OVERHEAD CONSTRUCTION
SURVEYING


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ENGINEERING INSTRUCTIONS
LINES
OVERHEAD
B 1301
Issue 5, 23.2.60


SURVEYS FOR AERIAL LINES
Detailed Survey

1. General
When it has been decided to erect an overhead line, a Detailed Survey is undertaken. This will generally follow the Preliminary Survey and the authorization of the Preliminary Estimate. Such a sequence of events is assumed to have taken place for the purposes of this Instruction. It will often be found, however, that a Preliminary Survey, and sometimes a Preliminary Estimate, is unnecessary. The survey then made must meet the relative requirements of both Preliminary (see B 1101) and Detailed Surveys. A somewhat liberal interpretation and combination of the instructions on these two types of survey is therefore demanded.

2. Objects of the survey
The objects of the Detailed Survey are to:-

  1. decide, from a knowledge of the ultimate capacity of the line, upon the length and class of poles and the number and type of fittings necessary.
  2. select and mark the precise sites for poles.
  3. ascertain whether the new route may be attached to low-voltage power poles at any points.
  4. decide upon the length of the poles so as to avoid obstructions.
  5. advise on the method of excavating the pole-holes and of erecting the poles.
  6. obtain particulars of stays, struts and other means of strengthening.
  7. determine the transposition points.
  8. obtain wayleave details relating to the smaller properties.
  9. secure details of protective measures necessary at power crossings.
  10. decide upon means of cartage, delivery, and layout of materials and of any special tools likely to be required.
  11. enable a Works Estimate to be prepared.

3. Importance of the survey
It is essential that the survey be made with great care and attention to detail. The ultimate appearance of a line depends largely upon the foresight and judgement of the Surveying Officer, and the cost of subsequent alterations to accommodate additional wires may be avoided by care in making the survey. A well-planned line also minimizes the difficulties of both construction and maintenance. The importance of a careful and accurate survey cannot, therefore, be too strongly emphasized.

4. Capacity of the line
Before the actual survey is commenced, a development forecast, in respect of the exchanges and subscribers to be served, should be obtained from the Sales Division. The possibility that the route might be jointly erected with the Electricity Board should also be considered (see B 1101). The ultimate capacity of the line, as forecasted, should be used to decide on the length and class of poles required.

5. Advice Note work
If the survey is in respect of an Advice Note work, the Survey Officer or the officer advising him, e.g. the Installation Control Routing Officer, should be aware, or should ascertain, if any development schemes are in hand or are contemplated in the vicinity of the proposed overhead line. The effect of such a scheme on the proposed line must be taken into account and inquiry made to this end will often enable the capacity of the line to be decided forthwith. If the Advice Note work is urgent and a forecast cannot be obtained, a liberal, intelligent estimate of the ultimate capacity must perforce be made.

6. Class of poles and spacing
Poles and stays of sufficient strength to meet the ultimate capacity of the line should be erected in accordance with C 3001 and D 3026. As indicated in C 3001, poles at angles should be heavier than those erected on straight portions of the line. A 63yds. spacing should be adopted for lines on which light and medium poles are used for the straight portions; 55yds. spacing should be used for lines on which stout poles are required for the straight portions. (For spacing on curves, see pars. 26 and 28.)

7. Arms
In general, the number of arms fitted should meet initial requirements. Six and eight way arms should not be used (see C 3501). Careful note should be made of points where special methods or fittings, e.g. cross-arms or side-arms, are necessary.

8. Equipment required
The Surveying Officer should have a measuring chain or tape, a measured length of sash-line or a measuring wheel, and a set of surveying rods. A pocket Clinometer may be carried to determine the heights of obstructions (see B 1326). Forms A524 should be used for recording the requisite particulars and a map-print is required for marking the chosen pole sites. The intended route should be marked in pencil on the map-print before the survey is commenced, so as to show desirable positions for poles and transposition points. Spacing irregularities may thus be equalized to some extent and the pencilled route then forms a general basis on which the Surveying Officer may conduct his investigations. As the survey proceeds, any modification of the marked route or pole positions can be indicated in coloured pencil on the map-print. For surveys in respect of Advice Notes, the tentative route may be planned by the Routing Officer. Otherwise the marking may be done by the officer who made the Preliminary Survey.

9. Record of Information
All the information gathered during the survey (which will be used by the officer preparing the Works Estimate and, subsequently, to guide the Foreman of the construction gang) should be entered in the Survey Report (A 524) and the pole positions should be marked clearly on the map-print. These records should be so complete that, with their aid, the proposed route, however intricate, may be followed without difficulty, and so that the chosen site for every pole may be determined even if all marks made by the Surveying Officer have disappeared. Moreover, if a situation demands the application of a particular method of construction or a particular sequence of operations, instructions must be given in the Survey Report with such clarity and detail as will ensure that no other method will be applied by the Foreman of the construction gang. When preparing the Works Estimate, it will not be necessary to revisit the route of the proposed line if the Survey Report has been carefully compiled.

Where an aerial cable is to be erected on an existing pole line, forms A825 and A572 (the use of which is described in F 1101) and form A524 should be completed. To keep the forms clean and tidy they should be enclosed in covers A 1047 which should be re-used on successive works until worn out.

10. Selection of sites
Every effort should be made to place fixtures upon public property, since security of tenure on private property depends upon circumstances over which the Dept. has no control. Accessibility for both extension and maintenance of the line is easier if the fixtures are on public property, and prospective subscribers are usually nearer. Furthermore, the negotiation of wayleaves for subsequent additional wires is very often necessary if private property is concerned, whereas for public property this is not required.

11. Nevertheless, fixtures should be placed on private property if the requisite stability for the line can be obtained by this means only. For example, to avoid an excessive line-angle or to obtain adequate spread of stay, a wayleave for the placing of a pole or stay on, or wires over, private property should be negotiated. In this connexion it should be remembered that flying-wire powers ' will often obviate the seeking of private wayleave consents.

12. Consultation with road and other authorities
The surveyor to the appropriate road authority should be consulted orally to learn his views on, and to secure (if possible) prior agreement to, the pole positions selected on roads for which he is responsible. Confirmation of any proposals for road widening (see B 1101) must be obtained at the same time. Such consultation is valuable in that wayleave negotiation can be thereby considerably simplified and accelerated, and the personal contacts thus established often improve the relations existing between the Dept. and the road authorities.

13. Position of underground plant (sewers, gas, water and power)
The opportunity should be taken, at this stage, to ascertain the whereabouts of service connexions to sewers, or to gas or water mains. (See WAYLEAVES, Public, B 0012 or B 0013.)

14. The exact location of underground power plant (including service cables) can be ascertained by inquiry of the tramway, railway, or electricity supply authority concerned. Requests for such information should preferably be made by personal interview and should be supported by a map-print showing the location of the proposed overhead line. This procedure is supplementary to that indicated in B 1101 and will be unnecessary if it was possible to obtain the information when the Preliminary Survey was made. (WAY-LEAVES, Public, C 0013 or C 0016 refer.)

15. All sites should be chosen so as to avoid the risk of damage to pipes or cables during erection or recovery of the poles or supports and to minimize the danger of the stability of the poles being affected by subsequent excavation work on such pipes or cables (see A 0011). If circumstances will not permit of ample clearance being given, the proximity of the pipes and cables must be indicated in the Survey Report and on the map-print, so that the pot-holes, etc., may be excavated with special care. Where it will be necessary to bring underground cable to a pole, e.g. at a Distribution Pole, the pole site should be selected so as to comply with the provisions of Underground, A 3901 as regards clearance to power plant.

16. Consideration of amenities
Wherever possible, pole sites should be selected so as to detract as little as possible from the amenities of a neighbourhood. Aesthetic considerations are exerting an increasing influence on the layout of modern housing estates, and more thought is also being given to town and country planning in general. It is desirable, therefore, to render the Dept.'s overhead plant as unobtrusive as possible and the careful selection of pole sites, coupled with the use of the minimum length of pole necessary, will greatly facilitate the achievement of such an end, and will thereby reduce the possibility of public objection to overhead lines.

17. Poles should not be erected in front of the windows of a house and, in selecting a site, advantage may sometimes be taken of the screening effect of adjacent, though not overhanging, trees.

18. Contamination of water by creosote
Newly creosoted poles should not be erected near watercress beds, drinking ponds or wells, as the creosote from the poles may contaminate the water. If it is impossible to avoid such a site, a note should be made in the Survey Report to use poles on which the creosote has dried.

19. Lines near oil companies' depots
Wires crossing above, or in close proximity to, oil or petrol storage tanks or fuelling sheds introduce a risk of fire from lighting, or by sparking from a broken wire. Consequently, lines should not be erected in such situations unless the alternatives would involve a substantial increase in cost. In this event a report should be submitted to the Eng. Dept. (Cn2) as soon as complete details and proposals can be supplied.

20. Effect of winds
It should be borne in mind that a strong wind may blow wires horizontally to a distance nearly equal to their sag. In selecting pole sites, therefore, sufficient space should be allowed to ensure that wires will not be blown into contact with vehicles, trees, buildings, etc.

21. Poles in carriageways
A pole should not be erected in, or partly in, a carriageway unless its position, e.g. close to a wall or fence, is such that it will not cause risk to road traffic and is specifically agreed to by the road surveyor.

22. Poles near carriageways (including poles in foot-ways)
These poles should be placed in such a position as to avoid causing obstruction to pedestrians or road traffic. The views of the road surveyor should be ascertained and taken into account if possible, especially with regard to poles on the footway. Whilst some authorities prefer the poles to be erected at the edge of a footway which is nearest to the kerb, others indicate the back of the footpath (that is the edge which is adjacent to the hedge, wall or building) as affording less obstruction to pedestrians. Care should be taken when selecting pole sites to ensure that the poles when erected will not constitute a road hazard by obstructing or restricting a driver's field of vision.

23. Poles in grass margins
In a grass margin, poles should be set as far back as is consistent with the minimum height of pole and freedom from overhanging of private property where the P.M.G.'s flying-wire powers ' would not apply.

24. Clearance between poles and carriageway boundary

  1. Where the carriageway has an appreciable camber and is likely to be used by high vehicles, e.g. double-decker buses, the poles should be placed, if possible, at a distance of at least 18in. from the carriageway boundary. If the road surveyor raises objection to the selection of such a position, he should be advised that the reason is to avoid the risk of accident, due to the use of the road by high vehicles.
     
  2. In other circumstances a minimum clearance of 12in. between the outside edge of the pole and the carriageway boundary should be given, subject to the requirements of the road surveyor. If there is a kerb, the pole should be placed so as not to encroach upon it.
     
  3. The clearance between the pole (including fittings) and the vertical plane of the carriageway boundary should not be less, at any point up to 16 ft. from the ground, than the desirable minimum clearances of 18in. or 12in. given in (a) or (b). Where the clearance at ground level is less than the desirable minimum quoted, the clearance up to 16ft. above ground should not be less than that at ground level.
     
  4. Up to 20ft. from the ground neither pole nor pole-fittings should encroach beyond the vertical plane of the carriageway boundary. In other words, no arm, step, crutch pole-fitting or wire should overhang a carriageway at a height less than 20ft., midsummer conditions being allowed for in the case of wires (see pars. 31 to 35).

25. Straight roads
The side of a straight road to be selected for a line of poles should generally be that on which there is less danger of the poles being blown on to the roadway.

26. Tortuous roads
On curves and on tortuous roads long stretches of wire overhanging the road should be avoided by increasing the number of poles if necessary. At awkward bends the pull-on-pole ' should be reduced as much as possible by careful choice of pole sites, i.e. by reducing the length of the spans and arranging for two or more poles to share the deviation instead of allowing the line to turn through the whole angle at one pole. Fig. 1 shows how both overhang and deviation are reduced simultaneously.



FIG. 1
SHOWING REDUCTION OF DEVIATION AND OVERHANG EFFECTED BY SUITABLE CHOICE
OF POLE SITES AND USE OF SHORTER SPANS AT A SHARP BEND

27. Other considerations
A line should not cross from side to side of a road if it is avoidable; thus the expense of rearrangement will be obviated if, at a later date, it becomes necessary to use both sides of the road. Under present-day conditions, when more than one authority may wish to erect poles on a public road, it is incumbent upon the first-comer to select pole sites on one side only. The side selected will normally be influenced by the general facilities offered for staying, avoidance of trees where permission to cut cannot be obtained, and by wayleave difficulties.

28. The position for stayed or strutted poles should be selected with due regard to the facility offered by convenient boundary hedges, buildings, etc. for guarding the support. In these positions it is often found that increased spread of stay can be obtained, without offering obstruction to agricultural operations, grazing animals, etc. If irregular spacing results, it cannot be counted a serious disadvantage and may conveniently be adjusted over the adjacent spans.

29. Marking pole sites
A few spans having been measured off, either by using the chain, tape, or sash-line, or else by striding the distances if the length of the stride is known, the rods should be set up each in the position to be subsequently occupied by a pole. This procedure is especially useful at sharp or awkward curves, where alternative pole sites are possible, or where a number of obstructions (gateways, trees, windows of houses, etc.) make a compromise necessary in the form of uneven spacing. The sighting of rods actually erected is an immense help in picturing the line as it will be when the poles are in situ, will often avoid much duplication of work, and will go far to ensure that the best possible sites have been chosen. Before the surveying rods are finally removed, the positions for the poles should be marked by pegs driven well into the ground, broad arrows on walls or fences, by the removal of a square of turf, or by other suitable means and recorded in the Survey Report.

30. For poles which are to be erected using either the earth-auger or the bar and spoon, since no kicking' can be done once the hole has been excavated without losing the advantages of the method, it is particularly important that the Surveying Officer should mark the position precisely. If, for instance, the pole is to be placed in a grass margin, a square of turf may be removed (using a grafting or other convenient tool) to indicate the exact position for the pole.

31. Length of poles
In estimating the length of pole required in a particular situation, it should be remembered that certain minimum lengths are necessary to give the clearances over roads, etc., which are indicated in the following paragraphs and in Table 1. These clearances should be adhered to closely, so that construction costs will be kept low and also so that the line will be as unobtrusive as possible. In a suburban district 28ft. poles will generally be found to suffice. The method of determining the heights of obstructions is described in B 1326.

32. Clearances
Poles should be of such a length that, when loaded to their ultimate capacity, the lowest wire at the mid-point of a span at the maximum summer temperature should be not less than 12ft. from the ground along the side of main roads. Where it is necessary to erect wires across roads, tramways and railways or where wires overhang, the minimum clearances shown in Table 1 should be given. The clearances refer to maximum summer temperature and, if measurements on existing wires are being taken at this temperature, it may be assumed that the wires will be 1ft. higher under extremely cold winter conditions. A proportional correction should be made when taking measurements at intermediate temperatures. Particulars of the changes in dip are given in E 3065 and E 3155.

TABLE 1
SHOWING MINIMUM PERMISSIBLE CLEARANCES

  Crossing Minimum clearance above ground-level of lowest wire for ultimate capacity of
line
A Over trolley wires 28ft.
B Streets in towns 20ft. (or more if possibility of high local traffic, e.g. fire escape exceeding 20 ft.)
C Over railway 22ft.
D Country roads and encroachment over carriageway of 2 ft. or more 20ft.
E Encroachment over carriageway of less than 2 ft. 20ft.
F Entrance to field 18ft.
G Elsewhere (i.e. where unaffected by traffic). Ordinary construction 12 to 18ft.
H Elsewhere (i.e. where unaffected by traffic). Light construction 10 to 18ft. according to circumstances
J Over canals So as not to impede canal traffic
K Railway crossings in goods yards where mobile cranes operate 30ft. above rails

33. The minimum clearance to be provided in crossing trolley wires is 6ft. It will be seen from PROTECTION, Power, B 1001 that the normal height of such wires is 21 to 22ft. above the level of the road or rail. Hence the minimum height above road-level for the erection of wires is 28ft. as shown in Table 1 (see also par. 41).

34. Height of obstructions
Methods of estimating and determining heights of obstructions are given in B 1326 and B 1327. In using the P.M.G.'s flying-wire powers ' to carry the proposed line over a dwelling-house it must be remembered (see WAY-LEAVES, Public, B 0012 or B 0013) that, if the owner insists, a clearance of 6ft. above the roof must be given. If tentative and tactful inquiry shows that such objection is likely to be raised, and if the alternatives are costly or difficult, the length of pole erected should be such as to provide for the necessary clearance the ultimate capacity of the line being taken into account.

35. Change of level
On hilly roads, abrupt differences of level in the wires should be avoided by grading the length of poles employed. Where a change of level is made so as to clear obstacles, e.g. in crossing a railway, the difference in the length of adjacent poles should not exceed 5ft. A method of measuring a change in level is given in B 1326.

36. Method of excavating holes
It will often be possible for the Surveying Officer to indicate in his report the method to be used when excavating the pole-holes, i.e. by digging, blasting, bar and spoon, rabbiting-spade or earth-auger. Such information not only greatly assists the Foreman of the construction gang in laying out his tools, but also is an aid in estimating for special conditions likely to be encountered, e.g. running sand, rock.

37. Method of erecting poles
A note should be made of any poles which require special methods of erection, e.g. by derrick, tackle, or winch.

38. Strengthening and the use of stays, struts, etc.
The method of determining the number and strength of stays is given in D 3026. The surveying rods may conveniently be used at angles by placing them in the proposed pole positions, to determine the 'pull-on-pole' and also to bisect the angle in order to find the anchoring point for stays or the foundation point for struts. (Struts are referred to in D 3126.) The judgement of the Surveying Officer should be exercised in suggesting any alternative methods of strengthening to be used, such as blocking, where local conditions do not favour the use of stays or struts. Such information obtained at the time of the survey will often avert a departure from the estimate at a later date. The line should be stayed to withstand the stresses which will be imposed by its ultimate capacity. When a line is exposed to special risk of damage from falling trees or branches, Wire, Suspension, 7/14 may be erected above the open wires or aerial cable in the sections affected. Terminal stays at the ends of the strengthened sections and stays at intermediate points should be provided as described in F 3101. The suspension wire should be erected as described in F 3126. Longitudinal stays may also be provided at the ends of the sections affected and at intermediate points as considered desirable (see F 3101).

39. Transposition points
These points should be carefully selected, in accordance with E 3003, and the need for provision of double-longitudinal stays at all such points should not be overlooked.

40. Wayleaves
Particulars of the highway authorities and of the larger properties will have been obtained if a Preliminary Survey has been made (see B 1101). All details relating to smaller properties or to any other wayleave which has not already been obtained, e.g. due to enforced deviation from the route originally proposed or because a Preliminary Survey was unnecessary, should be secured while making the Detailed Survey. Wayleave procedure is dealt with in WAYLEAVES, Public, C 0013 or C 0016 and in WAYLEAVES, Private, C 0011.

41. Erection of wires across or near overhead power circuits
In addition to the special construction referred to in the following paragraphs, special precautionary methods are necessary when erecting wires near power circuits (see J 1201). The position chosen for the proposed line should therefore be such as will avoid unnecessary proximity to such circuits. Since the erection of lines in the vicinity of power circuits will involve special methods, note should be made in the Survey Report so that suitable adjustment of rates can be made when estimating.

42. Tramways, light railways, and trolley-vehicle systems
The selection of D.P. area boundaries which will generally avoid the crossing of tramways, light railways, and trolley-vehicle systems is dealt with in General, D 1002, and protective measures are indicated in PROTECTION, Power, B 1010, B 1011, and B 1012.

43. Low- and medium-voltage power lines
The various methods of protecting telegraphic lines from low- and medium-voltage power circuits are described in Memorandum A80B (see also PROTECTION, Power, D 0016).

44. High-voltage power lines
High-voltage power lines are dealt with in form A231 and PROTECTION, Power, E 0011. The authority of the Eng. Dept. (Cn2/2) must be obtained before overhead lines used for carrier working are diverted underground.

45. High-voltage neon signs
It is desirable to avoid erecting open wires close to a high-voltage luminous-discharge tube sign (e.g. a sign of the neon tube type), since there are usually exposed high-voltage terminals and connecting wires at the ends of the tube. There is also some risk of the sheath of the high-voltage cable and other metalwork of the sign assuming a dangerous potential above earth by leakage. Neon signs should be considered as high-voltage lines, and separations and clearances in accordance with form A231 will apply (see also J 1201 for the precautions to be observed when working near neon signs). Instructions on leading-in near neon signs are given in G 3020.

46. Cartage, delivery and layout of materials
A note should be made in the Survey Report of suitable points en route at which poles may be deposited and stacked ready for erection.

 

 
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Last revised: April 03, 2021

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