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L8

Caveat

Due to the limits of the available data, details on the pylon pages are incomplete and in places inaccurate or incorrect. Pages typically only cover standard tower types and common variants. This information is provided for enthusiasts and those of a curious mind and is not authoritative. Additional source material is welcome, in particular drawings. Most data and illustrations provided are sourced from industry-related material. See the sources page for a list of sources and the series page for a list of series.

Contents

Overview

L8 is a UK electricity pylon (steel lattice transmission tower) series.

L8 comes in at least two forms: tall (46 metre) 400 kV towers, and shorter and slimmer 275 kV towers. The 275 kV towers seem to have an “R” prefix to the tower type. L8 RD looks like a cross between L3 and L12, and can be found on the new 275 kV line between Dounreay Substation (58.576° N 3.754° W) to Spittal Substation (58.480° N 3.452° W) via Thurso South Substation (58.5647° N 3.513° W). This 275 kV line replaced an older 132 kV line (PL16) that has since been removed but is still shown on Open Infrastructure Map.

Just as with L6, and unlike L2 that it is said to have replaced, L8 does not have a separate D10 type. Instead, there are separate foundation dimensions for the 0–10° and 10–30° versions of the D30 tower.

Single-circuit-adapted L8 can be seen on the short 2.3 mile run (2 miles as the crow flies) that connects the 819 MW Sutton Bridge Power Station (52.757° N 0.192° E) to the nearby Walpole Substation. National Grid’s terminology page claims that “a 400 kV National Grid circuit may carry 1 kA in each of its three phases, thus transmitting a power of 700 MW”; possibly this L8 line has higher rated conductors, as the power station is rated slightly higher than this. The 0° towers on this line appear to be standard D towers with the bottom crossarm and half of the top crossarm deleted; this is the same adaptation as seen with L4(m). The deviation towers are less clear; from what little can be seen of them, there is a suggestion that they are customised for lower height.

L8 is said to be a replacement for L2, although this is yet to be confirmed from official literature. Like L2, there are 15° and 45° earthwire changeover (EC/EWCO) types. Chart 35/8744 covering L8(c) indicates that all towers are to Blaw-Knox design.

L8(c) is the metric conversion of L8. The height adaptations are thus listed in both imperial and metric, e.g. E24 and E7.3.

General data

Some height information is given in [East Coast 400 kV EIA] for “L8”. The tower heights in that material are rounded to the nearest metre; analysis would imply however that the height extensions and reductions are in feet. It thus appears that height adaptations were in multiples of four feet, and that the figures relate to the original Imperial L8 instead of the metric L8(c).

Known subtypes L8
L8(c) (metric conversion)
Height (straight line tower) 46.4 m (400 kV)
ca. 40 m (275 kV)
Voltage 400 kV, 275 kV
Dates back to 1966 (specification)
Conductors Twin
Known heights (400 kV) M24, M12, STD (all)
E12, E24, E36, E48, E60 (not SF60)
Additional options for D
See table below for more details

Details on the 275 kV version are very limited. The “L8” type given in [Dounreay to Mybster]—clearly an L8 RD variant—is shown with a height of 33123 mm and a base width of 5486 mm; the height is described thus:

The towers used for the 275 kV OHL will vary in height between 32m and 52m to accommodate changes in topography and where the line crosses roads and rivers, although the height will generally be in the region of 40m.

The L8 RD M4.9 diagram comes from [NeSTS SSEN003]. No height is given for this tower, so it was scaled using the diagram in [Dounreay to Mybster], through which it comes out at around 35.6 m. Adding the 4.9 m from the height reduction gives 40.5 m, very close to the implied size of the standard tower at 40 m.

Known conductor arrangements
Designation Phase Earthwire Voltages used Per
Twin 400 mm² ACSR (Zebra) 175 mm² ACSR (Lynx) 400 kV 35/8744 (1975 or 1977)
Known height extensions and reductions (400 kV type)
Suffix Tower type
Imperial Metric DD30D60D90DTDJTSTSF60
M24M7.3
M16
M12M3.7
M8M2.4
M4
STD
E4
E8
E12E3.7
E16
E20
E24E7.3
E28E8.5
E36E11.0
E48E14.6
E52E15.9
E60E18.3
E90E27.4
E120E36.6
E150E45.7

“M” (minus) figures are reductions in height, and “E” figures are extensions in height. The metric suffixes correspond to L8(c) metric conversion.

Entries marked † are from [East Coast 400 kV EIA] and are given for “L8”. The remaining data is from L8(c) drawing 35/8744.

Tower forms

The following diagrams are shown to scale at 12 pixels per metre. First, the full-size 400 kV towers:

L8(c) D STD
L8(c) D30 STD
L8(c) D60 STD
L8(c) D90 STD
L8(c) DT STD
L8(c) DJT STD
L8(c) ST M7.3 (M24′)
L8(c) SF60 STD 0–20°
L8(c) SF60 STD 20–60°

There are two versions of the SF60 tower, for 0–20° and 20–60° deviation ranges.

Middle phase:
0°–20° deviation use centre line of tower
20–60° deviation offset 1.905 m and long extension crossarm

The smaller 275 kV towers:

L8 RD STD
L8 RD M4.9 (M16′)
L8 RD M7.3 (M24′)

The third image is taken to be M7.3 as it is around 2.5 m taller than M4.9, in line with the expected 2.438 m difference (8 feet). The standard height (STD) drawing is a combination of the D STD and RD M7.3 and drawings. In practice, it seems that these towers tend to be built in M sizes, possibly because at 275 kV the voltage is lower than the 400 kV design voltage of L8 and thus the full height is not necessary.

Like L4(m), L8 has been adapted into single-circuit form; as with L4(m), the designations are not known and those given below are only suggestions:

L8 S STD

Tower details

Crossarm width is the total width across the widest arms. Base width is back-to-back of the stubs at ground level; dimension over concrete at ground level is around 400–500 mm greater.

There is no D10 type; there are instead 0–10° and 10–30° variants of the D30 that differ in the dimensions of the foundations, sharing the same tower design and crossarm lengths.

Type Source Height Base width Crossarm width Weight
L8(c) D STD 35/8744 46.4 m 7.5 m 17.1 m 13.0 tonnes
L8(c) D30 STD 35/8744 43.8 m 9.6 m 17.5 m 21.0 tonnes
L8(c) D60 STD 35/8744 44.5 m 10.8 m 18.2 m 24.0 tonnes
L8(c) D90 STD 35/8744 49.4 m 16.2 m 21.8 m 34.3 tonnes
L8(c) DT STD 35/8744 46.3 m 10.8 m 19.5 m (standard width)
25.6 m (extended)
38.1 tonnes (standard)
L8(c) DJT STD 35/8744 48.2 m 14.2 m 26.1 m (standard width)
31.1 m (extended)
43.9 tonnes (standard)
L8(c) ST STD 35/8744 44.3 m 9.6 m 22.1 tonnes
L8(c) SF60 STD 35/8744 21.1 m 11.0 × 6.3 m 19.4 m (standard width)

Examples

Bricket Wood

Two extended-height L8-series D30 towers were inserted into the Sundon–Elstree 400 kV route just outside of Bricket Wood, Hertfordshire; the route passes alongside M25 junction 21A (Bricket Wood Interchange), 51.715° N 0.371° W. Sundon–Elstree is an L2 route, and it seems likely that the L8 towers were introduced as replacements in order to increase clearance when constructing the M25 motorway. The comparatively sunny picture is from 13th March 2021, and the dismal photos are from 30th July 2022.

National Grid’s GIS data still lists the two L8 towers as L2 from 1956, as this is an L2 line.

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L2 D10 (foreground) and the two L8 D30 towers (background), seen from Meadow Close
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L8 D30 (tower ZA502) alongside the M25 motorway
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ZA502, detail
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ZA502, side view
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Second L8 D30, opposite side of the A405 North Orbital Road (tower ZA501)
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ZA501, front view, with the regular L2 towers in the background

Documentation

See also