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L7 and L7(c)

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Overview

L7 is a UK electricity pylon (steel lattice transmission tower) series. L7 is the only 132 kV type designed for twin conductors, and the tower structure is visibly more sturdy than the older PL generation of single-conductor types.

L7 is based around a CEGB specification issued in 1963. The design was a joint project between BICC and J L Eve, with each contractor responsible for specific towers. D30, D60, DT and ST are Eve’s design and resemble the design of Eve’s L6 variant, and the four towers effectively share the same tower body. BICC designed the D, D90 and DJT towers; D90 and DJT also share the same tower body and the DJT tower is the same style as that of BICC L6. The SF60 gantry seems to be a joint BICC/Balfour Beatty design.

L7(c) is the metricated version; according to the L7(c) standard, L7(c) uses the same exact dimensions and the steel members are are the nearest metric size.

General data

Designer J L Eve and BICC jointly
Found Kenya
Height (straight line tower) 26.9 m
Voltage 132 kV
Dates back to 1963 (specification), 1964–68 (drawings)
Conductors Single, twin
Normal span 1000′ (305 m), twin 0.175
785′ (239 m), twin 0.4
Earthwire shade 45°
Known conductor arrangements (all 132 kV)
Designation Phase Earthwire
Twin 0.175
Twin 0.4″ (reduced span)
L7 Twin Lynx (175 mm² ACSR) Lynx
L7/1 Zebra (400 mm² ACSR)
L7/2 Twin 200 mm² ACSR
L7 (H) Twin Zebra
Twin Upas (300 mm² AAAC)
Totara (425 mm² AAAC)
L7(c) Twin Lynx Lynx
L7(c)/1 Zebra
L7(c)/2 Twin Zebra
L7(c)/3 Finch (560 mm² ACSR)
L7(c)/4 Rubus (500 mm² AAAC) Keziah (160 mm² AACSR)
L7(c)/x

There are two definitions of L7/2, and the more realistic one is given above; the higher weight one may be an error but does correspond to L7 (H).

Lines

Kenya

Tower forms

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

L7 or L7c D (BICC)
L7(c) D30 (J L Eve)
L7(c) D60 (J L Eve)
L7(c) D90 (BICC)
L7(c) DT (J L Eve)
L7(c) DT (basic crossarms)
L7(c) DJT (BICC)
L7(c) ST (J L Eve)
L7(c) SF60 0–30° (BICC/BB)
L7(c) SF60 30–60° (BICC/BB)

The D30, D60, D90, DJT, ST and SF60 diagrams were redrawn from a chart posted to the RMweb forum. Due to the poor scan quality, invalid cropping (all chart details needlessly removed) and poor reproduction quality of the original material (itself simplified compare to the actual towers), these diagrams contain errors and guesswork. L7(c) is being assumed here as the details are in metric, but no tower heights are available as the person who scanned them chose to remove all that information. The D diagram has been adjusted to match a diagram in the L7(c) specification that is fairly accurate but omits the crossarm tips.

Some alterations in bracing notwithstanding, D30, D60, DJT and ST appear to share the same tower body. D90 and DJT are confirmed to share the same tower body.

SF60 30–60° is an approximation because only the 0–30° version is included in the basic charts and in the L7(c) standard.

Double earthwire

Three short lines of double earthwire L7(c) towers can be found at Lynemouth, insulated for around 66 kV and carrying only 24 kV with twin conductor bundles.

The diagrams below are based solely on Google Street View.

L7(c) D double earthwire
L7(c) D30 double earthwire

D30 and DT

L7 D30 and DT towers are very similar. The most prominent difference is the plan of the crossarms: D30 crossarms are symmetrical front-to-back, while DT crossarms have a back that is flat and perpendicular to the incoming line direction. As not all DT towers have the top crossarm extension and top crossarm extensions can be retrofitted to D30 towers, this distinction will allow the two types to be differentiated.

This difference is the key to understanding that the converted downlead tower in Slip End near Luton is based on a D30; it also explains why the top crossarm extension on that tower is a special design: the normal extension does not fit D30 crossarms. Likewise, it also demonstrates that the disconnected through tower at Redbourn is a DT, as it has a DT-only crossarm extension and DT crossarms.

The two crossarm styles are illustrated in the diagrams below:

The vertical bracing of the crossarms also differs between the two types; this difference is a little more subtle but also quite distinct.

Examples

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L7 D
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L7 D30
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L7 D60
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L7 DT
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L7 DJT

Tower details

Crossarm width is the total width across the longest crossarm pair.

Tower dimensions
Type Source Height Base width Crossarm width Weight
L7(c) D ENA TS 43-9 26.898 m 4.877 m 9.398 m 4.751 t
L7(c) D30 26.212 m 6.722 m 9.906 m 8.704 t
L7(c) D60 26.212 m 6.722 m 9.906 m 11.215 t
L7(c) D90 28.118 m 8.230 m 12.192 m 15.912 t
L7(c) DJT 28.118 m 8.230 m 12.764 m (with standard top crossarms)
16.460 m (with extended top crossarms)
17.643 t
L7(c) DT 26.212 m 6.722 m 9.448 m (basic)
14.326 m (with crossarm extensions)
13.374 t
L7(c) ST 26.491 m 6.722 m 7.468 m 8.532 t
L7(c) SF60 17.100 m 6.533 × 3.267 m 12.150 m 6.937 t
Line entry limits
Type Limits
L7(c) D20EC Mean line deviation 0–20°; maximum angle of separation 20°
L7(c) D40EC Mean line deviation 0–40°; maximum angle of separation 20° (0–20° deviation) or 40° (20–40° deviation)
L7(c) DJT Entry angle 0–45° as terminal tower; more complex limits for junctions
L7(c) DT Entry angle 0–5°
L7(c) ST Entry angle 0–45°

Examples

The following examples represent opinion only. No tower types were obtained from any official material and thus all designations are speculative. “L7” in the image captions and filenames refers to whichever of L7 or L7(c) is depicted, as presently there is no known means to differentiate them.

Sundon

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Eve D10 (foreground) and L7 D and L2 towers in the background
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L7 D and Eve D90 towers
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Eve D10 and L7 D towers
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L7 D detail

Redbourn

The Nickey Line rail trail from Harpenden to Hemel Hempstead, Hertfordshire, passes under part of the 132 kV Elstree–Sundon network as well as the 400 kV Sundon–Elstree route. Largely a SEE PL1a route, two L7 or L7c towers straddle the Nickey Line, crossing it at 51.788° N 0.410° W. Only a D10 is needed for either tower, so it’s not clear whether these are D10 or D30, due to a lack of details on L7. Southwards, the route returns to PL1a.

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L7 D, along with a DT (right) and an L7(c) DJT (left)
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First of the two L7 angle towers straddling the Nickey Line, shown as seen from Gaddesden Lane (25/09/2021)
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First angle tower
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First angle tower, alternative angle
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Second L7 angle tower, located at the entrance of the tunnel under the M1 on the Nickey Line, seen in better weather (30/10/2021)

Luton

There are several L7 towers south-west of Luton. Judging by the routeing, the 132 kV lines originally ran from Sundon Substation to Luton. At the edge of Luton, east of Pepperstock, the line was teed off within a sealing end compound and resurfaced on the far side of the A1081 at an L4 sealing end platform, where the line continued via Redbourn to Elstree Substation. Both the sealing end compound and sealing end platform at Luton have had their downleads removed, and the former DT tower at Redbourn (now a through tower to an adjacent DJT) is also disconnected, leaving the entire line from Redbourn to Luton de-energised.

The DJT tower at Redbourn and DJT-derived tower at Luton are the same design and both are found in the vicinity of L7 towers, and considering the resemblance to L7, is being deemed L7 DJT for now. The other tower types at each location are PL1a, PL16 and L4, and these towers are not the DJT from any of those types.

Nearby there is a brand new sealing end platform being built (at the time of writing: it may now be completed) around a D30 through tower; a custom top crossarm extension has been fitted to take the top downleads, as the standard top crossarm extension is designed around the special crossarms of DT towers.

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L7 DJT without the tee arms
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L7 DJT side view
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L7 DJT base
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L7 DJT detail
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L7 DJT portrait view
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L7 D60
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L7 D60
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L7 D30 with custom crossarm extension added to accept downleads
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Adapted L7 D30 detail
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Adapted L7 D30 side view

See also