L4(m)

Contents
Overview
L4(m) is a 132 kV UK electricity pylon (steel lattice transmission tower) series. The “(m)” suffix suggests an earlier, imperial L4 type. Although there is no evidence to support an imperial version of L4(m), see the L4 page for historical notes on the L4 designation.
Blaw Knox designed the towers.
RMweb forum comment 3209904 indicates that “L4 metric” was introduced in 1975, replacing PL16. [Protean and POC-MAST] gives both 1975 and 1977 as introduction dates for L4M, referring to it as “L4”, “L4m”, “L4M” and “L4 (M)” (one instance of each).
There is no separate 10° angle tower design: the D30 tower has separate 0–10° and 10–30° foundation arrangements. There is one known type of earthwire changeover tower, D40EC.
L4(m) 66
L4(m) 66 is a derivative suite designed to replace obsolete 66 kV tower types.
General data
Contractor | Blaw Knox |
---|---|
Found | |
Height (straight line tower) | 26.1 m |
Voltage | 132 kV |
Dates back to | 1972 (specification), 1973–1975 (drawings) |
Conductors | Single |
Earthwire shade | 45° |
Designation | Phase | Earthwire |
---|---|---|
L4 | Lynx (175 mm² ACSR) | Horse (70 mm² ACSR) |
L4/1 | Zebra (400 mm² ACSR) | Lynx |
L4 (H) | ||
L4M | Upas (300 mm² AAAC) | Keziah (160 mm² AACSR) |
L4(m) | Lynx | Horse |
L4(m)/1 | Zebra | Lynx |
L4(m)/2 | Upas | Horse |
L4(m)/3 | Upas | 73-AL5/43-ST1A (60 mm² AACSR) |
Single circuit
Like almost all modern tower types, L4(m) is a double-circuit-only design. However, in instances where existing single-circuit towers need to be replaced, L4(m) can be used as a substitute. There are at least two alternatives available. For small numbers of towers, standard double-circuit towers can be used, strung as single-circuit only. This has been used in Clapham in Bedfordshire for example, as depicted further down the page.
There is also a single-circuit adaptation with only three arms. The bottom crossarm is deleted, and the top crossarm is deleted on one side. Since the crossarms provide some of the tower bracing,
Single-circuit L4(m) towers can be seen on a line that runs from a junction tower just north of Pickup Bank at 53.707° N 2.412° W (east of Darwen, southeast of Blackburn in Lancashire) to where the line goes underground just outside of Blackburn Substation on a modified L4(m) DJT at 53.757° N 2.441° W. Most of this line uses single-circuit-adapted towers, although a few regular towers strung single-circuit can also be found.
A second type of single-circuit adaption has one half crossarm on each side, more like the PL1 towers that they replace. For example, there is an S30 in Madeley at 53.004° N 2.338° W. One span along from this is a regular D tower.
Tower forms
The following diagrams are shown to scale at 12 pixels per metre:
Note the open crossarms (no vertical bracing) and their nearly isosceles triangle shape. Note that D30, D60 and D90 have successively smaller peaks. D60 and D90 carry the earth wire off to the side of the tower on a short spur projection; this spur is distinctly larger on D90 compared to D60.
Some alterations have been made to National Grid’s drawings to match actual towers. The lower central tower bracing in National Grid’s D30 drawing was in essence “upside down”, and has been reversed based on a photograph. The outer centre arm of D60 has two bracing members omitted from the National Grid drawing.
The D90, ST and SF60 diagrams were redrawn from a chart posted to the RMweb forum. Due to the poor scan quality and poor reproduction quality of the original material, the D and D60 diagrams have not been corrected as the accuracy is likely to be no better than the existing diagrams.
The DT and DJT are very similar, but there are some signficant differences that allow for easy identification. The portion of the body between the bend (kink) line and the bottom crossarm has four complete diamonds in the DT, and only three and a half diamonds in the DJT. On the longitudinal sides, the DT peak is wedge-shaped just as with D through D90, while the DJT peak is square section. These details are illustrated below:
There are also specially-adapted single-circuit towers:
The formal designations are not known and are therefore not shown in bold above. Notice how the parts of the crossarms that tie the tower together horizontally have been retained where the crossarm is incomplete or deleted. Dedicated single-circuit towers can be shorter than double-circuit towers, but these towers do not take advantage of that, likely to avoid needing to design a whole new tower.
Super-strength L4(m) D E6 found in Scotland on non-L4(m) lines:
Examples
Tower details
Crossarm width is the total width across the widest arms; not all figures are given due to poor quality source material with unreadable figures. “SH” denotes standard height.
Type | Source | Height | Base width | Crossarm width | Mass |
---|---|---|---|---|---|
L4(m) D SH | ENA TS 43-7 | 26.1 m | 4.0 m | 8.4 m | 3.4 t |
L4(m) D30 SH | 26.0 m | 4.8 m | 8.7 m | 5.3 t | |
L4(m) D40EC SH | |||||
L4(m) D60 SH | 25.8 m | 5.3 m | 8.95 m | 6.7 t | |
L4(m) D90 SH | 25.7 m | 5.4 m | ? | 7.8 t | |
L4(m) DT SH | 26.3 m | 5.3 m | 13.6 m | 9.4 t | |
L4(m) DJT terminal 0–45° SH | 26.5 m | 5.4 m | ? | 11.0 t | |
L4(m) DJT junction SH | ? | 11.6 t | |||
L4(m) ST SH | 26.3 m | 4.8 m | — | — | |
L4(m) SF60 SH | 17.7 m | 5.0×3.0 m | ? | — |
Type | Limits |
---|---|
L4(m) D40EC | Mean line deviation 0–40°; maximum angle of separation 20° (0–20° deviation) or 40° (20–40° deviation) |
L4(m) DJT | Entry angle 0–45° as terminal tower; more complex limits for junctions |
L4(m) DT | Entry angle 0–5° |
L4(m) ST | Entry angle 0–45° |
Examples
The following examples represent opinion only. No tower types were obtained from any official material.
Luton South Substation
Part of the Elstree–Sundon 132 kV network, this branch between Sundon Substation and Elstree Substation provides power to Luton from the south by way of the Luton South Substation (51.874° N 0.400° W). Largely an L4(m) route, the final two towers are SEE PL1a. Just before the substation is a DJT tower standing in for a D60 for reasons unknown. Along with a D60 and a D90, this detour avoids suspending cables directly above the pitches of the Crawley Green Football Club. The pictures were taken on 11th September 2021 (cloudy) and 30th April 2022 (blue skies).

















Luton
In southwest Luton, following on from the aforementioned DT functioning as a D60, there are also two DTs—without the tee arms—functioning as D30s:



Clapham, Bedford
Two 132 kV lines pass between north Bedford and Clapham, one single circuit (appears to be SEE PL2) and one double-circuit (appears to be WGR). The double-circuit line changes to L4(m) and splits into two single-circuit lines by way of an L4(m) DJT; one fork goes all the way around the outside of Clapham to the south and the other heads away from Clapham. Initially both forks use PL16, but the northerly fork changes to L4(m) and then metal poles.






See also
- L4(m) DJT (unconfirmed), Sandridge, Wiltshire (Flickr)
- Alternative view of the above tower (Flickr)
- Special ST-style L4(m) tower with sealing end platform (Flickr)
- L4(m) ST (unconfirmed), serving as termination of an ENA TS 43-97 metal pole 132 kV line (Flickr)
- L4(m) D90 (Flickr)
- Symmetrical D60 tower, Lynemouth (Flickr)
- L4(m) double earthwire, Lynemouth (Alamy)
- L4(m) double earthwire, Lynemouth (Alamy)
- L4(m) double earthwire, Lynemouth (Alamy)