L2

Contents

• Overview
  • General data
  • Development
  • Extended middle crossarms
  • T1648
• Can be confused with
• Tower forms
  • Examples
  • Height reductions and extensions
• Tower details
• Examples
  • Holtsmere End Lane
  • Bricket Wood
• Documentation
• See also

Overview

L2 is a UK electricity pylon (steel lattice transmission tower) series. L2 makes up the bulk of the original Supergrid network following on from L66; it was soon followed by the higher capacity L6 which permits quad conductor bundles over L2’s limit of twin conductor bundles. Unlike the older 132 kV types, L2 towers have all three crossarms of very similar length, leaving L2 lines open to conductor clashing. This mistake was rectified with L6, as well as the L2 DMC types and the T1648 line towers that all extended the width of the middle crossarms.

The oldest drawings recovered for L2 date back to 1952, while Blaw-Knox advertised L2 in 1954. The first L2 lines were constructed in the mid 1950s. Originally built as 275 kV, L2 was intended from the start to support higher voltage with longer insulator strings. Both 380 kV and 400 kV were stated to be the higher voltage, with 400 kV being the chosen voltage in production.

L2 was superseded by L8, designed as a substitute for L2.

General data

Although 380 kV was intended as one of the operating voltages, it seems likely that no 380 kV lines were established.

Development

The design of L2 was found to be unsatisfactory. When the 400 kV grid was being designed, the untested idea of quad conductor bundles was selected to double the capacity over L2’s twin conductor bundles. For situations where twin bundles were sufficient, a replacement for L2 was conceived. The replacement for L2 would share as many components as possible with L2. Although unconfirmed, it seems highly likely that this replacement type was L8. At least two proposed designs of enhanced L2 tower were published, one of which was stated in Wires Pipes Pylons to be “BES–L2”, depicted as follows:

This has led to a misconception that such a design entered production. There is no evidence to this effect. Although both a variant and a derivative of L2 with extended middle crossarms do exist, the type shown in Wires Pipes Pylons appears to have been hypothetical only with the production design taking the form of L8.

Extended middle crossarms

The Coventry–Berkswell line uses a variation on L2 with extended middle crossarms on the D, D30 and D60 towers (a D10 in this form may exist but details of it are not known). These towers are still part of the T1011 suite from Blaw Knox to which conventional L2 belongs. There seems to be no other change to the size and shape of the towers. These towers originally had no suffix code, being described in the form “Outline of suspension tower type ‘D’ with extended middle crossarm”. The suffix “DMC”—of unknown expansion—was later applied to these towers.

T1648

An alternative line tower was designed for Scotland and designated under reference T1648. Although this would seem to put it in a different suite from L2 (T1011), these line towers are used alongside what appear to be L2 angle towers. The upper portion of the tower (down to the bottom crossarm) is covered on drawing XM3693/2F sheet 2, first drawn on 23/02/1961 and modified 14/06/1967 and is from the drawing series “North of Scotland Hydro Electric Board 275/380 K.V. Double Circuit Lines Twin 0.4 sq.in. S.C.A. Conductors”.

T1648 D is considerably slimmer than L2 and L8; L8 also extended the height of the earthwire peak and is compatible with L2’s height extensions, being the same size and of the same angles at the stubs for standard height.

Can be confused with

• L3
• L66

Tower forms

The following diagrams are shown to scale at 12 pixels per metre. Standard line, angle and earthwire changeover towers:

The Blaw Knox T1648 line tower is included in the set of images above as it was used in conjunction with what seem to be standard L2 angle towers.

Extended middle crossarm (later “DMC”) type used on Coventry–Berkswell:

These towers were originally annotated simply as “extended middle crossarm” but were later designated “DMC” of unknown expansion. They are the same overall size as conventional L2 towers.

Junction and terminal towers, and gantries:

D10 and D30 share the same tower body; D30 has longer crossarms (1′–9″ (53 cm) longer top and 2′ (61 cm) longer middle and bottom crossarms), and the crossarm bracing arrangement of D30 looks more “stretched” than that of D10. The crossarm bracing in the D10 and D30 diagrams is approximated from photographs due to a lack of source material. In practice they are difficult to differentiate. The diagram below illustrates the difference, although note that the bracing detail is not exact:

Above ground (i.e. not considering foundations and stubs) D15 EWCO appears to be essentially identical to D30; the only discernible difference is a single vertical bracing member located between the bottom crossarms. The D40 EWCO diagram is drawn on the assumption that (as with other series) D40 EWCO shares the same tower body as D60, as no D40 chart is available. The crossarm widths and bracing patterns are again determined from Google Street View imagery, of one of the aforementioned D40 EWCO towers in the Leaves Green area. (National Grid simply refer to these two towers as “D40”.)

L2 has two single-circuit terminal tower types: ST and 380ST. The ST towers are for 275 kV only, and have a phase-to-phase clearance of 20′ (6 m), the same as for L3 ST. 380ST is designed for 380 kV operation and is taller with 30′ (9 m) phase-to-phase clearance. STX is the same as ST but with a fourth crossarm to take the earthwire and thus a small increase in height of 5′ (1.5 m).

DJ, DJX, DT and DTU share the same tower body. DJX is a DJ with auxiliary crossarms. A DJ can only divide a single circuit into a tee; the second circuit runs alongside on an SFX gantry. The drawing below is based on two junctions between Kidderminster and Bromsgrove. The first junction is between ZN (400+275 kV) and YYC (275 kV) and uses DJ YYC032 and SFX ZN216 (along with D10 ZN215 and ZN217), and appears to allow the 275 kV YYC line to cross under the 400 kV side of ZN. The same arrangement is found between ZN and YYB, the latter also being 275 kV. This second junction uses DJ YYB026 and SFX ZN237A (with D10 ZN237 and ZN238).

The auxiliary crossarms on a DJX allow the second circuit to run through the same tower:

DT and DTU differ only in crossarm lengths. DT has the conventional nearly-equal-length crossarms characteristic of L2. DTU has longer top crossarms and shorter bottom crossarms compared to DT, making it suitable for use with a sealing end platform (rather than passing through to gantries or frames); a DTU with a sealing end platform can be seen at Beddington Substation (51.375° N 0.128° W) in Croydon, England. The derivation of “DTU” is unknown; in L3 the same arrangement is confusingly named “DTV” (although National Grid’s data lists only L3 DTU and not DTV). DJ has larger foundations than DT and DTU as well as different crossarm lengths: the top and middle crossarms are shorter than those of DT. DT45 is extra wide in order to allow an entry angle of 45°.

The SFX gantry type is used as a low-height tower in the vicinity of London Biggin Hill Airport, where three successive pairs of SFX towers carry the 400 kV line at a low height across the flight path. These SFX towers sit between two D40 EWCO towers at 51.3457° N 0.036° E near Leaves Green and 51.347° N 0.052° E in Farthing Lane.

Examples

Height reductions and extensions

For L66, Blaw Knox designed a modular height extension system that was also used on L2, Blaw Knox L3, T2175 and L8. Separate 8 foot, 16 foot and 24 foot legs were provided. These legs could be added to the minimum height tower (M24′ ) for reduced heights of M16′  and M8′  and to produce the standard height tower. The same legs were used in conjuction with body extensions to provide taller towers. (From around E44′  upwards—towers with 32′  and 36′  body extensions—the leg design remained unchanged but the drawing numbers differed, suggesting the use of heavier steel angles to accommodate the extra tower weight.)

Tower details

Crossarm width is the total width across the widest crossarm pair. Base width is back-to-back of the stubs at ground level; dimension over concrete at ground level is around 1 to 2 feet greater.

D10 and D30 share the same tower body; D30 has longer crossarms to accommodate the larger angle. The D30 crossarm size is taken from [Clearances] and converted from metric to imperial. DTU and DJ are missing from drawing 35/5542.

The ST/STX drawing gives crossarm widths that do not match the drawing and may indicate attachment points, as this is done sometimes. Thus, the precise crossarm widths are not given for ST. ST tower arrangements vary anyway. The remainder of the crossarm lengths are assumed to be to the end of the steelwork.

The “M” conductor for L2/6 was not defined in the source material; possibly it is Moose as that is 500 mm² ACSR and would be plausible.

Examples

Holtsmere End Lane

The 132 kV Elstree–Sundon PL16 route crosses under the 400 kV Sundon–Elstree L2 route in farmland between Holtsmere End Lane and Gaddesden Lane, between Redbourn and Hemel Hempstead, Hertfordshire, England, 51.792° N 0.4337° W. Instead of splitting the two 132 kV circuits out onto low-height gantries and rejoining them on the far side of the 400 kV line, a short 400 kV span between two extended-height L2 towers is used to maintain clearance between the two routes. The 132 kV Elstree–Sundon route terminates at the nearby substation in Picotts End, Hemel (51.774° N 0.484° W), with PL16 DD towers as seen at Welham Green.

The (comparatively) sunny photos were taken on 28th May 2022 from a public footpath near Holtsmere End Lane. The cloudy photos were taken on 25th September 2021 from the adjoining Gaddesden Lane.

Bricket Wood

The Sundon–Elstree 400 kV route (list in National Grid GIS data as 1956 construction) passes through Bricket Wood, Hertfordshire (51.709° N 0.363° W). Where the route crosses over the A405 North Orbital Road, two extended height L8 towers have been inserted, possibly for improved clearance when constructing the M25 motorway. The following pictures are from various dates in 2021 and 2022.

Documentation

• L2 Towers and Foundations (chart 35/5542, revised 1978), covering D, D10/D30, D60, D90, DT45, DT/DTU, 380ST and ST/STX
• L2 D90 Key Erection Diagram & Drawing Schedule (chart T1011-321^B, drawn 1961)
• L2 DT Key Erection Diagram & Drawing Schedule (chart BK35/24707, drawn 1952, revised 1968)
• L2 DTU Key Erection Diagram & Drawing Schedule (chart BK35/24726, drawn 1958, revised 1961)

See also

• Blaw Knox/Painter Brothers advertisement depicting L2 undergoing testing, 1954 (Flickr)
• L2 pylons album (Flickr)
• Ocker Hill Substation (Flickr):
  • L2 DT, 2 × 380ST, DT45
  • L2 DT45, 380ST, ST, SF60, 380ST, DT (the “SF60”, sat between the two 380ST towers, appears to be an SFX; L2 DT and 132 kV DT in the background)
• L2 D40 EWCO (Flickr)
• L2 pylons outside Tandragee, County Armagh, Northern Ireland (Flickr)
• L2 DJ (Flickr)