L8

Contents

• Overview
  • General data
  • Origin
• Can be confused with
• Tower forms
  • 400 kV
  • 275 kV
  • L8(c) Modified
• Tower details
• Examples
  • Bricket Wood
• Documentation
• See also

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

Contractor	Blaw Knox
Found	()
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

Origin

For the then-new 400 kV Supergrid lines, new towers were conceived. The principal tower suite was to be quad 0.4 in² SCA (Zebra), but it was recognised that a lower-capacity twin Zebra tower would be needed. At the time, the standard twin Zebra type was the L2, but a superior replacement to the L2 was desired.

As noted in [400 kV Grid]:

The principal circuits of the new 400 kV system will comprise quadruple 0·4 in² lines. Where circumstances do not justify the installation of such a heavy line a new design of 400 kV twin 0·4 in² line will be adopted. [ … ]

The general design basis for both lines is the same, and it is therefore possible to a large extent to use many interchangeable standard components such as conductor accessories, insulators and fittings.

[ … ]

Both designs will employ a single earth wire, but instead of continuing with a shielding angle of 45°, as with the existing 275 kV lines, this has been reduced to 35°, which, according to world statistics, should halve the number of lightning flashovers.

Both designs will also have their middle cross-arms off-set by 6 ft, which will reduce the possibility of clashing of the conductors of adjacent phases.

The depiction of the new twin bundle tower from [400 kV Grid] (Booth, Clark, Egginton and Forrest) is shown below. In the same year, Wires Pipes Pylons depicted a quite different design, describing it as “BES–L2 (1961)”.

Neither of the these proto-L8 types are likely to have entered production. Based on the timescales involved, it seems fairly likely that L8 was the realisation of this design goal: drawing 35/5163 rev. K, 13/07/1966 ([Goulty]) lists L8 as “BES–L8 (1966)”. The final design was produced by Blaw Knox as effectively a reduced-size version of their L6 tower suite.

Can be confused with

• L6, Blaw Knox type

Tower forms

The following diagrams are shown to scale at 12 pixels per metre.

400 kV

First, the full-size 400 kV towers:

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

275 kV

The smaller 275 kV towers:

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.

L8(c) Modified

Like L4(m), L8 has been adapted into single-circuit form, with the series name “L8(c) Modified”:

With the bottom crossarm deleted, the standard height is reduced. The S60 tower is based on the D30; the minimum angle of deviation is not known.

Tower 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.

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.

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).

“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.

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.

Documentation

• L8(c) Towers & Foundations (35/8744, drawn 1977, last revised 1983), covering D, D30, D60, D90, DT, DJT, ST, SF60 and D15 and D40 EWCO

See also

• L8 pylons album (Flickr)
• L8 D30, D60 and 2 × SF60 20–60°, and L66 D30 and D2, Harker to Stella (Flickr)
• L8 pylons album (Flickr)
• Symmetrical L8 D60 (tower ZF426) (Flickr)
• L8 DJT, Capenhurst Substation (Flickr)
• L8 DJT, Inverarnan Substation (Flickr)
• L8 DJT and DT, West Ham (Flickr)
• Asymmetric L8 DJT, Thurrock (Flickr)
• Another asymmetric L8 DJT, Thurrock Substation (Flickr)