Tower design history

Contents

• Overview
• Origin
• Increased clearances I
• Reduction in tension, increase in height
• 10° angle towers
• 85° earthwire swing
• Tension tower peak height
• Tension tower earthwire shade
• Double earthwire
• Increased angle tower earthwire shade
• New crossarms

Overview

The design of transmission towers in the UK has changed considerably since the earliest steel tower lines. These changes have been driven both by technical requirements and aesthetic decisions. Despite being utilitarian structures, British transmission tower design is distinctive and many types cannot be mistaken for those of any other country.

This page covers some of the changes in design that have taken place in the many decades of transmission tower design in the UK.

Origin

The first 132 kV transmission towers in the UK were designed in the late 1920s by the American firm Milliken Brothers who had offices in the UK. Architect Sir Reginald Blomfield selected the design out of a selection of entrants. The CEB 1928 annual report noted the following with regard this original design:

There is considerable scope for the structural designer in connection with the difficult question of determining the most suitable form of line supports, both with regard to the material to be used and the outline.

Ample proof of this is to be found by an inspection of the existing E.H.T. Transmission Lines throughout the world. Over-elaboration, for whatever reason, is to be deprecated, and true adherence to the strict engineering requirements will seldom result in a structure which will offend the eye. The economic factor has perforce to be fully weighed, and after exhaustive investigations of the claims of steel, reinforced concrete, narrow base, wide base and Eiffel towers, the Board, in consultation with Sir Reginald Blomfield, R.A., settled on a wide base straight line tower of the form depicted in Figs. 1 and 1A opposite.

Figures 1 and 1A in the report depict CS PL1. “Narrow base” describes the German preference, which in the UK was adopted but only for 33 and 66 kV. The meaning of “Eiffel towers” is uncertain, but it may refer simply to the tower outline rather than any involvement with Gustave Eiffel (who was already dead by this point). Transmission towers do exist in Ottawa that follow the same general outline as the Eiffel tower:

• Line tower with deviation
• Angle tower with suspension insulators

The section of line above runs along McRae Avenue, Laurentian View, Ottawa, Canada. This Canadian design simulates a continuous curve from the narrow peak to the wide ground-level outline, while 1920s to 1940s UK towers favoured a “bend line” or “kink line”, a sharp change in outline angle at a specific height, often at the level of the middle crossarm. More subtle outline shapes appeared in the 1950s in a manner more like the Eiffel tower, sometimes with a reverse curve where the tower tapers towards the top.

In any case, the Millken design used an excessively large base, a characteristic seldom repeated (and even then chiefly in Scotland where it may have been chosen for robustness against the elements). The South East England region used different towers (GEC and Callender’s) with considerably narrower profiles, and the various tower suites that followed continued with the narrower base dimensions.

Increased clearances I

The suite of towers designed for the Central Scotland region were largely used only in this region; they were also found in south-east England (in particular Bedford to Little Barford) and later in County Antrim as the angle towers in Milliken T169. A revised set of tower designs were created for the remainder of Great Britain (except South East England and Mid-East England). For the suspension towers, the bottom crossarms were raised by 6 inches, and the top crossarms were raised by another six inches, giving a foot of extra height. The tension towers were six feet taller, added to the height of the top crossarms. The suspension tower peaks were braced, and the S10 tower was completely redesigned. A corresponding D10 tower was added for regions that desired it. So far, no explanations are known for any of these changes. South East England and Mid-East England used different towers and these retained the 12 foot crossarm spacing of the Central Scotland type.

Reduction in tension, increase in height

[Nicholls] (1946) notes that steel-cored aluminium (SCA) lines were originally strung with a maximum working tension of 8000 lbf at 22 °F (under wind and ice conditions) equivalent to 3450 lbf at 40 °F in still air (15.3 kN at 4 °C). This load proved excessive as it resulted in strands of aluminium breaking under load due to vibration. Vibration dampers of the Stockbridge type were fitted to the conductors to halt the damage.

Going forwards, the conductor tension was reduced to 2560 lbf (11.4 kN) at 40 °F; vibration dampers were still fitted as a precaution and have continued to be used on tower lines ever since. The reduction in tension increased the slack in the conductors, resulting in an increase in sag at maximum operating temperature from 22′–0″ (6.7 m) to 27′–3″ (8.3 m). This was addressed by new tower designs where the height of the bottom crossarm was increased accordingly. With CEB towers this change seems to have started with CE PL3 (1936) and continued with PL4/WGR (1936), PL7 (1938) and PL16 (1944) amongst others.

The overall tower heights vary for other reasons, each of which is explained separately.

10° angle towers

Each tower series contains a number of angle tower types. For 132 kV, North Scotland preferred 20° and 60°, while the CEB preferred 30°, 60° and 90°. In some regions, 10° angle towers were also included. In the early years of the grid, 10° angle towers typically used suspension insulators, often attached to large brackets (rather than the droppers of line towers). This practice has continued outside of the UK, but within the UK it did not survive long. 10° towers became standard within the CEB in the mid 1930s (from CE PL3 onwards) and in all cases used tension insulators just as with higher deviation angles.

The reason for choosing tension versus suspension for 10° angles isn’t known.

85° earthwire swing

A very visible change took place in the mid-1930s. Early towers used triangular pyramid peaks. On suspension towers this posed a problem for the earthwire: if it were to swing too far it would collide with the tower peak. Newer designs opted for flat-topped peaks that allowed the earthwire to swing up to 85° to either side of the centre position. The original design of MIDESCO 66 kV tower, Blaw Knox K1373, used a pyramid peak, but only one line (Spondon to Willoughby on the Wolds) was built to this design. The remainder of the lines were built with earthwire peaks allowing 85° earthwire swing. The first 132 kV type known to follow this pattern was CE PL3 (1936).

The original two light construction types in north Scotland (J L Eve C772 and Blaw Knox T1498) reverted to pyramid peaks as neither type supported an earthwire. When each type was adapted to carry an earthwire (J L Eve C1415 and Blaw Knox T2639), the peaks returned to flat-topped. T2639 also included a peak design described as “alternative transverse face for special earthwire clearance”, although the standard peak already supported 85° swing; the special earthwire clearance type has yet to be observed.

Tension tower peak height

Angle towers were typically designed with either a 4-foot or a 6-foot peak. For SEE PL1(b), Callender’s opted for 6-foot peaks, while all other designs used 4-foot peaks. The revised Milliken design however incorporated 6-foot peaks in specific cases, including some terminal and junction towers. In Skewen in Wales there is an SWE PL1 D90 with a 6-foot peak.

Tension tower earthwire shade

The original tower suites provided very little earthwire shade on the tension towers.

CE PL3 appears to be the first type designed to accommodate the increase in sag resulting from reduced conductor tension, dating to 1936. CE PL4 was designed the same year; it appears to be identical to CE PL3 except that the earthwire on angle towers was raised up to provide much better earthwire shade.

Double earthwire

From around 1936 it was common for new tower lines to carry two earthwires for the first mile out from the substation at each end. This was done to increase the level of protection for the substation against lightning strikes. New 132 kV tower series typically included both single and double earthwire towers. Double earthwire was less common on 33 and 66 kV lines; examples include the unidentified type used for Willoughby-on-the-Wolds to Newark-on-Trent and for Rannoch power station to Bridge of Orchy, Blaw Knox K1373 and K4611, and the Preston Corporation type. [Nicholls] (1945) reports that double earthwire was introduced in 1938 but the towers for CE PL3, which included double earthwire, were designed in 1936.

Nicholls also gives 1938 as the year that the double earthwire arrangement was redesigned, moving the twin earthwires much further apart. This ties in with SEE PL7, the type he used as an illustration, which was designed in that year. Blaw Knox would continue to use the old arrangement with K5735 for NWE PL7, adopting the new arrangement around 1945 for the SEE PL16 towers.

Improvement in substation technology made double earthwire obsolete with time, and it was not used for double circuit 275 kV towers (L66, L2, L3 etc) or for the later 132 kV types (in particular L7 and L4(m)). Many PL16 lines were built with all single earthwire.

Increased angle tower earthwire shade

The early tower designs offered limited earthwire shade on angle towers, especially those with only 4 foot peaks. CE PL3 (designed in 1936) was significant for being (seemingly) the first type designed for reduced conductor tension (and thus greater ground clearance) and support for double earthwire. Earthwire shade on the PL3 angle towers remained poor, with around a 73° shielding angle for the D10. The same year, the towers were redesigned with taller angle tower peaks, giving a more respectable shielding angle of around 45° for the D10.