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L66/L89

Overview

L66 is a UK electricity pylon (steel lattice transmission tower) series, designed by Blaw-Knox. “L66” is not a series designation: it is the number of one of the contracts to build a 275 kV power line (T(12)-BEA-L66 was the contract code for Clydesmill–Carlisle–Stella), and the towers have ended up being officially named after this contract. Contract L89 used the same towers as contract L66, and the metricated drawings describe the towers as “L66/89(c)”. [Standard Tower Types] refers to this type as “L1 (or L66)”, although an RMWeb forum comment indicates that “L1” is a low-voltage wood pole type, hence lattice towers starting at L2.

L66 is considered the first production 275 kV tower type in the UK; it followed the single-circuit L34 type, considered to be a prototype.

Although both designed for 275 kV, the little-used L66 and widely-used L3 are quite different. The phase clearances (bottom to middle, middle to top) of L66 D2 are 22′–6″ (6.9 m) and 22′–0″ (6.7 m), while for Blaw Knox L3 the clearances are only 20′–0″ (6.1 m). The top crossarms on L66 D are 18′–3″ (5.6 m) long, compared to 18′–0″ (5.5 m) for L2 D and only 13′–3″ (4.0 m) for L3 D. Yet, the ground clearance is markedly less: the height from the ground to the bottom crossarm is 63′–3″ (19.3 m) for L66 D2, 11 foot shorter than L3 D’s 74′–0″ (22.6 m). The ground clearance taking into account the insulators would suggest that L66 was designed around a 1000 foot span (distance between towers) the same as 132 kV towers, instead of the longer 1200 foot span used for L2, L3 and L6. Tentatively, National Grid’s GIS data would seem to bear this out, with Harker–Stella having only nine spans of 1200 feet or more between its 270 towers; the average span on that line is 1018 feet. Page 247 (PDF page 254) of YG-DCO-048 effectively confirms this.

Can be confused with

General data

Contractor Blaw Knox
Found England Scotland
Height (straight line tower) 115′–3″ (34.3 m)
Voltage 275 kV
Dates back to 1950–1952 (drawings)
Conductors Twin
Normal span 1000′ (305 m)
Known conductor arrangements
Designation Voltage Phase Earthwire
L66 275 kV Twin 175 mm² ACSR 175 mm² ACSR

Lines

L66/L89 was only used for two contracts:

Tower forms

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

L66 D2 STD
L66 D30 STD
L66 D60 STD

Tower details

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

D10 and D30 share the same tower body and appear to also share the same crossarms.

No DT drawing has been recovered and no DT tower is known.

On page 14 of National Grid’s The RICA Project: WP3 - Preliminary Design Considerations V1 document (dated 30th June 2021), the dimensions of L66 are shown to be as follows:

The height, and the width across the top crossarms, differ from the dimensions given on Blaw Knox’s own drawings.

Tower dimensions
Type Source Height Base width Crossarm width
L66 D2 BK35/25923 115′–3″ (34.3 m) 22′–0″ (6.7 m) 41′–0″ (12.5 m)
L66 D10 109′–0″ (33.2 m) 25′–0″ (7.6 m) 40′–0″ (12.2 m)
L66 D30
L66 D60 113′–0″ (34.4 m) 28′–0″ (8.5 m) 38′–0″ (11.6 m)
L66 DT

See also