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P.O. ENGINEERING DEPT.
ENGINEERING INSTRUCTIONS
LINES
OVERHEAD
E 3130
Issue 3, 14.8.39
OVERHEAD DISTRIBUTION IN RESIDENTIAL AREAS
Open-wire Ring-type Pole-head D.P.s
1. General
The increasing opposition which is being experienced, from both public and
private wayleave grantors, to the erection of poles for subscribers'
distribution makes it extremely desirable that careful consideration be given to
the appearance of overhead distribution lines in suburban areas.
2. Common types of construction types are:-
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The erection of "through" open-wire lines from a D.P. along
a road, with services - open or drop-wire - to subscribers at intervals
along the line.
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Armed distribution poles with no "through" wires, serving
subscribers radially from arm positions or fittings attached to the ends of
the arms.
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Cross-armed distribution poles, with or without "through"
wires.
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Drop-wire distribution from pole-head distribution poles.
FIG. 1
SINGLE-RING TYPE - GENERAL VIEW
3. Type (a)
These are well adapted for the needs of an area with small development, but
should not be used in suburban areas where reasonably good development is
anticipated.
4. Type (b)
These can frequently lead to unsightly distribution poles, as well as to
considerable technical inconvenience in arranging for the required spans to
subscribers' premises. Arms were designed for through lines, and are ill-adapted
for radial distribution.
5. Type (c)
These are unsightly in suburban streets, requires taller poles, and provides for
a much greater capacity than is either necessary or desirable under modern
conditions.
6. Type (d)
Whilst providing in many cases a sound and economic method of distribution,
frequently leads to complaints due to the greater visibility of the covered wire
in comparison with open wires. It should not be employed in situations where
amenities are of primary importance.
7. Principles of distribution in residential areas
In order to meet present-day requirements and to render overhead distribution
less liable to objection, the following general principles should be observed in
designing local distribution plant in residential areas:-
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Where scanty development is anticipated, type (a), par. 2,
should be employed.
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When the ultimate development is at least 7 circuits per 100
yards of road, distribution should be effected by ring-type distribution
poles erected at suitable intervals along the streets. A spacing of 80 to
120 yards will meet the average case. Each pole being served by cable, wires
should not be erected between poles.
8. Open-wire ring-type distribution pole
To meet the objections outlined in pars. 3 to 6, a type of distribution pole
employing a "Pole Head, 15-way, Ring-type" has been developed, and should be
used generally in good-class residential districts where the conditions of par.
7 (b) apply. Fig. 1 shows a typical case.
9. Normally, D.P.s should not carry more than 15 circuits but,
in exceptional circumstances, the number may be increased to 20 circuits. One
ring per D.P. will, therefore, generally suffice. It should be borne in mind,
however, that only half the ring is available to each side of the road, and a
second ring may therefore be required if the circuits cannot be accommodated in
their appropriate positions on the single ring (see par. 16).
10. Height of pole
For the sake of appearance, the height of the poles should be the minimum which
will provide the necessary clearance for the wires. The average case will be met
by the use of 28' or 30' medium poles.
11. Fitting of pole-heads
When only one ring is to be provided, it should be fixed 15in. below the top of
the pole. When two rings are required, the upper ring should be 15 in. below the
top of the pole, and the lower ring 15in. below the upper. This provides for the
rings to be lower than would be required by purely technical considerations, but
it is desirable from the point of view of appearance.
12. The wrought-iron fixing strips should be adjusted, by
bending, so that the ring is accurately centred on the pole. Coach screws should
be used for fixing. The ring should be so fitted that the slot is uppermost and
central with the terminal block.
13. Finials should always be fitted to poles which carry
ring-type pole-heads.
14. Terminal blocks
The terminal block should be fitted so that its upper edge is flush with the top
of the pole, the block facing squarely up or down the street. If it should be
necessary to fit a second block on a pole, it should be fixed between the two
rings, immediately below the first block (see Underground, K3102).
15. Spindles and insulators
For the sake of appearance, white "Insulators No. 16" should be used
exclusively, together with "Spindles No. 7". Requisitions should be specially
endorsed to show that white insulators are required.
16. Careful attention should be paid to the choice of spindle
positions. Spindles should be fitted in holes as nearly as possible in radial
alignment with the subscribers to be served. It is unnecessary to equip the
rings initially with spare spindles and insulators; but, to ensure satisfactory
clearance between adjacent circuits, such spindles as are fitted should be in
positions which will leave suitable spaces for the circuits which may be
expected subsequently.
17. Considerations of design have limited the space available
for access to the terminal block, and it may be advantageous to defer the use of
the two holes adjacent to the slot until the ring is reaching its full capacity.
18. In all cases, the plane of the "Spindle No. 7" should
coincide with the direction of the pull of the wires. No attempt should be made
to obtain clearance between neighbouring pairs by "splaying out" adjacent
spindles. To facilitate the fitting of the spindles a "Spanner, Box, in. square"
should be used.
19. Pole leads should be run tidily and in a systematic manner
between the terminal block and the spindles. The leads should be run beside the
P.C. cable from the terminal block to the slot in the ring, and then between the
flanges of the channel to the spindles. On the pole, they should be secured by
means of "Clips, Pole-lead, No. 2" (see G3904) or by "Lead Strip, 1in., 5lb."
(see G3001); in the channel, by "Clips, Pole-lead, No. 3"; and to the spindles,
by "Lead Strip, 1in.".
20. Since the pole-head has been introduced largely for the sake
of improved appearance, it is important that the finished job should not be
marred by untidy leads.
21. Pole steps should be fitted as described in C 3151.
22. Protection of cable on pole
To prevent damage to the P.C. cable by the wiremen's boots, a 15in, length of
capping, known as "Capping, Steel, No. 3" should be fixed over the cable close
to the top steps, as shown in Figs. 3 and 4. The capping should be secured by
"Spikes, Dog" two of which should be fixed against the lower edge of the capping
to prevent it slipping downwards. The outside of the capping should be painted
black to tone with the colour of the pole.
FIG. 2
TWO-RING TYPE - GENERAL VIEW
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FIG. 3
DETAIL OF SINGLE-RING TYPE |
FIG. 4
DETAIL OF TWO-RING TYPE |
23. Technical advantage of pole-head
Apart from improved appearance, the ring-type pole-head presents certain
engineering advantages, viz:-
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The finished job is more rigid than arms, terminal irons,
"Spindles, Nos. 7.A. and 10" etc.
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Subscribers' open-wire drops will always be run with the
wires of the pair in vertical formation.
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The running of pole leads is simplified, and the leads are
less liable to disturbance than those on arms.
24. Wayleave negotiations
In negotiating way-leaves it will occasionally be advantageous to indicate to
wayleave grantors the type of D.P. which can be erected. For this purpose Fig. 1
of this Instruction has been published in suitable leaflet form as TE2096,
which can be obtained on requisition from the Stores Dept. As only a limited
quantity of the leaflet has been printed, copies should be obtained and held
only by those officers directly concerned with way-leave negotiations.
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