L6

Contents

• Overview
  • General data
  • Origin
  • Insulator count
  • L6(c)
• Tower forms
  • Balfour Beatty design
  • BICC design
  • Blaw-Knox design
  • J L Eve design
  • L6(c)
  • L6m
  • Adaptation
  • Recognition points
  • Examples
• Tower details
• Examples
  • Sundon–Wymondley
    • Sundon
  • Sundon–East Claydon
    • Chalton
• Documentation
• See also

Overview

L6 is a UK electricity pylon (steel lattice transmission tower) series. L6 appears to be the joint tallest tower type in the United Kingdom, with the standard height suspension towers ranging from 49.1 to 50.6 metres tall, compared to 50.5 m for SSE400. (The later L8 and L12 types that replaced L2 and L6 are only 46 metres tall.) L6 can be seen with both quad and triple conductor bundles.

In his article [Transmission tower development], author C Lomas reported:

Initially four separate detailed designs of the L6 tower were in existence but these were later rationalized into one new metric design when metric replaced the old imperial measures in the UK.

The four original L6 types were those of Blaw Knox (BK), BICC, Balfour Beatty (BB) and J L Eve. Although all four types met the same specification, each one was a quite different design and they are readily distinguishable from each other. Lomas missed out a stage of L6’s evolution: in between the original imperial designs and the unified L6m, there is also L6(c), which appears to have been a joint contract between BICC to J L Eve to each metricate a portion of their own design.

L6m is a new design that appears to resemble BICC’s design more closely than the other types. Drawing SP4072489 does not name the designer of L6m.

Lomas’s article also noted:

The maximum span of the L6 suspension tower was still 540 m with the maximum sum of adjacent spans being limited to 800 m. The L6 suspension tower weighed 23.2 tonnes.

L9 is the low-height version of L6, designed by J L Eve.

As of February 2023, National Grid’s approach to new heavy-duty lines is said to be D and D30 from L6 and all other tower types from L12, with steelwork upgrades to L12 as required.

General data

Origin

For the 400 kV grid, additional capacity was obtained by building the new power lines with quad Zebra conductor bundles instead of twin Zebra bundles. This required much stronger and therefore heftier towers. The requirement for new towers also afforded the opportunity to increase the phase clearances, re-extend the middle crossarms to reduce conductor clashing, and increase the earthwire shielding by reducing the shielding angle from 45° to 35° “which, according to world statistics, should halve the number of lightning flashovers.” [400 kV Grid].

The images below show the progression of how the design of L6 was presented. Where official overview drawings—showing all tower types in production—only depicted Blaw Knox L3 (and not Eve L3), for L6 they only depicted the Balfour Beatty type.

Insulator count

L6 towers were designed to share the same insulators as L8. With L6’s quad Zebra conductor bundles (versus twin Zebra bundles on L8 and L2), twice as many insulators were required per phase to support the load. On suspension towers, these insulator pairs are known as “duplex” sets.

As lines are reconductored, the 190 kN insulators are replaced with 400 kN insulators, halving the number of insulators per phase. This change is evident when only one circuit has been reconductored.

• Balfour Beatty D30 quad insulators for one circuit, double insulators for the other (Flickr)

L6(c)

L6(c) is a metric conversion of L6. Drawing 35/8173 from November 1977 indicates that L6(c) comprised the following:

• BICC D
• J L Eve D30
• BICC D60
• J L Eve D90
• BICC DT
• J L Eve DJT
• J L Eve ST
• BICC SF60

This same working arrangement was used with L7, which was also a joint BICC–Eve design again using a mixture of BICC and Eve tower designs. The L7 towers bear corresponding similarities to those in L6.

Tower forms

L6 does not have separate D10 towers. The D30 type is divided into 0–10° and 10–30° forms that differ only in the design of the foundations. Above ground the two D30 types are identical; the D30 10–30° type has deeper front foundations and stubs and shallower back foundations and stubs than the 0–10° type.

The known L6 sub-series are:

• L6 BB (Balfour Beatty)
• L6 BICC
• L6 BK (Blaw Knox)
• L6 JLE (J L Eve)
• L6(c)
• L6m

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

Balfour Beatty design

Note that the centre crossarm bracing design of D30 and D60 varies. The solid lines represent D30 and D60 as shown on drawing 35/7291, redrawn 1966. D20EC, D30 and D60 towers matching this design can be found on the two 275 kV lines between Monk Fryston Substation and Ferrybridge Substation, to the north of Pontefract, England. The dashed lines show the extra bracing per drawing 35/6070 of illegible age, and this is the more common form taken by these towers. Drawing 35/6937, again of illegible date, has no vertical crossarm bracing on the D60 at all but this design is presently unconfirmed. D20EC and D30 appear to be identical above ground.

The details in red in the DJT diagram are based an example depicted in Flickr photo Aston Station - Electricity pylons, due to the incompleteness and inaccuracy of the drawings on chart 35/7291. Chart 35/6070 is much better but the copy on the RMWeb forum is too small to use due to inability to comprehend the forum’s upload dimensions cap of 1500×1500 pixels. The area enclosed in red on the SF60 diagram is likely to have additional bracing but this detail was again omitted on the chart.

BICC design

The diagrams below were taken from an overly-shrunk copy of a poorly-reproduced copy (again due to use of the RMWeb forum) of an inaccurately-drawn L6 BICC drawing of unknown date and age. SF60 has been omitted due to chart inaccuracy.

Some towers were re-used in L6(c) below.

The D20EC earthwire changeover tower supports deviation up to 20° and a total angle of separation (between the outgoing circuits) of 40°. D40EC allows deviation up to 40°, with the total angle of separation still capped at 40° as with D20EC. The DT tower allows an angle of entry from 0–5° while the DJT and ST towers each allow an angle of entry of 0–45°.

Blaw-Knox design

J L Eve design

Some drawings pending chart availability. Some towers were re-used in L6(c) below.

The drawings above are from Eve’s C896 drawing series. The D, D20EC, D30, D40EC and SF60 drawings are from the POWE 14/1690 file held at the University of Hertfordshire that contains material relating to the Thorpe Marsh–Stalybridge line. Of these, only the D20EC, D30 and SF60 drawings show all the bracing; the D drawing shows some bracing and the rest was taken from Paul Rees’s Eve L6 summary illustration of unknown source, with all additional bracing members connected to the ½ and ⅓ interval points of the respective members. The D60 and DJT drawings were obtained separately as the POWE 14/1690 versions omitted the bracing.

Note that D20EC and D30 are virtually identical but have different bracing in two places: the top crossarm (downwards-facing instead of upwards-facing in D20EC) and the bottom two body sections (symmetrical in D20EC and asymmetrical in D30).

An Eve SF60 tower has yet to be seen; L6 SF60s appear to be almost always BICC. The earthwire peaks in the SF60 diagram above have been significantly shortened in order to fit the dimensions given in Eve’s diagram, as their drawing and the dimensions therein do not correspond.

L6(c)

L6(c) was a joint project between BICC and Eve to each metricate a portion of their tower suite.

The D STD drawing on chart chart 35/8173 is erroneous: the crossarm bracing is not the same on either side, and neither side matches BICC L6. The crossarm bracing is taken from photographs of actual BICC L6 towers and is thus drawn only approximately due to perspective. Any changes to the crossarm bracing between L6 and L6(c) are therefore not taken into account.

BICC’s SF60 tower has three positions for the horizontal section depending on deviation angle. The L6(c) chart also notes that if a low earthwire is used (rather than the high earthwire peak) then an extended gantry is required, with 4.953 m of extra width added to the inside angle. This length corresponds to three upright triangles of the horizontal section, i.e. 50% more width than the intial overhang of the 0–15° arrangement. In practice, most L6 SF60s are the BICC type and they are all reduced in height significantly (either M20 or M30); no standard size SF60s are yet encountered.

There are at least two D40EC (written D40° E/C) variations: 10° deviation and 20° deviation. These have an “offset” of 1.270 m and 1.346 m respectively, compared to a 2.590 m offset for D60 and a 5.182 m offset for D90.

L6m

L6m metric adaptation:

The drawings above are redrawn from SP Energy Networks drawing SP4072489 “L6M SPEC TOWERS DIMENSIONS”. This preliminary drawing does not include D90 or SF60.

L6m D height adaptations, taken from incomplete plans shown within an RMweb forum comment:

The reason for a hillside-only M3 is not explained, and neither is the difference between the two E3 options.

Adaptation

Reduced-height, “headless” towers can be found at Dungeness:

Leading up to the Dungeness substation are two parallel lines of BICC L6 towers. This includes two side-by-side runs of closely-spaced reduced-height deviation towers without earthwire peaks. The earthwire is still there—attached at the top of each tower—but there is very little earthwire shade. These low-height towers are BICC D30 with what appears to be a 20-foot height reduction at the base (not counting the removal of the tower peak). The drawing above is only approximate due to the lack of a good BICC L6 chart.

See also:

• Pylons from Dungeness Power Station (Geograph photo 1)
• Pylons from Dungeness Power Station (Geograph photo 2)

Recognition points

The diagrams below will help with identifying the manufacturer of a particular L6 line.

Balfour Beatty suspension towers have five “K” bracing sections in the tower base:

Balfour Beatty D30 and D60 deviation towers are recognisable by their unique crossarm bracing style (the only types known to use horizontal bracing members).

All Blaw Knox L6 tower types have arch-like bracing of the base:

J L Eve D suspension towers have virtually parallel sides of the top of the tower, from the top to the bottom crossarm. The tension types have a very slender peak with a flat top and zig-zag bracing.

Examples

Tower details

Crossarm width is the width across the widest crossarm; this may (as is the case with Eve) be only the width between the centres of the conductor attachment points, rather than to the end of the steelwork.

The BICC L6 angle limits and tower dimensions come from a CEGB drawing bearing the text “TYPICAL RANGE OF TOWERS FOR DOUBLE CIRCUIT 400 KV. LINES WITH 4×400 SQ.mm NOM.AL . A.C.S.R. CONDUCTORS PER PHASE (STANDARD HEIGHT TOWERS SHOWN)” of unknown title, number and date (the bottom is cut off), noted as source “?” in the dimensions table.

Examples

Sundon–Wymondley

The Sundon–Wymondley 400 kV line, route 4TA, ends at Sundon substation. It uses Balfour Beatty (BB) L6 towers with quad conductor bundles.

Sundon

The line terminates at Sundon substation where a DJT tower splits the line onto an ST and an SF60, the latter via an intermediate ST. The tower types are confirmed from National Grid data.

Sundon–East Claydon

The Sundon–East Claydon 400 kV line is route 4YJ using Balfour Beatty L6 towers with triple conductor bundles.

Chalton

The Sundon–East Claydon line starts at Sundon Substation. The following pictures are from Chalton, near to the substation. Here, L6 line 4YJ and L2 line ZA cross over PL16 line PU. The photographs were taken on 23/09/2023, mostly while the sun was behind the clouds unfortunately.

Documentation

• L6 Blaw Knox Towers, Dimensions (preliminary chart SP4072513, drawn 2010)
• BEBS L6 400 kV Towers and Foundations, Balfour Beatty (chart 35/7291, revised 1966)
• L6(c) Towers and Foundations (chart 35/8173, drawn 1977, revised 1978)
• L6m Spec Towers, Dimensions (preliminary chart SP4072489, drawn 2010)

See also

Additional designs and details (note that some BICC and Eve examples could be L6(c) as they are not all verified against adjacent towers):

• Balfour Beatty:
  • Balfour Beatty L6 D40 at Burwell Substation (Flickr); deemed to be non-standard due to its symmetric design
  • Balfour Beatty L6 DT (Flickr)
  • Balfour Beatty L6 DJT (tower 4YL073), where line 4YLA tees off from 4YL, Twinstead, Essex (Flickr)
  • Balfour Beatty L6 D90 (Alamy)
• Blaw Knox:
  • Blaw Knox L6 D (Flickr)
  • Blaw Knox L6 D towers (Flickr)
  • Blaw Knox L6 D and D30 (Flickr)
  • Blaw Knox L6 D30 (Flickr)
  • Blaw Knox L6 D60 STD (Flickr)
  • Blaw Knox L6 D60 STD (Flickr)
  • Side view of the above tower (Alamy)
  • Blaw Knox L6 DJT STD at Burwell Substation (Flickr)
  • Blaw Knox L6 DJT STD at Ebbsfleet (Alamy)
• BICC:
  • BICC L6 D60 reduced height (Flickr)
  • BICC L6 DJT variant (Flickr)
• Eve:
  • J L Eve L6 D towers (Flickr)
  • Three back-to-back “L12 DJT” towers with unused top crossarm extensions (towers 4YX149, 4YX149A and 4YX149B) outside Portskewett, Monmouthshire, Wales (Flickr)
  • J L Eve L6 DT (4ZO164R) within a sealing end compound, one end of the undergrounded section (between Carlecote and Woodhead) of line 4ZO (Flickr)
  • Customised J L Eve L6 D in Halifax serving as a height-extended 132 kV LD60 carrying twin earthwires … (Flickr)
• L6(c):
  • L6(c) D30 and D (Flickr)
  • Various L6(c) towers at Medway substation, with DT 4TL001 and DJT 4TK001 in the foreground; behind 4TL001 appears to be an L12 DT (4TL003) mislabelled by National Grid as an L6 D30 (Flickr)

Other material:

• The Pylon Men (British Pathé, 1966), showing construction of a BICC L6 line (YouTube)

Related types:

• L9: low height version
• L12: replacement type for L6
• L13: direct substitute for L6