AS4 (ASTI SSE400)

Contents

• Overview
  • General data
  • Development
  • Size increase over SSE400
• Lines
• Tower forms
• Tower details
• References

Overview

AS4 is a UK electricity pylon (steel lattice transmission tower) series under development (as of December 2025) within SSEN Transmission. It is the proposed type for several new SSEN transmission lines within Scotland. “AS4” stands for “ASTI SSE400” [1]: AS4 is a larger version of the SSE400 tower series used for the Beauly–Denny line. “ASTI” in turn stands for Ofgem’s Accelerated Strategic Transmission Investment framework, investment from which is being used for the new lines (see SSEN Transmission news article from 2022). AS4 is the tower series created specifically for these new 400 kV onshore lines. SSE400 was designed by Balfour Beatty (as was the Coylton–Auchencrosh type) and one could reasonably assume that they were responsible for AS4 also, but this is not confirmed.

Possibly to separate SSE400 from AS4, almost all AS4 tower designations have an “A” prefix: AD, AD10, AD25 etc. The one exception is “BD”, depicted identical to type AD. The actual derivation of the “A” and “B” prefixes is not known.

When constructed, this will be the largest tower type in the UK.

General data

Designed for	SSE
Designer	Balfour Beatty? (unconfirmed)
Found
Height (straight line tower)	54.6 m
Voltage	400 kV
Dates from	2025
Conductors	Triple
Circuit count	Double
Design conductor	Triple 700 mm² AAAC (Araucaria)
Design earthwire	?
Normal span	?

Development

Compared to SSE400, AS4 exhibits the following changes:

• Conductor support increased from twin bundles to triple bundles; AS4 carries triple 700 mm² AAAC (Araucaria) conductors [1]
• Increased height to the bottom crossarm from the ground line
• Increased phase separation (vertical spacing between conductors)
• Increased taper angle below the bend line, giving greater resistance against toppling (higher overturning moment); the towers thus have a wider footprint at ground level

The AD and BD line towers have a maximum deviation of 2°. [1]

Size increase over SSE400

There is no single percentage increase over SSE400. The table below summarises the increases across three key dimensions:

Note that the average is across the suite of tower types; the average across an actual power line will be biased towards the line tower (most common type) and will also be affected by the geography along the route (in terms of the height extensions required for the towers).

Lines

The following lines are intended to use AS4 towers when the lines are constructed:

• Beauly–Blackhillock–New Deer–Peterhead (ca. 186 km; estimated completion 2030)
• Spittal–Loch Buidhe–Beauly (ca. 173 km)
• Kintore–Tealing (ca. 106 km; estimated completion 2030)

Tower forms

The diagrams below are manually traced from SSEN’s conceptual key diagrams; as such, the design of the final towers may differ. All bracing shown in dashed form in SSEN’s key diagrams has been included as standard bracing; potentially some towers may omit some bracing. As the diagrams below are taken from key diagrams (simple wireframe outlines), no crossarm tips are shown, unlike with SSE400 where it was possible to approximate these from views of actual towers.

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

There are two line towers, designated “AD” and “BD”. The key diagrams depict them as identical.

The AD90, ADJ and ADT are essentially the same tower. ADT gains 500 mm outriggers on the bottom crossarms. ADJ and ADT have additional bracing compared to AD90 as well as a taller peak.

Beauly–Peterhead Section 37 Application Volume 2 (Main Report) Chapter 3 (Project Description) section 3.7.12 (page 3-11) indicates that the optional top crossarm extension for the ADT tower is intended for transposition, something not used in the UK since the 1930s. Section 3.7.12 notes:

Due to the length of the OHL for the Proposed Development, voltage unbalance can occur between the different phases (conductor arrangements on either side or the tower). To reduce voltage unbalance transposition of the phases is required i.e. the conductors are moved to connect to different arms of the towers; this is illustrated in Plate 3.2. For the Proposed Development this will take place at two locations; Towers CB6-4A to CB6-4B and Towers CB14-24A to CB14-24B. To facilitate this, two terminal towers with extended top crossarms, as can be seen in Plate 3.3, will be positioned approximately 100 m apart; the location of transposition towers is presented on Figure 3.1: Site Layout.

Tower details

References

1. Spittal–Loch Buidhe–Beauly Section 37 Application: Electric & Magnetic Field Study Report (Part A)