Blaw Knox K1373 and K4611

Contents

• Overview
  • General data
• Tower forms
  • Single circuit
  • Double circuit
  • Three circuit
• See also

Overview

Blaw Knox K1371 is an unusual suite of towers. The contract produced two separate sets of towers: 66 kV single and double circuit towers, and three-circuit towers that carry two 66 kV circuits and one 33 kV circuit below them. The appearance of the crossarms on the double circuit line towers earned the design the nickname “Christmas Tree”: the middle crossarms are not extended in length.

K1371 was used for various schemes in Derbyshire and Nottinghamshire. A related Blaw Knox K4611 contract covered Willoughby–Queniborough in Leicestershire using a subset of the K1371 towers. Although the double earthwire designs are somewhat different in K4611, the K4611 designs can also be found on lines that appear to fall under K1371.

K1371 was designed for 66 kV, but no lines presently operate at that voltage. Some lines have been uprated to 132 kV, perhaps possible due to what seems to be very generous clearances in the tower design (albeit smaller than those of regular 132 kV towers). The design of the K4611 DC-type tower for Willoughby–Queniborough was amended with outriggers for 132 kV operation but in reality this line is only strung for 33 kV; the outriggers can however be seen on other 132 kV lines.

Neither K1371 nor K4611 uses standard tower designations. Instead, a single letter denotes the tower body up to roughly the bend line and covers every purpose fulfilled by that tower body; K4611 added a “D” prefix to this letter (“A” → “DA” and so on). The tower body above the bend line—including crossarm spacing—can vary without affecting the designation; in standard nomenclature the tower type designation describes the function of the tower without regard to which components may be shared with other tower types. The designations are as follows (standard double circuit designations in bold type):

The tower angles are derived from the stress sheets for K1373 on the basis that the angles chosen for the working loads are the maximum angles intended for each tower type. The resulting angles tie in with other 33 kV and 66 kV tower types.

General data

Contractor	Blaw Knox
Found
Height (straight line tower)	77′–0″ (23.5 m) (type A double-circuit line tower) 76′–0″ (23.2 m) (type S three-circuit line tower)
Voltage	66 kV, 66 + 33 kV (design) 132 kV (uprated lines)
Dates back to	1935–37 (drawings)
Known conductors	Single
Circuit count	Single-circuit 66 kV Double-circuit 66 kV Double-circuit 66 kV plus single-circuit 33 kV (three total) Double-circuit 132 kV (uprated lines)

Tower forms

Single circuit

Standard single circuit towers. These are said to be from K4611, but no material has been recovered; the diagrams below are traced from Google Street View imagery and scaled according to the equivalent double circuit towers. The tower designations are not known, nor are the rated deviations.

Although there is no corroboration to the claim that these are K4611, these towers being K4611 would offer an explanation for the K4611 tower designations. K1373 type A is type DA in K4611, making way for a K4611 SA for single circuit, with SB and SC likewise.

Double circuit

Standard double circuit towers. Designed for 66 kV but capable of operating at 132 kV.

The line from Willoughby to Ratcliffe-on-Soar uses a revised tower peak resembling that of PL16. This appears to correspond with the drawing depicting a revised peak that allows for 85° earthwire swing.

Neither terminal type is presently known to have survived. The standard A-type double earthwire type is also not known. Transposition towers remain on the Willoughby to Ratcliffe line.

The B-type transposition tower rigging is based on Google Street View depictions of “Baby PL16” 66 kV transposition towers; these use a crossarm arrangement virtually identical to K1373 and the rigging set-up corresponds sufficiently with the K4611 tower schematics.

K4611 drawing XM1631-50A from 1958 covers the addition of outriggers to the DC-type tower to support 132 kV uprating. Such outriggers are not known from that line (which is only strung for 33 kV) but it is known from other lines that were uprated to 132 kV. Outriggers are not used on all C-type towers; as type C covers all angles from around 23° to 67°, the outriggers only appear to be needed on larger angles.

Several custom towers are presently known. The line from Morley to Grassmoor now terminates part-way along the route at Belper. A B-type (20°) tower has been converted to a terminal tower by the addition of bracing to the crossarms. The B-type body is not designed for termination but as this is a 33 kV line the conductors may be of a lower weight that can be handled on a B-type tower. This tower is depicted below, with its 20-foot height extension.

The first two towers on the line from Willoughby to Queniborough have unusually long top crossarms that are not covered in any of the recovered drawings. The tower functioning as a terminal tower has a 60° body instead of a terminal body (minor difference in bracing) and may have been converted. This is one of the few towers found to have a lightning conductor fitted. K1373 and K4611 towers were all designed to take lightning conductors but they were only scheduled to be fitted to selected towers. Rannoch–Abernethy towers are another type that use lightning conductors.

There is also a 132 kV line from Willoughby to Ratcliffe-on-Soar. The terminal tower at Willoughby is a PL16, followed by a double earthwire C-type tower, a variant not known from any recovered drawings.

Three circuit

These towers carry three circuits, by design 66 kV double circuit above a single circuit of 33 kV. No lines are known now that use the 33 kV crossarms for anything.

Full transposition uses M-type towers. There are also S-type towers used for transposing only the 33 kV circuit, suggesting that 33 kV was held to need transposing more frequently along the power line than 66 kV.

The following towers are of unclear purpose:

The four-tier type can be found along Spondon–Morley; the type with special crossarms may no longer exist. Spondon–Morley is only strung as double-circuit 33 kV.

“Derby & Notts” tower no. 1 is a custom design with bizarre crossarms. The top crossarm is square-ended on top and pointed below. The top (earthwire and top phase) and middle crossarm levels have auxiliary crossarms but the bottom level does not. The bottom crossarm is pointed but the middle crossarm is square-ended and splayed on one side.

The location of tower no. 1 is not known, but a tower matching this design exists at Spondon substation, Derby. Although Google Earth is not capable of modelling transmission towers properly, the four crossarm planforms are clearly visible and match the drawings (including the mismatched top and bottom planes of the top crossarm and splayed middle crossarm), although there are presently no auxiliary crossarms on that tower.

See also

• A-type tower, double earthwire, downrated to 33 kV, Willoughby to Queniborough line (Flickr)