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Issue 5, 23.2.60
SURVEYS FOR AERIAL LINES
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 B1101) 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:-
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.
select and mark the precise sites for poles.
ascertain whether the new route may be attached to low-voltage power poles at any points.
decide upon the length of the poles so as to avoid obstructions.
advise on the method of excavating the pole-holes and of erecting the poles.
obtain particulars of stays, struts and other means of strengthening.
determine the transposition points.
obtain wayleave details relating to the smaller
secure details of protective measures necessary at power crossings.
decide upon means of cartage, delivery, and layout of materials and of any special tools likely to be required.
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 B1101). The ultimate capacity
of the line, as forecasted, should be used to decide on the length and class of
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 C3001 and D3026. As indicated in C3001,
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.)
In general, the number of arms fitted should meet initial requirements. Six and
eight way arms should not be used (see C3501). Careful note should be made of
points where special methods or fittings, e.g. cross-arms or side-arms, are
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 B1326).
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
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 (A524) 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 F1101) and form A524 should be
completed. To keep the forms clean and tidy they should be enclosed in covers A1047 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 B1101) 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
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,
B0012 or B0013.)
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 B1101 and will be unnecessary if it was possible to obtain the information
when the Preliminary Survey was made. (WAY-LEAVES, Public, C0013 or C0016
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 A0011). 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, A3901 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
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
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.
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.
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.
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
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
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
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
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 B1326.
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 E3065 and E3155.
SHOWING MINIMUM PERMISSIBLE CLEARANCES
||Minimum clearance above ground-level of lowest wire for ultimate capacity of
||Over trolley wires
||Streets in towns
||20ft. (or more if possibility of high local traffic, e.g. fire escape exceeding
||Country roads and encroachment over carriageway of 2 ft. or more
carriageway of less than 2 ft.
||Entrance to field
||Elsewhere (i.e. where unaffected by traffic). Ordinary construction
||12 to 18ft.
||Elsewhere (i.e. where unaffected by traffic). Light construction
||10 to 18ft. according to circumstances
||So as not to impede canal traffic
||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 B1326 and B1327. 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, B0012 or B0013) 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 D3026.
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 D3126.) 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 F3101. The suspension wire should be erected as described in F3126. Longitudinal
stays may also be provided at the ends of the sections affected and at
intermediate points as considered desirable (see F3101).
39. Transposition points
These points should be carefully selected, in accordance with E3003, and the
need for provision of double-longitudinal stays at all such points should not be
Particulars of the highway authorities and of the larger properties will have
been obtained if a Preliminary Survey has been made (see B1101). 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, C0013 or C0016 and in WAYLEAVES, Private, C0011.
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
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.