipment
SEC
Sealing end compound
SEP
Sealing end platform

Terms

Deviation
A change in direction of a route. These direction changes occur at deviation towers, where the incoming and outgoing cables face different directions.