L3

Contents
Overview
L3 is a 275 kV UK electricity pylon (steel lattice transmission tower) specification. There are two L3 ranges: Blaw Knox and J L Eve. The original Blaw Knox L3 design was imperial; L3(c) is the metric Blaw Knox version.
L3 is a 275 kV-only type. Like the preceding L2, the crossarms on each side are all roughly the same length.
The shorter clearances needed for 275 kV means that L3 towers are smaller than those of L2 by just over 10%. L3 was only designed for twin 0.175□″ SCA (ACSR) conductors—the typical conductor used with 132 kV towers—versus L2’s twin 0.4□″ SCA, which may be why L3 towers are also slimmer than L2. L2 D and L3 D can also be differentiated by the bracing patterns.
Both the Blaw Knox and Eve types later gained line towers with extended middle crossarms to reduce conductor clashing in high winds. In both ranges, this change was not extended to the angle and terminal towers.
L7 towers can be used as a substitute for L3 on 132 kV lines.
National Grid’s Policy Statement (Transmission) PS(T)081 from 2007 prohibits the construction of new L3 towers due to insufficient clearances. L66 is a suggested alternative for existing L3 lines where towers need to be replaced or where the line needs to be diverted, with the caveat that L66 towers may need to be strengthened if alternative conductors—such as 300 mm² AAAC—have been adopted. Overhead Lines Safety Bulletin 03-2006 also limits the level of tower climbing permitted on a live L3 tower, again due to limited clearances. However, L3 towers have been constructed since this time (e.g. XD line in 2015), so the edict appears to have been rescinded.
General data
Contractor |
Blaw Knox J L Eve |
---|---|
Known subtypes |
Blaw Knox L3 (imperial) Blaw Knox L3(c) (metric conversion) J L Eve L3 (C673) J L Eve L3 (C864) |
Height (straight line tower) |
121′–0″ (36.9 m) (Blaw Knox L3 D) 125′–0″ (38.1 m) (Blaw Knox L3/L3(c) DS) 118′–3½″ (36.1 m) (J L Eve D, C673) 124′–9″ (38.0 m) (J L Eve D, C864) |
Voltage | 275 kV |
Dates back to | 1953 or 1956 (specification) |
Conductors | Twin |
Design conductor | 0.175□″ SCA |
Normal span | 1200′ (366 m) |
Versions
Original
Two original designs of L3 exist: Blaw Knox and J L Eve.
The older Blaw Knox L3 towers are similar to those of L2. L3 D10 and D30 have proportionally stouter crossarms than the equivalent L2 towers.
Blaw Knox L3 is fairly common. The Eve version of L3 is rare; there is only one complete line known to use the original towers: Stourport–Ludlow in England (132 kV). This line runs from Ludlow Substation outside Ludlow to Bishops Wood Substation at Crossway Green near Stourport, around 23 miles south of Bishops Wood, but the line goes underground at Bishops Wood Substation and appears above ground near Crossway Green. “L3” is not mentioned on the tower drawings for these contracts, but CEGB drawing 35/4829 from 06/11/1958 does describe the type as “B.E.S._L3 (J.L.EVE DESIGN)” [sic]; there is also a National Grid drawing schedule for contract C673 which gives “J L Eve Reference: L3 (C673)” and “Series L3 (C673)”.
The C673 line tower drawings indicate contract T(8) K&D L8, where T(8) denotes the South Wales Electricity Board. The only known Eve L3 in South Wales is a short section of the otherwise Blaw Knox L3 line XM that runs from Pyle Supergrid substation to Severn power station. At Pyle there are two ST towers at the substation that appear to be Eve L3, followed by an earthwire changeover tower that looks the same as an Eve L3 D30 and is listed by National Grid as a D30. The next five towers (D30, D10, D60, D60 and D) are Eve L3, after which the remainder of the line is all Blaw Knox L3. The Eve section seems far too short to justify designing a new tower suite.
Drawing C.673/12A for the line tower is also marked C.916/37, indicating a third contract for these towers. The standard height line tower drawing is also annotated “CONTRACT WM(EL) L27 / STOURPORT-LUDLOW LINE”, which may be C916.
Enhancements
In 1960, the two line tower types (Blaw Knox and Eve) were enhanced with extended middle crossarms and greater clearances. The Blaw Knox enhanced line tower was placed into its own contract, T1521; no other towers are known within contract T1521. L2 was enhanced the next year in the same manner, with the new line tower placed into T1648.
The Eve type was used for Beauly–Blackhillock–Kintore in Scotland, Eve contract C864. C864 is not known to be documented as L3 but the line is still twin Lynx as per original L3. The tower drawings for Beauly–Blackhillock–Kintore are in the C864 number range, but the D10, D30 and D60 drawings were reproduced from those of C673 and bear numbers from both ranges. As with the Blaw Knox type, only the line tower was modified. Although the DT drawings in C864 seem to be new, the terminal tower at Ludlow appears to be the same general design, but with various customisations to the crossarms including auxiliary crossarms at the rear. (The Stourport end of the Stourport–Ludlow line terminates away from the substation on a Blaw Knox L3 DTV45.)
Beauly–Blackhillock–Kintore is said to have been constructed in the 1960s, per Caithness Business Index article Refurbishment of 1960s transmission line complete from November 2015. This would match the 1960 date on the line tower drawing.
In the late 1960s, Blaw Knox were contracted to design towers for the line between Dounreay and Beauly. These towers form Blaw Knox contract T2175. This line uses heavier smooth-bodied 0.52□″ SCA instead of L3’s twin 0.175□″ SCA. The line tower for T2175 is the same tower as that of T1521, while the angle towers have revised crossarm bracing. Single 0.52□″ SCA should be heavier than twin 0.175□″ SCA, leading to questions about how this extra load is supported, especially when the line tower was not modified at all.
Around 1968 (based on the drawing date), the enhanced Blaw Knox line tower was adopted into the L3 tower suite as type “DS”, where “S” might denote “Scottish”, “supplemental” or “substitute” (nobody knows).
Metric
The Blaw Knox version was metricated as L3(c) and, according to an unverified 2008 members-only post by Jules Seifert at the Pylon Appreciation Society forum, L3(m) also exists. Metricated Eve L3 is not presently known.
L3(c) is essentially identical to imperial L3. L3(c) appears to have re-designated D10° as D30°(0°–10°) and D30° as D30°(10°–30°); L3(c) chart 35/8192 only depicts D30, while imperial L3 chart 35/5543 depicted both D10 and D30 with their different crossarms.
Tentatively it appears that L3 D was never metricated, being fully replaced by L3(c) DS as of metrication.
Considerations for uprating
Per [400 kV Grid]:
The existing 275 kV system uses both twin 0·175 in² and twin 0·4 in² lines. The former cannot be reinsulated for 400 kV operation with the same conductors because the corona discharge at that voltage would be unacceptable. The latter, shown in Fig. 17, have twin 0·4 in² conductors per phase and can be converted to 400 kV.
Although one might have assumed that L3 was limited to 275 kV by the vertical clearances, this paper implies that, were L3 strong enough to take Zebra conductor, it could be uprated to 400 kV. Considering National Grid’s concerns for clearances even at 275 kV, this notion seems unlikely now, although possibly with higher-diameter AAAC conductor or similar it could be made to work.
Contract summary
Contract | Drawing dates | Contents |
---|---|---|
Blaw Knox T1120 | 1953–59 | Imperial L3 |
J L Eve C673 | 1955–56 | Imperial L3 (South Wales) |
Blaw Knox T1521 | 1960 | Enhanced D tower |
J L Eve C864 | 1960 | Beauly–Blackhillock–Kintore (with enhanced D tower) |
Blaw Knox K2175 | 1967–69 | Modified L3, takes 0.52□″ smooth-bodied SCA conductor |
Blaw Knox MT1120 | 1973–75 | L3(c) |
Can be confused with
Tower forms
The following diagrams are shown to scale at 12 pixels per metre:
Blaw Knox
Details on terminal and junction towers are scant and contradictory. The only complete drawings are L3 ST and “DJ DT DTV 45°” (chart 35/5543) and L3(c) DTV 45°, ST and DT 45° (chart 35/8192). The charts indicate that DJX is “similar to DJ”. National Grid’s public network map has no record of a DJX, DTV45 or DT45; instead, it uses DTU45 and there are two DJT towers listed in Wales, one of which being in reality an L8.
The DT drawing above is a combination of the “DJ DT DTV 45°” from chart 35/5543 and the L3 DT chart in [Clearances]: the latter source has more detailed crossarm dimensions and a much more accurate tower shape although it is only the most basic outline. There is no drawing for DJ or DJX. The DJ drawing is based on the only known DJ tower, at Stella West Substation. The only known DJT is at Cardiff East Substation, and that tower was used as the basis for the DJT drawing. Neither of the L3 charts mentions DJT and it may be that DJT is a redesignation of DJX. L3 lines are comparatively uncommon so there are few towers to examine.
The key erection diagram for Blaw Knox terminal type DTV45 bears the comment, “PRIMARILY FOR VERTICAL & EXCEEDING 45∡ DOWNLEADS”; this is the only clue as to what “DTV45” represents. The comment does not indicate where the 45° angle is measured from and whether it is vertical angle or deviation angle.
According to “lesc” at the Pylon Appreciation Society in 2013, there is a D5° type that “… appears unique to Scotland …” No indications are given as to what D5 might look like or where such a tower may be found.
J L Eve
Examples
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. Some dimensions may be incorrect due to the terribly poor legibility of the source material due to incorrect upload practice. Junction and terminal tower crossarm dimensions unclear due to confusing, contradictory and incomplete source material. (L3 “DJ DT DTV 45°” is depicted with the same crossarms as L3(c) “DT45°”, where L3(c) DT45° is not a documented type within the same drawing.)
The L2 ST 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 L3 ST in case the same applies (pending a more detailed L3 ST drawing). ST tower configurations vary anyway. The remainder of the crossarm lengths are assumed to be to the end of the steelwork.
D 0° EWCO is only included in L3 drawing 35/5543; L3(c) drawing 35/8192 only lists the standard EWCO angles of 15° and 40°.
Although L3 is an imperial series, L3 drawing 35/5543 is all metric, just as is L3(c) drawing 35/8192.
Type | Source | Height | Base width | Crossarm width | Weight |
---|---|---|---|---|---|
Blaw Knox L3 D | 35/5543 | 121′–0″ (36.881 m) | 6.401 m | 9.144 m | 6.9 t |
J L Eve L3 D (C673) STD | C.673/12A + C.673/49 | 118′–3½″ (36.1 m) | 18′–10″ (5.7 m) | 31′–0″ (9.4 m) | |
J L Eve L3 D (C864) STD | C.864/4J + C.864/6C | 124′–9″ (38 m) | 20′–77⁄16″ (6.3 m) | 43′–6″ (13.3 m) | |
Blaw Knox L3/L3(c) DS | 35/8192 | 125′–0″ (38.091 m) | 7.010 m | 13.258 m | 7.7 t |
Blaw Knox L3 D0 EWCO | 35/5543 | Similar to D10 | |||
Blaw Knox L3 D10 | 35/5543 | 36.347 m | 7.620 m | 9.504 m | 9.1 t |
Blaw Knox L3(c) D30 (0–10°) | 35/8192 | 11.8 t | |||
J L Eve L3 D10 STD | C.864/48 + C.864/55 | 120′–2″ (36.6 m) | 25′–51⁄16″ (7.7 m) | 32′–10″ (10 m) | |
Blaw Knox L3 D15 EWCO | 35/5543 | Similar to D30 | |||
Blaw Knox L3(c) D15 EWCO | 35/8192 | ||||
Blaw Knox L3 D30 | 35/5543 | 36.347 m | 7.620 m | 10.668 m | 11.8 t |
Blaw Knox L3(c) D30 (10–30°) | 35/8192 | ||||
J L Eve L3 D30 STD | C.864/68 + C.864/76 | 120′–2″ (36.6 m) | 25′–51⁄16″ (7.7 m) | 33′–3″ (10.1 m) | |
Blaw Knox L3 D40 EWCO | 35/5543 | Similar to D60 | |||
Blaw Knox L3(c) D40 EWCO | 35/8192 | ||||
Blaw Knox L3 D60 | 35/5543 | 37.413 m | 10.058 m | 11.582 m | 15.2 t |
Blaw Knox L3(c) D60 | 35/8192 | ||||
J L Eve L3 D60 STD | C.864/88 + C.864/102 | 127′–2″ (38.8 m) | 31′–35⁄16″ (9.5 m) | ? | |
Blaw Knox L3 DJ | 35/5543 | 38.709 m | 11.277 m | ? | 21.0 t |
Blaw Knox L3(c) DJ | 35/8192 | ||||
Blaw Knox L3 DJX | 35/5543 | Similar to DJ | |||
Blaw Knox L3(c) DJX | 35/8192 | ||||
Blaw Knox L3 DT | 35/5543 | 38.709 m | 11.277 m | ? | 21.0 t |
Blaw Knox L3(c) DT | 35/8192 | ||||
J L Eve L3 DT STD | C.864/137A + C.864/154C | 122′–8″ (37.4 m) | ca. 30′–9″ (9.4 m) | 54′–0″ (16.5 m) | |
Blaw Knox L3 DTV 45° (0–5°) | 35/5543 | 38.709 m | 11.277 m | ? | 21.0 t |
Blaw Knox L3(c) DTV 45° | 35/8192 | ||||
Blaw Knox L3 ST | 35/5543 | 37.413 m | 7.620 m | ca. 5.3 m | 12.2 t |
Blaw Knox L3(c) ST | 35/8192 | ||||
Blaw Knox L3(c) DT45 | 35/8192 | ? | ? | 18.136 m | ? |
Designation | Voltage | Phase | Earthwire |
---|---|---|---|
275 kV | Twin 0.175□″ | ||
L3 | Twin Lynx (175 mm² ACSR) | Lynx | |
L3C | |||
L3/1 | Araucaria (700 mm² AAAC) | Keziah (160 mm² AACSR) | |
L3/2 | Twin Upas (300 mm² AAAC) | ||
L3/2R |
Examples
Maybe another day.
See also
-
Blaw Knox:
- L3 pylons album (Flickr)
- L3 pylons album (Flickr)
- L3(c) DS (Flickr)
- L3 D60 (Flickr)
-
J L Eve:
- Beauly to Blackhillock (Flickr album)