Glossary for British pylons and high-voltage power lines

Abbreviations
Terms

This page is not exhaustive and is not in any way authoritative. The information on this page is provided solely to offer a little bit of background knowledge to anyone curious about pylons. The details herein may not all be correct and may be misleading or not explained as well as they could be.

Abbreviations

Tower types

For tower types please see the guide page.

Technical terms

AAAC: All-Aluminium Alloy Conductor (conductor type); within the UK, each type is named after a type of tree
ACCC: Aluminium Conductor Composite Core, a type of HTLS conductor; each type is named after a city
ACCR: Aluminium Conductor Composite Reinforced, a 3M proprietary type of HTLS conductor; each type is named after a type of bird
ACSR: Aluminium Conductor Steel Reinforced (conductor type); within the UK, each type is named after a type of mammal
AACSR: Aluminium Alloy Conductor Steel Reinforced (conductor type)
ACAR: Aluminium Conductor Alloy Reinforced (conductor type)
CCT: Abbreviation for “circuit”
CSE: Cable sealing end (“sealing end”)
DC: Double circuit; Direct currernt
DEW: Double earthwire
EW: Earthwire
GTACSR: Gap-type (“G”) ACSR conductor, thermal resistant (“T”) HTLS conductor
GZTACSR: Gap-type HTLS ACSR conductor, super thermal resistant (“ZT”) HTLS conductor
HTLS: High Temperature Low Sag, any of a number of conductor types designed to run at very high temperatures (up to beyond 200 °C) with low sag, for example ACCC, ACCR and G(Z)TACSR
OPPC
OPGW: Optical Phase Conductor and Optical Ground Wire conductors support optical fibres within a tube that replaces one of the core strands
SCA: Steel-cored aluminium; an older term for ACSR
SEC: Sealing end compound
SEP: Sealing end platform
SEW: Single earthwire
SH
STD
STD HT: Standard height

Organisations

BB: Balfour Beatty (tower design, power line construction)
BEBS: British Electricity Boards Specifications
BICC: Official abbreviation of British Insulated Callender’s Cables (tower design, power line construction)
BK: Blaw Knox (tower design)
BEA: British Electricity Authority (1947–1955)
CE: “Central Electricity” (but see also 1930s electricity schemes below)
CEB: Central Electricity Board (1927–1948)
CEGB: Central Electricity Generating Board (1958 until privatisation in the 1990s)
JLE: J L Eve Construction (tower design, power line construction)
K&D: Kennedy & Donkin (consultant engineering)
M&M: Merz & McLellan (consultant engineering)
SWE: South Western Electricity (but see 1930s electricity schemes also below)

1930s electricity schemes

The following prefixes can be found on tower designations, indicating the applicable region of Great Britain.

CE: Central England
CS: Central Scotland
EE: East England
MEE: Mid East England
NEE: North East England
NWE: North West England and North Wales
SEE: South East England
SS: South Scotland
SWE: South West England and South Wales

Terms

Note that, as with entire power section of the site, the definitions below are for lay people and are not necessarily accurate.

Approach: The first mile of a power line out from the substation.
Approach gap: A shorter gap between the arcing horns in the approach to a substation, to help protect the substation from lightning damage.
Arcing horn: Curved metal rods typically placed at each end of a string of insulators to guide flashovers through the air and away from the insulators. This is done to prevent damage to the insulator set. The arc gap is reduced for the first mile out from each end of the line, replacing the older practice of double earthwire [NSP/004/127]. When a power line has been down-rated without replacing the insulator sets, new arcing horns are required to reduce the arc gap according to the new operating voltage. Per [NSP/004/127], arcing horns also “improve the corona performance of the insulator set”.
Angle tower: A tower used where an overhead line changes direction. These towers have sufficient strength and stability to withstand the resulting horizontal pull from the conductors. Angle towers are tension towers and, so it has been alleged, are referred to by engineers as tension towers (although “angle tower” is what appears on tower drawings), with “angle tower” being a linesman’s term only. The width at the base increases with supported deviation angle, and the crossarms generally become less symmetrical: longer on the outside of the angle and shorter on the inside. Changes in elevation are generally accommodated by suspension towers. Angle towers are occasionally described as “deviation towers”. Angle towers can be used with zero deviation at the boundary between sections of line.
Bar mark: See erection mark.
Bend line: In the first couple decades of tower development in the UK, the tower’s body (base) and top hamper each had a single taper angle, meeting at the “waist” or “bend line”; J L Eve also used the term “kink level” for this point. The tower body had a greater taper angle than the top hamper: the base is quite visibly flared. Typically the bend line was at the height of the middle crossarm. In more recent decades, towers often have simulated curved profiles using multiple changes in taper angle; for these, the bend line or kink level is often much closer to standard height ground level and denotes the lowest change in taper angle. All height extensions continue the taper angle below the bend line.
Bundle: See conductor bundle.
Cable: Underground cable: buried power lines. Overhead lines use “wires”, referred to as “conductors” (load-carrying wires) and “earthwires”.
Compression front face: In J L Eve terminology, the front face (transverse face) of a terminal tower facing the incoming line; the opposite face is termed the tension front face.
Conductor: Any of the wires that are strung from a transmission tower that carry power, also known as “line conductors” and “phase conductors”. Each conductor carries one phase. For voltages up to 132 kV there are normally three or six conductors depending on whether the line is single circuit or double circuit. For higher voltages each phase is typically formed of a bundle of two to four conductors.
Conductor bundle: Two or more conductors grouped together to carry the same phase, held a fixed distance apart by spacers. Typically found on 275 and 400 kV lines, bundles have two, three or four conductors in the UK (China uses as many as eight). The 275 kV Supergrid lines (L34, L66/89, L2 and L3) were twin-conductor bundles, while L6 was designed to carry quad-conductor bundles at 400 kV to offer twice the current-carrying capacity of L2 lines at a greater voltage (although many L2 lines were uprated to 400 kV). L7 towers support twin-conductor bundle 132 kV lines. As L6 lines are refurbished, they are typically converted to triple bundles of higher-capacity conductors.
Conductor clashing: This is where the conductors from different circuits or phases collide under strong winds. To minimise this risk, many double-circuit towers have middle crossarms that are significantly wider than the top and bottom crossarms. In theory it should be sufficient for conductors to simply get close enough to each other to flash over (arc across), but there seems to be no instance of the definition of the term including this situation.
Crossarm: Any of the arms on a tower that support the conductors. Depending on usage, the term may refer to a single arm, or a pair of arms as a complete assembly, and may or may not include the angled bars that support the horizontal bars; modern usage describes a complete arm, with a double circuit tower having six (not three) crossarms.
Crossarm tie: Another term for raker.
Design family: See tower series.
Deviation: A change in compass direction of the route that the overhead lines follow. These direction changes occur at angle tower, where the incoming and outgoing wires face different directions.
Deviation tower: Non-standard term for angle tower.
Diamond crossing: This is a crossing between two lines, where one line is split onto two gantries (or reduced-height towers) and routed around a tower of the other line. Not to be confused with diamond crossings on railways.
Dolly: See pilot set.
Downdropper: A wire connecting a downlead to a cable sealing end or other equipment.
Downlead: A wire connecting one of the phase conductors on the tower to ground-level equipment, found at sealing end compounds, sealing end platforms and substations.
Dropper: A wire link, typically asymmetrical, forming a shackle for suspension insulators. Droppers are suspended from a pair of longitudinal bars attached to the end of the crossarm. Droppers for 132 kV are typically around 11–15 inches tall including the attachment brackets. Although plenty of droppers remain, when lines are reconductored the droppers are often if not always removed and replaced with simple links. Droppers were used up to and including PL16 but have not been used on tower types since. Droppers are sometimes described as “suspension hangers” (e.g. NWE PL2 drawing E111-1), not to be confused with rakers, also known as hangers. Within the industry, droppers are also known as “Milliken hangers”, for example in [NSP/004/030].
Earthwire: One or more wires placed above the conductors of a power line to protect the line against lightning strikes or keep all towers at the same earth potential. The earthwire is bonded to each tower in order to maintain a good electrical connection. Lines designed for 132 kV and above typically use one earthwire; some older lines have two earthwires side-by-side for the first mile out from the terminal tower at each end. 11 kV lines seldom if ever use an earthwire; 33 kV lines may use an earthwire on lattice towers, but seldom on wood poles. 132 kV lines on “Trident” wood poles don’t use an earthwire. Some power lines in the Netherlands use three earthwires.
Earthwire shade: The protection offered by the earthwire against lighting strikes hitting the conductors below. A narrower angle is better, thus the earthwire should be significantly higher than the top crossarm. The earthwire shade angle has also been referred to as “shielding angle” and “screening angle” [400 kV Grid].
Erection diagram: A technical drawing of a tower or one or more parts of a tower (e.g. crossarm plans, height extension) showing the erection marks of each bar and plate, from which the tower may be assembled. Each erection diagram is paired with a general arrangement drawing showing the cross-section dimensions of the parts. The erection diagram and general arrangement may be combined into a single drawing. The outline and clearance dimensions are also included. On Blaw Knox drawings, the erection diagram typically gives the base width as the outside distance across the top of the stubs.
Erection mark: An alphanumeric code stamped into each piece of steelwork to identify its position on the tower in order to facilitate assembly. These marks count up from 1, and each tower type (e.g. D, D30) has a prefix applied to these numbers. During metrication, an “M” prefix may be added to the erection marks. For example, the erection marks for Blaw Knox T1521 D use prefix “FG” (FG1, FG2, FG3 and so forth), while the metricated version (L3(c) DS) uses prefix “MFG”. Where two or more bars on a tower are identical, they can share the same erection mark. The alternate term for these codes is “bar mark”, but this is confusing as these codes are also applied to plates in addition to the bars. The erection diagram for a tower depicts the tower along with all the erection marks, to show where each piece is located.
Family: See tower series.
Front face: In J L Eve terminology, either of the transverse faces of a tower.
General arrangement: A technical drawing of a tower or one or more parts of a tower (e.g. crossarm plans, height extension) showing the cross-section details of all the bars and plates, e.g. 5″ × 5″ × ⅜″ L, ¼″ plate or 2″ × ¼″ flat. In addition to basic dimensions (e.g. base width, crossarm widths, vertical spacings) there may be other dimensions including panel size, bolt spacings etc. Especially in Blaw Knox practice, the identities of each bar and plate is found on the erection diagram, but both drawings are often combined. On Blaw Knox drawings, the general arrangement drawing typically gives the base width as the outside distance across the stubs at ground level (excluding concrete muffs).
Hanger: Appears to be another term for raker; however, the term “Milliken hanger” refers to droppers.
Hamper: See top hamper.
Insulator set: Traditionally, a chain of two or more cap-and-pin insulator units. The number of units depends on the voltage and environmental conditions. Tension insulator sets tend to have slightly more units than suspension insulator sets. There are also polymeric insulators, plus pin and post insulators that project upwards (such as the pin insulators on 11 kV poles and post insulators on “Trident” 132 kV poles).
Intermediate tower: Another name for suspension tower (straight line tower): a tower at an intermediate point in a wire run, where angle/section and terminal towers terminate wire runs.
Kink level: See bend line.
Line conductor: See conductor.
Line tower: Another name for suspension tower.
Longitudinal face: Either of the two faces of a tower to which the standard crossarms are attached, colloquially the “sides” of the tower; see transverse face.
Milliken hanger: Another term for dropper, presumably only on Milliken/Blaw Knox towers.
Outrigger: A projecting bar used to extend a crossarm or an arm used for attaching a pilot set. Typically found on terminal towers.
Phase: The electricity supplied by any one of the three wires that make up a three-phase circuit; each phase’s alternating current cycle is shifted by 120° relative to the other two phases. Each phase is carried by one conductor (“wire”) or conductor bundle. A set of three phases makes up a single circuit; towers often carry two three-phase circuits.
Phase conductor: See conductor.
Pilot set: A pilot insulator set (pilot set, also “dolly”) is a suspension insulator set used for routing conductors around a tower and for maintaining clearances. Pilot sets are found on tension towers. In the Republic of Ireland, all single circuit angle towers use pilot sets for the centre phase as the tower body presents an obstruction and the wire must pass around it.
Pole step: The steps added to utility poles for the purpose of climbing; Callender’s used “pole steps” instead of “step bolts” for SEE PL1(b).
Raker: Either of the upper, downward-sloping bars of a crossarm. Also known as “hangers” (see hanger for the alternative use of this term) and (by Callender’s and Watsham’s) “crossarm ties”.
Sag: How much a conductor hangs down between towers. This amount changes with temperature as the conductor expands and contracts with the heat and cold. Sag is unavoidable: if you tried to pull the conductor horizontal it would snap under the strain.
Sag adjuster: These fittings “regulate sags or align insulator strings on twin insulator sets” [NSP/004/127].
Screening angle: See earthwire shade.
Section: A single run of conductor, terminated at each end by tension towers (terminal towers or section towers). Within a section, all towers are suspension towers. There is a limit to how long each run of conductor can be, and where the power line is longer than this, adjacent sections are joined back-to-back on section towers.
Section tower: An angle tower used in straight line configuration to terminate an adjacent pair of sections; found when a straight line run exceeds the conductor length limit.
Series: See tower series.
Shackle: A metal link from which an insulator string or earthwire shoe is suspended. Superseded by swivels.
Shielding angle: See earthwire shade.
Span: The section of overhead line between adjacent towers. The standard span for 132 kV was 900 feet, i.e. each tower was around 900 feet away from the next one; this was increased to 1000 feet for 132 kV and extended to 1200 feet for 275 and 400 kV. The positioning of towers and thus the length of each span is determined by various factors including land availability, land occupancy and terrain. It is possible to exceed the span limit between a pair of towers, although this will reduce the limit of the next span accordingly.
Step bolt: Special bolts inserted into the tower legs at fixed intervals for the purpose of climbing the tower, typically on two opposing corners. Callender’s used the more generic term “pole steps”. The lowest step bolts are omitted and are temporarily inserted during maintenance.
Straight line tower: Another name for suspension tower.
Suite: See tower series.
Support: A structure from which a power line is suspended; these are most commonly poles or towers.
Suspension tower: A tower where the conductors are suspended from vertical strings of insulators. Also referred to as a straight line tower, intermediate tower or line tower, suspension towers are contrasted with angle towers by not allowing a change in line direction. (With that said, older designs of suspension tower do allow deviation up to 2°, 3° or 10° depending on the type, using suspension insulator sets and without terminating wire runs.)
Swivel: A pivoting metal plate to which an insulator string is attached; successor to shackles.
Tension front face: In J L Eve terminology, the front face (transverse face) of a terminal tower facing away from the incoming line; the opposite face is termed the compression front face.
Tension tower: A tower where the conductors are attached to nearly-horizontal strings of insulators under tension. See angle tower, terminal tower. Opposite to suspension tower. Depending on what you believe, “tension tower” is an engineer’s term in the UK for what linesmen call angle towers and treated as distinct from terminal towers.
Terminal tower: A tower where the line comes to an end and is terminated onto ground-level equipment.
Top hamper: In overseas usage, and some UK types, the crossarms all attach above the bend line (waist). This upper portion is known as the “top-hamper” or “top hamper” (or “cage” overseas), a term of nautical origin; the portion below the bend line is the “body”. In the UK it was standard practice to have the bend line at the level of the middle crossarm, but more recent designs often have multiple changes in taper angle (including reverse curves) and the lowest change in taper angle is much lower down. “Top hamper” is confirmed to be used with PL16 per the K1124 drawing schedule and with SEE SL3, and likely refers to the portion of the tower above the bend line.
Tower: The industry term for pylon, even though the term pylon was specifically chosen for steel lattice transmission towers!
Tower series: A set of tower designs encompassing the full range of towers needed for a power line. Also known as a family or tower suite. A single tower series will typically contain a design for the line tower, at least two angle towers (e.g. 20° and 60°) and the terminal tower with their respective height extensions and reductions; other types that may be included are junction towers, transposition towers and gantries. A tower series may contain designs for single circuit or double circuit towers or both.
Tower suite: See tower series.
Transverse face: Either of the two sides of a tower that face into the power line, colloquially the “front” and “back” of the tower; see longitudinal face.