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Essentially these are two of poles erected in the style of the letters A or H.  It was found that a single pole lacked the strength required for the weight of wiring and infrastructure required on some Trunk routes.  Trunk poles had the heaviest wires which were specified to overcome transmission difficulties because of the distances involved.

C 3251
Issue 1, 21.9.33


Fig. 1 - A Pole

1. Use of A-poles
A-poles are used to give additional stability to a line of single poles carrying a heavy load of wire. Under normal conditions on such lines every fifth pole should be an A-pole, but on winding roads it should be so arranged that the A-poles are erected at the angles. A-poles may be used with advantage instead of strutted, poles, especially where the space available for strutting is very limited and/or where the pole carries a large number of arms.

2, H-poles
H-poles have been used in the past for lines which were required to carry more than 48 heavy gauge wires. H-pole construction should not be used for new work without the sanction of the Engineer-in-Chief.

3. A-poles, Method of Construction
The poles forming an A-pole should be approximately of the same dimensions and as straight as possible. If the poles, however, are not straight, the curvature should be at right angles to the plane of the "A". A-poles should be constructed as shown in Fig. 1. Each leg should be scarfed at the upper end so that, when the two legs are fitted together, an isosceles triangle is formed, with a separation of 12 inches between the inner surfaces of the poles at a distance of 20 feet from the top. The diameter at the top of each leg should be reduced by a full 21 inches, thus providing a scarfed joint 6 feet long.

4. Scarfed Joint
To scarf the pole, first cut the top ends off "square" so that the poles are of exactly equal length, then arrange the poles together side by side with the tips supported on trestles. Twist the poles into such a position that the proposed scarves will be vertical and parallel to each other. With the poles in this position, draw a vertical line on each tip at a distance of 21 inches from the point of contact. Then, on the inner side of each pole, mark off a distance of 6 feet from the top, and, by means of a chalked line, make lines from these marks to both ends of the vertical lines on the tops of the poles.

5. The scarves should first be cut with an "Adze, Carpenter's" and a "Plane, Jack, 2 inch" then used to true up the surfaces, to ensure a close contact throughout the joint. Before the two legs are fixed together, the cut surfaces should be thoroughly coated with "Creosote and Tar". The two scarfed surfaces should then be placed together and secured by means of two Finch arm bolts, one fitted 1 feet 4 inches and the other 5 feet 5 inches from the top of the pole, as shown in Fig. 1. ("Bolts, Arm, Nos. 14 and 19" will usually be found suitable for most stout poles.)

6. Tie Bolt
A tie bolt of suitable length (see Table 1) with four "Washers, Galvd., No. 6" and a distance piece cut from "Tube, Galvd., 1 inch" should be fitted, as shown by the full lines in Fig. 1, halfway between the bottom of the scarfed joint and the middle of the pole brace. When a cattle guard is provided, however, the tie bolt should be fitted as detailed in para. 16.


7. Measurements
The measurements given in Table 1 refer to stout poles of average diameter. The measurements and items shown in columns 9 and 10 are based on the assumption that 8 arms will be fitted to the 30 and 32 foot poles, 10 arms to the 34-foot poles, and 12 arms to the poles of greater lengths. If it should be necessary to fit the tie bolts in positions other than those indicated, approximately 1 inch should be added to, or taken from, the widths given in columns 4, 7 and 9 for each foot by which the tie bolt position is lowered or raised.

8. Pole Brace
At a point 12 inches from the bottom of each leg, slots should be cut for the pole brace. These slots should be approximately 4 inches deep, with a slight increase for poles of the larger diameters, and should be of such a width as to ensure that the pole brace is a driving fit. The pole brace should be secured by two "Bolts, Arm" of suitable length (usually "Bolts, Arm, Nos. 17, 18 or 19") with four "Washers, Galvd., No. 8" in place of the ordinary arm-bolt washers. "Braces, Pole, 6 foot" should be used for A-poles up to and including 32 feet in length, and "Braces, Pole, 8-foot" for poles exceeding 32 feet. "Kicking" blocks (shown in Fig. 1) should be provided as laid down in para. 14.

9. Arms
The structure is further strengthened by the arms, which are slotted into the poles as described in C 3551, each arm being secured by a bolt through each leg. Appropriate arms for A-poles are shown in Table 2.


Arm position 4-way 6-way 8-way
Arms 1 to 8 Length 57 inches
"Arms, Wood, No. 9"
"Arms, Wood, No. 10" or
"Arms, Wood, No. 18"
Length 82 inches
"Arms, Wood, No. 26"
"Arms, Wood, No. 27"
Length 87 inches
"Arms, Wood, No. 31"
"Arms, Wood, No. 32"
Arms 9 to 12 Length 60 inches.
"Arms, Wood, No. 15"
"Arms, Wood, No. 16"
- -
Arms 13 and upwards Length 66 inches.
"Arms, Wood, No. 29"
"Arms, Wood, No. 30"
- -

Note:- These arms to be used only for wires of 150lb. per mile and over on terminal poles.

10. Pole Roof.
The slopes for the pole roof should be cut as described in C 3151, and a "Roof, No. 4, 6 or 8" fitted - according to the width of the top of the A-pole. When a saddle is required, however, a "Roof, No. 3, 5 or 7" should be used.

11. Saddle
When a saddle is to be fitted to an A-pole, a saddle stay should always be used, to make the attachment more secure. "Stays, Saddle, No. 2" should be used in conjunction with "Roofs, No. 3" and "Stays, Saddle, No. 3" with "Roofs, No. 5 and 7". When a "Roof, No. 7" (14-inch) is required, the "Stay, Saddle, No. 3" (12-inch) should be altered locally. Saddles should be fitted as described in C 3151.

12. Earth Wire
Earth Wires should be provided in accordance with C 3601. When it is necessary to earth-wire an A-pole, an earth-wire should be fitted to one leg only.

13. Care in construction
In order that A-poles should retain their rigidity, all parts should be carefully fitted together, particular attention being paid to the scarfed joint and the fitting of the arms and pole brace.

14. Kicking Blocks
These are employed to provide an additional bearing surface against transverse stresses. Generally, the kicking block should consist of a "Stayblock, Heavy"; alternatively, if more convenient, a recovered "Brace, Pole, 8 foot" may be cut to provide two blocks, each 4 feet in length. Two kicking blocks - one at each end - should be placed across the pole brace.

At angle poles, the block on the inside of the angle should be placed under the pole brace, and the block on the outside should be placed upon it. The normal arrangement of the kicking blocks on straight sections of line along a road is to place one block above and one below the pole brace; the upper block being placed on the end remote from the road to prevent overturning across the road. When the prevailing wind will tend to blow the line away from the road, however, the block on the roadside may also be placed above, instead of below, the pole brace. The procedure to be followed when fitting kicking blocks is described in paras. 24 and 26.

15. Cattle Guards
Where A-poles present a slight opening, insufficient for cattle to pass through but into which they might get jammed, a vertical guard should be provided in the aperture (see Fig. 1). The guard should usually be made of sound, seconds, creosoted timber, 6 or 8 feet long, buried to a depth of 18 to 24 inches. It should be secured to the pole by means of a tie-bolt of suitable length (see Table 1), six "Washers, Galvanized, No. 6" and two lengths of "Tube, Galvanized, 1 inch" fitted 12 inches from the top of the guard - which should be pointed. The distance from the butt of the pole to the point at which the tie-bolt should usually be fitted is given in Table 1, which also gives the length of tie-bolt required for stout poles, under average conditions. Generally, the cattle guard should be bolted to the A-pole before the pole is erected.

16. When a cattle guard is fitted to a new pole, the upper (normal) tie-bolt, specified in para. 6, should be fitted in a position halfway between the tie-bolt securing the cattle guard and the ultimate lowest arm. These items are indicated by dotted lines in Fig. 1.

17. Protection, as described in para. 15, should be provided where a dangerous space is left between a pole and a wall, or other solid boundary.

Fig. 2 - H Pole

18. Construction of H-poles
Fig. 2 illustrates the method of construction adopted for H-poles. The two poles forming an H-pole should be approximately of the same dimensions and as straight as possible. They should be spaced at a uniform distance of 18 inches apart at the top and at the ground line. The poles are secured at the top by the arms - fitted in accordance with C 3551. "Arms, Wood, No. 12" - which are 6-way - are usually employed; "Arms, Wood, No. 20" should be used only for wires 150lb. per mile and over, on terminal poles.

19. The poles are joined together one foot from the butt by a "Brace, Pole" which is fitted as for an A-pole (see para. 8). Kicking blocks should be placed across the ends of the pole brace, as shown in Fig. 2. The blocks should be provided as described in para. 14.

20. To prevent lateral displacement of the structure, a system of trussing is adopted, to convert what would otherwise be two independent supports into the equivalent of a latticed girder. This consists of sets of four truss rods 4 feet in length, bolted to a steel truss ring of channel section, and secured to the poles by means of tie-bolts. (These items are known as "Rods, Truss, No. 2," "Rings, Truss" and "Bolts, Tie, No. 4" respectively.) "Blocks, for Rings, Truss" (galvanized iron blocks which fit into the channel of the truss ring) are used to afford a bearing surface for the nuts of the truss rods. The lowest tie-bolt is fixed 18 inches above the ground line.

21. "Washers, Galvanized, No. 6"
3 inches by 2 inches by inch - are fitted under the heads and nuts of all the tie-bolts and, also, between the truss rods and the pole, on the intermediate tie-bolt (or bolts). "Washers, Galvanized, No. 7" - 8 inches by 2 inches by inch - each secured by means of two "Screws, Coach" are fitted on the inside of the pole to both the top and bottom tie-bolts, to prevent them from crushing into the fibre of the poles (which would result in the whole structure becoming curved under transverse stress).

22. Ordinarily, two sets of truss rods are employed but, where exceptionally high poles are necessary to clear obstacles, three or more sets should be provided if it is considered that the stability of the line demands such a measure.

23. Pole Roofs, Saddles, Earth Wires.
Pole roofs and saddles, when required,  should be fitted to H-poles by the method described in C 3151. Earth wires should be provided in accordance with C 3601. When an H-pole is earth-wired two wires, one on each pole, should be fitted.

24. Holes for A- and H-poles
These will generally be excavated as shown in Figs. 3 and 4, which represent a typical hole for a 36 foot A-pole at an angle. Where turf exists, it should be removed carefully and laid away from the soil excavated ; large stones removed during excavation should also be placed apart from the soil. The bottom of the hole should be shaped to admit the butt of the pole, allowance being made for slight adjustments to bring the pole into correct alignment. One side of the hole should be cut away to provide a slope, as shown in Fig. 3, to facilitate the erection of the pole. The sides of the hole should be under-cut, as required, to admit the kicking blocks. When a kicking block is required below the pole brace, it should be fitted in position before the pole is erected, the block being placed on solid, undisturbed earth, so that the pole brace will rest firmly upon it, as shown in Fig. 4. When both kicking blocks are to be placed above the pole brace, each end of the pole brace must rest on solid earth. The depth at which A- and H-poles should generally be set in the ground is given in Table 1.

Fig. 4 - A-Pole In position.
Typical Excavation for 36ft. A-Pole at an angle position.

25. Erection of A- and H-poles
The method employed in erecting A- and H-poles will depend upon local conditions. The lighter poles may be erected with both pole lifters and stout ladders in the same manner as employed for single poles between 30 and 50 feet in length (see C 3201), and pole lifting jacks may also be used to advantage during the erection. Two pole-raising boards will be required. A- and H-poles above 40 feet in length should be erected by means of a derrick, as described in C 3226.

26. Filling-in.
As comparatively large holes are required for A- and H-poles, it is essential that the replaced soil should be very carefully consolidated. The soil should be replaced in small quantities, and thoroughly punned as the hole is filled in. Any stones removed during the excavation should be tightly packed and rammed at the butt of the pole, and also round the poles near the top of the hole, so as to provide an extended bearing surface. The kicking block (or blocks) required above the pole brace should be placed in position when the hole has been filled in to a point level with the upper surface of the pole brace. Finally, any turf removed at the commencement of the excavation should be replaced neatly.

27. Care in Excavation and Filling-in
The foundations of A- and H-poles are important, and the strength of the structure may be lost if proper attention is not paid to the shape of the hole, the fitting of the kicking blocks, and the efficient reinstatement of the soil.

Fig. 4 - A-Pole In position.
Typical Excavation for 36ft. A-Pole at an angle position.

28. Numbering A- and H-poles
When filling-in has been completed, A- and H-poles should be numbered, in accordance with C 3151.

29. General Instructions relating to overhead work will be found under Section A, and matters relating to "Precautions against Accidents" under Section J. These instructions should in all cases be strictly observed.

Trunk Pole Pictures

Dated 1946

Trunk Pole Route - H Pole at Penarth, Glamorgan (1905).


Trunk Pole Route - H Poles at Tottenham (1931).


Trunk Pole Route - H Poles at Tottenham (1931).


Trunk Pole Route - London to Brighton  aerial cable route (1931).


Trunk Pole Route - London to Brighton aerial cable route.
Picture taken at Handscross, West Sussex - Pole 478 (1931).


Trunk Pole Route - London to Brighton aerial cable route.
Picture taken at Malehouse Road, Crawley, Sussex - Pole 596 (1931).


Trunk Pole Route - London to Brighton aerial cable route.
Picture taken at Povey - Pole 414 (1931).


Trunk Pole Route - London to Brighton aerial cable route.  A Pole.
Picture taken at Povey - Pole 448 (1931).


Two pole routes
One on each side of the road
The poles with finials are tramway lines


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Last revised: March 12, 2022