Blaw Knox K1373 and K4611

Contents

• Overview
  • General data
  • Tower type designations
• Lines
• Tower forms
  • Single circuit (K4611)
  • Double circuit (K1373 and K4611)
    • Double-circuit terminal towers
      • K1373 double-circuit terminal towers
      • K4611 double-circuit terminal towers
    • 132 kV uprated towers
  • Three circuit (K1373)
  • Examples
• Tower details
• Examples
• See also

Overview

Blaw Knox K1373 is an unusual suite of towers. The contract produced two separate sets of towers: double circuit 66 kV, 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”: each crossarm extends two feet further than the one above. The K1373 drawings primarily indicate that it covered Morley to Grassmoor. Spondon to Willoughby is briefly mentioned, and the dual voltage towers were used between Spondon and Morley but not listed as such on the drawings.

A related Blaw Knox K4611 contract covered Willoughby–Queniborough–Kibworth in Leicestershire. K4611 uses a different design of tower peak, although it’s uncertain to what extent this alternative peak design was included in K1373 (so far only found as a special version of the line tower).

K1373 was designed for 66 kV, but no lines presently operate at that voltage. Most 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; these outriggers can however be seen on other uprated 132 kV lines using the same towers. The remaining lines have been de-rated to 33 or 11 kV.

K4611 is also highly unusual in having a dedicated single-circuit transposition (SX) type; this is the only tower type that uses a standard designation. No other tower series is presently known to have an SX type.

General data

Designed for	MIDESCO (Midland Electric Supply Company)
Design contract	Blaw Knox K1373 and K4611
Found
Height (straight line tower)	77′–0″ (23.5 m) (type A double-circuit line tower) 79′–4″ (24.2 m) (type DA double-circuit line tower) 76′–0″ (23.2 m) (type S three-circuit line tower) 68′–3″ (20.8 m) (type SA single-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)
Normal span	700 feet

The wind span for the K1373 and K4611 suspension towers is 700 feet (and 1000 feet for the angle towers); the only tower type for which the span is presently known is K4611 SA, where 700 feet is also given.

Tower type designations

Neither K1373 nor K4611 use 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 prefix to this letter to indicate whether the tower was single (“S”) or double (“D”) circuit. 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.

Lines

• Spondon to Morley (K1373): originally dual 66 kV + single 33 kV, now de-rated dual 33 kV on the 66 kV crossarms
• Morley to Grassmoor (K1373): originally double circuit 66 kV, now split into:
  • Double circuit 33 kV between Morley and Belper
  • Demolished from Belper to Alfreton
  • Uprated to double circuit 132 kV between Alfreton and Grassmoor
  • Extended from Grassmoor to Chesterfield on PL16 towers
• Willoughby on the Wolds to Spondon (K1373): uprated to 132 kV from Willoughby to Ratcliffe-on-Soar, and demolished from Ratcliffe to Spondon; the line towers use 85° earthwire swing peaks
• Willoughby on the Wolds to Queniborough (K4611): de-rated to 33 kV and partially demolished
• Queniborough to Kibworth: K4611-style single circuit towers; now de-rated to 11 kV and demolished north of Kibworth
• Spondon to East Kirkby (Annesley), now split at Loscoe: uprated to 132 kV; K4611-style towers
• East Kirkby to Staythorpe: uprated to 132 kV; K4611-style towers
• Staythorpe to Hawton (Newark-on-Trent): uprated to 132 kV; K4611-style towers

A single K4611 tower is used at Corby to terminate a 33 kV steel tower line.

Tower forms

Single circuit (K4611)

K4611 single circuit towers (no single circuit in K1373):

Willoughby terminal tower 1B no longer exists.

Double circuit (K1373 and K4611)

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.

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.

A tower held to be the former terminal tower (now a through tower to a PL16 DT) at Annesley substation appears to be a DC with a custom top hamper. The same type is found in Newstead, Stanley Common and Loscoe for angles above 60°. The deviation angles are 67° for Loscoe and 72° for Newstead and Stanley Common. The higher-angle diagram above is a Street View-derived approximation, with weak assumptions made about the vertical dimensions owing to perspective distortion. The terminal tower at Annesley is possibly a DT45.

The existence of a combined transposition–angle tower was unexpected; this diagram is traced from a photograph from Zak Martin, of a tower found in Stanley Common.

Double-circuit terminal towers

K1373 double-circuit terminal towers

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 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.

K4611 double-circuit terminal towers

Willoughby substation:

Terminal tower 1A 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.

Terminal tower 1C no longer exists.

South Croxton substation (Queniborough):

132 kV uprated towers

The clearances on K1373 and K4611 towers are somehow great enough to allow the towers to be uprated to 132 kV operation. For larger angles, however, the C and DC towers require outriggers to maintain jumper clearances. K4611 drawing XM1631-50A from 1958 covers the addition of outriggers to the DC-type tower to support 132 kV uprating; a K1373 C tower at Willoughby is also adapted in this fashion.

Three circuit (K1373)

These towers carry three circuits, by design 66 kV double circuit above a single circuit of 33 kV. The 66 kV portion of the tower is symmetrical, with four phases across the middle crossarm tier, two per side. The 33 kV level is asymmetric, with two phases to one side and the third phase on the opposite side. The unusually wide spacing of the 33 kV phases is presumably to avoid conductor clashing rather than any need for such a wide air gap: the spans are longer than those of 33 kV wood pole lines.

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 Spondon terminal 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 following towers are of unclear purpose:

Examples

Out of range.

Tower details

See dimensions on the site guide page for a key to the tables.

For some towers the dimensions vary between drawings:

1. Depending on the drawing, K1373 A top crossarm to peak is given as 4′–0″ foot (wire clearance diagram and GA 85° earthwire swing), 4′–6″ (stress diagram), 5′–3″ (GA) and 5′–6″ (ED); for double earthwire, it’s 4′–10″. It’s meant to be 4′ to the earthwire position, but some drawings take this to be the height to the underside of the peak cap.
2. There is conflicting data for all the vertical clearances of K1373 S.

Examples

Out of range.

See also

• A-type tower, double earthwire, downrated to 33 kV, Willoughby to Queniborough line (Flickr)
• Spondon power station, showing Spondon–Morley tower 1