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PL16

Caveat

Due to the limits of the available data, details on the pylon pages are incomplete and in places inaccurate or incorrect. Pages typically only cover standard tower types and common variants. This information is provided for enthusiasts and those of a curious mind and is not authoritative. Additional source material is welcome, in particular drawings. Most data and illustrations provided are sourced from industry-related material. See the sources page for a list of sources and the series page for a list of series.

Contents

Overview

“PL16” is an unclear term covering an unspecified assortment of 132 kV UK electricity pylon (steel lattice transmission tower) types, including the basic double circuit suite that is found extensively across the UK. The name originates from the South West England and South Wales (SWE) contract PL16 for which Blaw Knox provided schematics for a suite of double circuit towers. The same tower design was used in Scotland not long afterwards, compliant with the STL1 specification. It’s not known whether the STL1 towers were in any way different from the L132 ones used for SWE PL16.

For Scotland, Blaw Knox also designed single circuit towers, as well as a different design of line tower. These types are described in the [Tower Bible] merely as “Scottish”, whereas the remainder are called “SWE PL16”. However, a copy of CEGB document 35/5522—which does not name the tower type—has been found hand-annotated “Ref PL16 Single Lynx” and this includes only the Scottish-style line tower, the form used by most PL16 lines. This chart shows only the six basic double circuit, single earthwire towers: D2°(S), D10°, D30°, D60°, D90°, DT and (in name only) DT90°. This chart dates to March 1962, when PL16 was still the UK’s standard 132 kV type.

The scope of the term “PL16” is unclear. The term is alleged to be a catch-all for other 132 kV types, although no clear details of this claim have ever emerged, including no examples of any types from other suites conclusively shown to be classified under the PL16 umbrella. Contract K1124/K1124B is classified by National Grid as “PL16” and includes the double circuit towers; this does not prevent other contracts being placed under PL16. Thus far there is no actual evidence to place any single circuit towers under PL16.

Blaw Knox’s successor design to PL16 was L4(m), introduced in the 1970s; L4(m) replaced PL16 as the UK’s standard 132 kV tower type.

SWE PL16

In the 1940s, the Milliken Brothers branch of Blaw Knox was contracted to design towers compliant with the CEB L132 and STL1 specifications. South West England and South Wales (SWE) scheme PL16 was a single-circuit power line from Andover to Bournemouth via Salisbury, built on double-circuit towers. This project was scheduled to commence on the 29th of September 1944 for completion on 30th September 1946. The contractor for the line was Callender’s, later BICC, but the towers were a Blaw Knox design. It is not known whether Blaw Knox specifically designed the tower suite for the PL16 scheme, but this is the scheme by which the suite has come to be known. The towers appear to be a derivative of the earlier Blaw Knox K5735 suite.

STL1

The Blaw Knox STL1 type is both single and double circuit. The Blaw Knox L132 type so far is only known to be double circuit, although a few single-circuit towers do exist in England. In most cases the STL1 and L132 Blaw Knox towers appear to be the same design. However, the two types used different line towers. The STL1 design of line towers came to be used outside of North Scotland and were renamed from D2° and DD2° to D2°S and DD2°S to differentiate them from the English types; the Scottish design of line towers became the standard line tower types in PL16.

Contracts

The overall family of towers is defined by a number of related Blaw Knox contracts. Exactly what the term “PL16” now covers is unclear. The SWE PL16 contract was double circuit only.

The table below lists the known contracts, along with tower types known to be in each one. The lists of tower types under each contract are limited to the extent of recovered material; for example, K8323 surely contained the DT and DDT towers but those drawings were not recovered.

PL16 itself appears to be covered within two contracts: K8323 (SWE PL16) and K1124/K1124B. The difference between the two contracts is not known. K1124B appears to contain additional types, and is where the Scottish-style line towers were placed when they were integrated into PL16.

Contract Years Tower types covered Description
K8323 1944–45 D2, DD2, D10, DD10, D30, DD30, D60, DD60, D90, DD90 SWE PL16
K9857 1945–47 D2 and DD2 (Scottish type), D10, DD10, D30, DD30, D60, DD60, D90, DD90, DDT NoSHEB STL1
K1201 1947–50 Standard formation towers: S2°, S10°, S30°, SS2°, SS30°, SST, ST90?
(extent of drawings recovered)
NoSHEB STL1
K1420 1947–1950s Flat formation towers: S10, S30, S60 NoSHEB STL1
Flat formation: ST CEA 132 kV single circuit lines
K1124 ? D2S, DD2S, D10, DD10, D30, DD30, D60, DD60, D90, DD90, DT, DDT, substation gantries PL16
K1124B 1948 K… D2° as D2°S
1955 D90 junction “PL16” (hand-annotated)
1960 DT through-line

J L Eve PL16

Several tower suites were built according to the CEB L132 specification. PL16 and Eve 0.175 were designed for 0.175″ SCA (now 175 mm² “Lynx”) conductors. There was also the “L16” type which takes heavier 0.4″ SCA (now 400 mm² “Zebra”) conductors. The term “J L Eve PL16” is sometimes seen, and confusingly both enthusiasts and SSEN (see [Elmwood-Glenagnes]) use it to refer to the L16 type that is not compatible with the Blaw Knox towers. If anything, “J L Eve PL16” should refer to Eve’s 0.175 conductor type that in some instances is actually intermixed with Blaw Knox “PL16” towers.

General data

Designer Blaw Knox, Milliken Brothers branch
Known subtypes SWE PL16, STL1 “Scottish”
Height (straight line tower) 74′–3″ (22.6 m) (STL1 S2)
86′–9″ (26.4 m) (PL16 D2)
86′–3″ (26.3 m) (D2S)
Voltage 132 kV
Dates back to 1944–45 (drawings)
Conductors Single
Circuit count Single, double
Design conductor 0.175″ SCA (now 175 mm² Lynx ACSR)
Normal span 900′ (274 m) (original)
1000′ (305 m) (revised)
Known heights STD
E10 (+10′)
E20 (+20′)
E30 (+30′) (not listed for DT or DDT)
E40 (+40′) (only listed for DT, DDT and SWE D2)

Note that no height options were given for D90.

Known conductor arrangements
Designation Voltage Phase Earthwire Rating Lines
132 kV Lynx (175 mm² ACSR) 89 MVA BG, BT, AL
PL16 175 mm² ACSR 70 mm ACSR

Can be confused with

Lines

The PL16 suite (including Blaw Knox STL1 in Scotland) was used extensively within the UK; it would not be practical or sensible to list out all PL16 lines. Some examples however are given below:

Tower forms

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

Double circuit

The dashed peaks on DD10, DD30, DD60 and DD90 represent the earthwire changeover (EWCO) types, where the double earthwires end and the single earthwire begins. DD90 EWCO was not originally part of the suite but is confirmed to exist.

SWE PL16 D2
SWE PL16 DD2
K9857 D2 (PL16 D2S)
K9857 DD2 (PL16 DD2S)
SWE PL16 D10
SWE PL16 DD10
SWE PL16 D30
SWE PL16 DD30
SWE PL16 D60
SWE PL16 DD60
SWE PL16 D90
SWE PL16 DD90
D60 Junction (unconfirmed)
K1124B D90 Junction
SWE PL16 DT
SWE PL16 DT90
SWE PL16 DDT
SWE PL16 DDT90
K1124B DT through-line

According to the L132 specification, the first route mile of a PL16 line uses double earthwires. The Tower Bible diagrams note that the tower on which the twin earthwires revert to a single earthwire is known as an “earthwire changeover” tower; such towers have the twin side peaks for the earthwire pair plus a normal single earthwire peak. In rare cases, the peak is omitted from the DT tower; the earthwire attaches below the peak but the peak is normally fitted regardless.

The CEB L132 specification does not allow for single earthwire terminal towers: the only permitted terminal towers are SST, SST90°, DDT and DDT90°. However, in practice, single earthwire DT towers are widespread and the dominant configuration of PL16 terminal tower.

Junction towers

PL16 junction towers are derived from the D60 and D90 towers and have symmetrical, square-ended crossarms. PL16 junction towers divide a double-circuit line into two single-circuit lines in a T formation. The reason for choosing a D60 vs D90 junction tower has not yet been determined; both seem to be used identically.

Towers that appear to be D60 Junction can be seen from Trevenson Moor road in Pool, Cornwall; Shoulton Lane in Hallow (near Worcester) and from Whitehaugh Road where the short line to Hawick substation tees off from Harker–Galashiels outside Hawick, Scotland.

D90 junction towers can be found at 53.707° N 2.412° W and 53.626° N 2.411° W. The former, on Jackson Heights Road near Pickup Bank, splits a 132 kV double-circuit line from a substation in south Blackburn into single-circuit NWE PL1 and L4(m) lines to Union Road Substation in Bolton and Blackburn Substation respectively; it has only six arms. The latter, on Chapeltown Road (B6391) between Bolton and Chapeltown, is between the first DJ tower and Union Road Substation and further tees off the line to a PL16 double-circuit line to Rochdale Substation. The latter tower has the auxiliary crossarms fitted but these face away from the tee and appear to be unused, possibly because of the split-outs to single-circuit lines only.

The D90 junction diagram above was adapted from the D90 and DT types to approximately match the actual towers and is pending being redrawn from the official drawing with accurate dimensions. D60 junction is a real type but the exact form is not conclusively known.

Single circuit

Single circuit towers were used for STL1 contract K1201. It is not known whether any additional contract numbers contained the single-circuit types. In the Tower Bible these types are listed simply as “Scottish”. Single circuit towers are so far not known to exist in the PL16 suite although they have occasionally been used in England.

K1201 S2
K1201 S60
K1201 ST
K1201 SST

T1073 rotated base towers were used on the Andover–Thatcham 132 kV line, according to the S30 drawing. They are also used at a PL16-under-L6 crossing at Burton Hastings near Nuneaton, England. These are presently being included on this page as they are not yet known elsewhere.

T1073 S30 (rotated base)
T1073 S60 (rotated base)

Examples

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PL16 S30 (miniature height)
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PL16 D2, Bricket Wood
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K9857 D2/PL16 D2S
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K9857 DD2/PL16 DD2S
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PL16 D10
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PL16 DD10 EWCO
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PL16 D30
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PL16 DD30
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PL16 DD30 EWCO
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PL16 D60
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PL16 DD60; photo courtesy Ian McAulay (CC-BY-NC)
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PL16 DD60 EWCO
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PL16 D90
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PL16 DD90
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PL16 D60 Junction (unconfirmed); photo courtesy Ian McAulay (CC-BY-NC)
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PL16 DT
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PL16 DDT
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PL16 DDT90 (left) and DDT; photo courtesy Ian McAulay (CC-BY-NC)
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PL16 DT through-line

Rail adaptation

PL16 towers with only two crossarms carry dual circuits from the Chapelcross Substation at the decommissioned Chapelcross nuclear power station (55.016° N 3.230° W) to the 132-to-25 kV West Coast Main Line railway substation outside Ecclefechan at 55.066° N 3.285° W. Each circuit is two out of the three 132 kV phases, forming 132 kV phase-to-phase that is then transformed to 25 kV for one road of the WCML. The bottom crossarm is unnecessary and thus omitted from each tower. Since the crossarms provide lateral bracing, the intra-tower portion is still present.

PL16 D2S, dual circuit two phase
PL16 D10, dual circuit two phase
PL16 D60, dual circuit two phase

Another approach, using L4(m) towers, was to simply leave the middle crossarms unpopulated.

Tower details

Crossarm width is the total width across the widest crossarm pair (double circuit) or across the longest crossarm on each side (single circuit).

Type Source Height Base width Crossarm width
STL1 S2 K1201 drawing 74′–3″ (22.6 m) ca. 11′–10″ (3.6 m) 24′–6″ (7.8 m)
STL1 S10 Tower Bible Chart 51173 missing
STL1 S30 Tower Bible Chart 51174 missing
STL1 S60 Tower Bible Chart 51175 missing
STL1 ST Tower Bible Chart 51177 missing
SWE PL16 D2 Tower Bible 86′–9″ (26.4 m) 13′–9″ (4.2 m) 30′–6″ (9.3 m)
STL1 D2/PL16 D2S Tower Bible 86′–3″ (26.3 m) 12′–6″ (3.8 m) 30′–6″ (9.3 m)
SWE PL16 DD2 Tower Bible 86′–9″ (26.4 m) 13′–9″ (4.2 m) 36′–6″ (11.1 m)
STL1 DD2/PL16 DD2S Tower Bible Chart 51111 missing
SWE PL16 D10 Tower Bible 84′–3″ (25.7 m) 17′–0″ (5.2 m) 27′–6″ (8.4 m)
SWE PL16 DD10 Tower Bible Chart 51118 missing
SWE PL16 D30 Tower Bible 84′–3″ (25.7 m) 20′–0″ (6.1 m) 30′–6″ (9.3 m)
SWE PL16 DD30 Tower Bible Chart 51126 missing
SWE PL16 D60 Tower Bible 84′–3″ (25.7 m) 21′–6″ (6.6 m) 32′–6″ (9.9 m)
SWE PL16 DD60 Tower Bible Chart 51136 missing
SWE PL16 D90 Tower Bible 84′–9″ (25.8 m) 22′–6″ (6.9 m) 38′–6″ (11.7 m)
SWE PL16 DD90 Tower Bible Chart 51142 missing
SWE PL16 DT Tower Bible 89′–9″ (27.4 m) 22′–6″ (6.9 m) 50′–0″ (15.2 m)
PL16 DT90 Observation Undocumented
SWE PL16 DDT Tower Bible 89′–9″ (27.4 m) 22′–6″ (6.9 m) 50′–0″ (15.2 m)
PL16 DDT90 Observation Undocumented

Examples

The following examples represent opinion only. No tower types were obtained from any official material and thus all designations are speculative.

Elstree–Sundon

Elstree–Sundon was a 132 kV link between Elstree Substation and Sundon Substation. Currently the section between Redbourn and Pepperstock is de-energised, leaving two separate lines: Elstree Substation to Piccotts End in Hemel Hempstead (51.774° N 0.484° W) by way of Bushey Mill Substation in Watford (this is a spur that originates at a DJT tower in Redbourn), and Sundon Substation to Luton South Substation, another spur originating near Pepperstock just south of Luton. PL16 DD towers are used at Piccotts End, as seen at Welham Green only properly wired with both earth conductors; the line terminates on a PL16 DDT.

Elstree–Sundon appears to have been constructed using SEE PL1a towers, although PL4/WGR, PL16, L4(m) and L7 towers have been substituted over time. Additional photos from the network can be seen on the respective tower pages.

Luton

To the south-west of Luton there is a single PL16 D2 tower, in between L7 and L4(m) towers. This is on the line between Sundon Substation and Luton South Substation.

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PL16 D2
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PL16 D2 detail, obscured by trees due to lack of a good vantage point
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PL16 D2 side view

Holtsmere End Lane

The Elstree–Sundon line between Elstree Substation and Piccotts End crosses under the 400 kV Sundon–Elstree L2 route in farmland between Holtsmere End Lane and Gaddesden Lane, between Redbourn and Hemel Hempstead, Hertfordshire, England, 51.792° N 0.4337° W. Instead of splitting the two 132 kV circuits out onto low-height gantries and rejoining them on the far side of the 400 kV line, a short 400 kV span between two extended-height L2 towers are used to maintain clearance between the two routes.

The (comparatively) sunny photos were taken on 28th May 2022 from a public footpath near Holtsmere End Lane. The cloudy photos were taken on 25th September 2021 from the adjoining Gaddesden Lane.

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View of the west side
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West side close-up; note the tension insulators on the bottom crossarms of the closer PL16 tower
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East side view, from Gaddesden Lane
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PL16 D2S straight line tower
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PL16 D10 tower
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PL16 D10 tower close-up

Garston

Garston is on the Elstree–Sundon 132 kV line between Elstree Substation and Piccotts End. SEE PL1a, L7 and PL16 towers can be found in Garston. The line passes through Bushey Mill Substation via a PL16 DT, with downleads allowing power to be tapped off for Watford and Bushey.

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PL16 D10
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PL16 D10 close-up
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PL16 D10 side
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PL16 D10 seen from below

Sundon–Aylesbury East

Chalton

At Chalton in Bedfordshire, the Sundon–Aylesbury East 132 kV line (route PU) forms a diamond crossing (51.931° N 0.508° W) under two 400 kV lines at once: Sundon–East Claydon (route 4YJ, L6 triple conductor) and Sundon–Elstree (route ZA, L2 twin conductor). The diamond crossing starts and ends on DD10 earthwire changeover towers, and miniature S30 towers are used as the crossunder towers. The the mini S30s look to be the same size as the special S60 in the enhanced tower bible, making them 51′–8″ (15.7 m) rather than the standard height of 72′–3″ (22 m). Within the diamond crossing is an L6 D60 and an L2 D60.

There is a public footpath that passes right underneath the crossing. The photographs were taken on 23/09/2023.

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Chalton diamond double crossing seen from a high vantage point
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PL16 line (foreground) passes under the L2 (background) and L6 (middle) 400 kV lines
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First DD10 earthwire changeover tower; D2S towers can be seen in the distance
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One of the two miniaturised PL16 S30 used as crossunder towers for the diamond crossing
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Second miniaturised PL16 S30
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Second DD10 earthwire changeover tower

Sundon substation

The photographs were taken on 23/09/2023.

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PL16 DT tower PU1 at Sundon Substation
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Tower PU1
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Tower PU1

Elstree–Hatch End Grid

Starting from Elstree Substation, the third tower (including the terminal tower) of Elstree–Hatch End Grid is a symmetrical DD60 bearing a deviation of only 6°! The photographs were taken on 26/08/2023.

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Symmetrical DD60
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Symmetrical DD60
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Symmetrical DD60

Elstree–Rye House

Colney Street

The photos below show part of the Elstree–Rye House 132 kV route, alongside a public footpath in fields to the east of Colney Street, Hertfordshire (51.706° N 0.311° W). The next part of this route eastwards uses L4(m) towers and can be seen on the L4(m) page. Beyond London Colney the route returns to PL16 towers. These photos were shot on 12th March 2022.

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Facing WNW towards the Tarmac concrete and asphalt works
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Facing ENE; this route will change to L4(m) after a few spans, then return to PL16 after London Colney
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Facing ENE directly along the route
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PL16 D2S tower
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PL16 D2S tower

Welham Green

Elstree–Rye House passes through Welham Green, Hertfordshire, England, 51.739° N 0.217° W. This part of the route is a mixture of D and DD. The DD towers are only strung as single earth, raising the question of why they were selected; the line may have since been reconfigured. The photographs were all taken on 7th August 2022, and are shown below in roughly the same order as the route itself.

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PL16 DD2S towers crossing farmland
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PL16 DD2S side view
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PL16 DD2S and DD30
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PL16 DD30 connected to a D2S
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PL16 DD30 earthwire changeover tower (three earthwire peaks)
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PL16 DD30 outer side view
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PL16 DD30 detail
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PL16 D30 (foreground: base of a D2S)
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PL16 D30 and a D2S in the distance
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PL16 D30
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PL16 D2S transverse face
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PL16 D2S transverse face detail
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PL16 D60
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PL16 D60 inner side view

Welwyn Substation

Welwyn Substation (51.788° N 0.210° W) is located to the south of Welwyn Garden City, north of Hatfield, in Hertfordshire, England. It terminates the other 132 kV spur on the Elstree–Rye House route, with a PL16 DT tower. Although generally a PL16 network (the spur to St Albans is JL Eve L16 or L55), the next tower from the PL16 DT is an L4(m) D90. The substation sits adjacent to the East Coast Main Line railway. These photos were taken on 7th May 2022.

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The L4(m) D90 visible beyond the railway
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Welwyn Substation
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Closer view; there is a lot more equipment beyond this
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PL16 DT; note that this variant lacks the wedge peak
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Tower base
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Tower detail
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The downleads from the terminal tower pass onto a pair of Eve universal crossunder gantries
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The downleads are nearly horizontal
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One of the two Eve gantries
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Eve gantry side view
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Eve gantry detail
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Residential area next to the substation
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A portion of the substation equipment
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Adjacent L4(m) D90

Earley

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PL16 D90 viewed from the train window at Earley railway station, August 2023

See also