Click here for Overhead
Originally open wires were made of bronze but circa 1936 the GPO
decided to replace all bronze wire of 40, 70, 159 and 300lbs per mile with
copper-cadmium of the same sizes.
The conductivity of copper-cadmium is approximately 1¾
time that of bronze. This allowed greater lengths of open wire to be run.
P.O. ENGINEERING DEPT.
Issue 2, 30.8.38
WIRING BARE OVERHEAD WIRES
Conditions of Use of Various Types
This Instruction details the conditions of use of the various types of bare
overhead wires listed in E 3011.
2. The choice of wire depends primarily on transmission
requirements, but modifications may have to be made in order to obtain the
necessary mechanical strength to withstand the probable windage stresses,
according to the degree of exposure of the line.
Transmission requirements are dealt with in TRANSMISSION, Telephone, Section B.
A summary of the transmission characteristics of various classes of conductor
will be found on Card TE 140.
4. Wires in situations which are abnormally exposed
In order to reduce the fault liability of wires in situations which are
abnormally exposed, wire of heavier gauge than that demanded purely by
transmission requirements should be used. Cadmium-copper should be used in
preference to copper, as it has a greater mechanical strength for a given gauge
of wire (see E 3011).
5. Subscribers' circuits
40lb. cadmium-copper wire should be used wherever this conductor will meet the
requirements of transmission and mechanical strength.
6. Junction circuits
Cadmium-copper wire of weight not less than 70lb. per mile should be used for
new junction circuits, even where the transmission requirements could be met by
the use of 40lb. cadmium-copper. The greater mechanical strength of 70lb. wire
will give these junction circuits a greater immunity from faults, and will
thereby help to eliminate the serious congestion of traffic which results from
faults on junctions serving small exchanges. (A factor rendering it particularly
desirable to minimize faults on U.A.X. junctions is that such faults lead to.
erroneous reporting of subscribers' circuits as faulty, due to the inability to
obtain connexion to the parent exchange).
7. Trunk circuits
Transmission requirements will demand the use of wire of at least 70lb. per
8. Telegraph circuits
400lb. and 200lb. G.I. and 150lb. copper wires have been largely used in the
past. Under present-day conditions, however, the use of open wires for
long-distance telegraph circuits has rapidly decreased, and there is no longer
any need to distinguish between wires for telegraphs and telephones.
9. Coastguard circuits
In sheltered situations 70lb. cadmium-copper, and in exposed situations 150lb.
cadmium-copper, should be used for circuits serving coastguard and similar
10. Railway crossings
Copper wire, of weight not less than 150lb. nor more than 400lb. per mile,
should normally be used (see E 3020). Circuits run elsewhere with 150lb.
cadmium-copper or bronze should be carried across railways by means of 200lb.
11. Long spans
For exceptional spans, of the order of 100-120yds. in length, 150lb.
cadmium-copper should be employed; such spans should be terminated at both ends.
12. Regulation tables for abnormal spans of this kind may be
constructed by the method outlined in E 1065.
P.O. ENGINEERING DEPT.
Issue 1, 8.4.63
Insulated Line Wires
This Instruction describes the insulated line wires in use at present. It also
gives the conditions in which they should be used together with the methods of
erection, tensioning, terminating etc.
2. Types of wire
Two types of insulated overhead line wire are in use at present, particulars
being given in Table 1. Details are also given in the table of the binding wire
used for the erection of insulated line wire.
3. Conditions of use
Wire, Cadmium-copper, 70lb. H.V. (see J 1201) should be used where P.O. wires:-
cross over or under power wires working at low or medium
voltages (see PROTECTION, Power, D 0016)
cross under power wires working at voltages up to 33kV r.m.s.
between phases or 19kV r.m.s. phase to earth (see PROTECTION, Power, E 0011)
are run in line of route on poles carrying power wires
working at low voltage (see PROTECTION, Power, D 0020) and on poles carrying
high-voltage conductors up to 11kV (see PROTECTION, Power, E 0020)
cross railways other than electrified railways where
adjacent power wires have to be crossed or exist as in (a) , (b) and (c
cross electrified railways operating at voltages up to 750V
d.c. with the third or fourth rail system
pass under bridges carrying electrified railways with
overhead power wires operating at voltages up to 1500V d.c. or 6.25kV 50 c/s
a.c. or are in proximity (as defined in PROTECTION, Power, C 1041) to such a
The P.V.C. covering (thickness of 0.065in.) is coloured blue to avoid confusion with
other types of wire or circuit and the Line Wires
blue type must always be used for any of the purposes given in (a) to (f).
Wire, Cadmium-copper, 70lb. H.V. should not be used where P.O.
routes pass under bridges carrying electrified railways with overhead power
lines operating at 25 kV 50 c/s a.c. (see PROTECTION, Power, C 1041).
4. Wire, Cadmium-copper, 70lb. P .V .C.
This is provided with a covering which is primarily intended to give protection
to the wire against mechanical damage or in corrosive areas where bare wire is
likely to deteriorate rapidly. The wire should be used:-
In localities where the life of bare wire is abnormally
short due to the corrosive atmosphere.
In coastal districts where bare wire is attacked by salt
spray or salt.
As a protection against electrical leakage and momentary
i) overhead power leads to P.B.X.s
ii) wires for a fire-alarm service working on the earthed-wire system when a
request for its use is received from the renter (see Tp. S.I. B4) or when
its use is justified as in (d) (see Utilization, P 3004).
To reduce fault liability due to fleeting contacts between
overhead wires which run through trees, where permission to cut cannot be
obtained or where interference by birds is experienced.
At crossings of railways other than electrified railways
where no power wire crossing is involved.
When the wire is used as in (d), the wire should be kept clear
of branches and rubbing contacts which will unduly chafe the wire covering and
defeat its purpose. The use of the wire should not be resorted to as a matter of
course to avoid systematic tree cutting (see E 5101) if there is no restriction
on tree cutting.
5. Carrying capacity of poles
Before covered wires are erected, it should be ascertained whether the carrying
capacity of existing poles (see C 3001) is likely to be exceeded by the
additional stress imposed under storm conditions and, if so, stronger poles
should be erected as necessary. The staying of the line should also be examined
and strengthened as required. The wind pressure on each type of covered wire due
to 40, 60 and 80m.p.h. winds is indicated in Table 1.
6. Wire erection
Wire, Cadmium-copper, 70lb. H.V. is a covered wire erected at power crossings or on
poles in line of route carrying power wires must be erected in a continuous
length without joints within the span. Where it is necessary to make a joint
between one span and another, the joint must be made within the cavity of an
Insulator No. 16 by means of Inserts, Insulator, No. 1 with one wire in each of
the common terminals. The insulator cover should be replaced by a Cover,
Protector for Insulators, No. 16. At intermediate insulator positions the wire
should be bound-in to the insulator with Wire, Binding, P.V.C. (see par. 14).
Where P.O. wires cross over power wires a termination or double
termination on an Insulator No. 16 should be made at each end of the crossing
span. The connexion between the tails of a double termination should be as for a
connexion between spans.
7. At the junction of bare wire and H.V. wire, each wire should
be terminated on an Insulator No. 16 mounted on a Spindle No. 13 or No. 10. At
intermediate poles where covered wire is to be jointed to the next span, both
spans should be terminated on one Insulator No. 16 mounted on a Spindle No. 4.
Elsewhere Insulators No. 7 or No. 3 should be used for H.V. or P.V.C. wire
respectively. The insulators used for supporting Wire, Cadmium-copper, 70lb.
H.V. may be white or black.
8. Wire, Cadmium-copper, 70lb. P.V.C.
Similar forms of construction to those described in pars. 6 and 7 should be
adopted when Wire, Cadmium-copper, 70lb. P.V.C. is to be erected, except that
the insulator covers should not be replaced by Covers, Protector for Insulators,
No. 16. The method of terminating, tensioning, cross-connecting etc. described
in the following paragraphs should be adopted for both types of covered wire
referred to in this Instruction.
WIRE, CADMIUM-COPPER, 70LB. H.V.
9. Terminating covered wire
Both types of covered wire should be terminated by the method described in E
3080 (as for 100lb. copper wire) using Wire, Binding, P.V.C. Sufficient line
wire should be left projecting from the termination to be fed through the cable
hole under the shed of the Insulator No. 16 to permit connexion to the insulator
Covered wire should be regulated to the tensions specified in Table 2. When
covered wires are erected on the same pole line as bare wires, the covered wires
should be placed below the bare wires and regulated to the tension shown in
Table 2. Covered wires must NOT be regulated to give the same dip or sag as the
bare wires since it would result in over tensioning and possible early failure
under storm conditions.
11. The general principle to be observed and the tools required
for tensioning covered wire are detailed in E 3064. The jaws of the wire vice of
the ratchets and tongs are not suitable for gripping covered wire since they
would damage the wire covering or slip. The difficulty may be overcome by means
of a length of 1½in. rope made up with a
spliced eye at one end as shown in Fig. 1. The other end of the rope should be
whipped to prevent the strands of the rope unravelling.
12. The method of applying the rope to the covered wire to
obtain a grip is as follows:-
Loop the spliced eye of the rope over the wire jaws of the
ratchets and tongs.
Twist the rope against the lay as at A (Fig. 1) in a
clockwise direction thereby separating the strands to receive the wire.
Lay the wire in the opened strands and pass the opened
section of rope around the wire in an anticlockwise direction as shown at B
Proceed as in (b) and (c) above until approximately 18 in.
of the wire is enclosed by the rope.
Proceed with tensioning in the normal way.
13. An alternative method is to plait a length of Tape, Linen
for about 18in. near the end of the covered wire and then apply the tension to
the free end of the tape. The plaiting should be done by laying the midpoint of
the tape across the covered wire, and crossing and re-crossing the ends over the
wire and over each other with a lay of about twice the width of the tape. The
ends of the tape should then be tied to the ratchet. The width of the tape
should be in. for tensions up to 37lb. and 2in. for tensions from 37 to 75lb.
At points where the wire is not to be jointed, the covered wire should be
bound-in to the insulator with Wire, Binding, P.V.C. by the following method.
The line wire should be whipped with binding wire over a length equal to the
diameter of the neck of the insulator. The whipped portion should then be placed
in the groove of the insulator and the two ends of the binder passed around the
groove of the insulator in opposite directions, given one and a half laps round
the line wire and the ends of the binder again passed round the groove in
opposite directions. The binding is then completed by making ten turns of the
binding wire on the covered line wire on each side of the insulator. A binder
60in. long is required for the purpose. The binding is illustrated in Fig. 2.
The joining of two spans of covered wire terminated on a single
insulator is performed by leading the tails into the insulator via the cable
holes and connecting them together by an Insert, Insulator, No. 1. See Fig. 4.
15. Cross connecting spans
At the junction of bare wire and covered wire, the bare wire tail from the wire
termination should be led into the insulator via the wire hole in the side and
connected to the terminal of an insulator insert fitted in the cavity of the
insulator on which the bare wire is terminated. The tail from the covered wire
termination should also be led to the insulator on which the bare wire is
terminated. The tail should be bound to the insulator spindle with scrap Cable,
Leading-in, 1 pair 12½ Flat and taken into
the insulator through a cable entry hole. The wires should be connected by means
of an Insert, Insulator, No. 1. The arrangement is illustrated in Fig. 3.
If Wire, Cadmium-copper, 70lb. H.V. is used on shared service
lines or is connected to Cable, Drop Wiring, Nos. 1 or 2 the cross-connexion
should be made with Cable, Leading-in, 1 pair 12½
Flat as described in par. 16.
If a shared service connexion is made with Wire, Cadmium-copper,
70lb. H.V. and two Cables, Drop Wiring, each drop wire should be terminated on
an Insert, Insulator, No. 2 in an Insulator No. 16 and the connexions made as
shown in Fig. 5.
For a connexion between Wire, Cadmium-copper, 70lb. H.V. and a single drop wire
the connexions should be as in Fig. 5 except that one drop wire and its
associated cross-connecting leads should be omitted.
16. Cross connecting covered wire to block terminals
Where it is necessary to cross-connect H.V. wire to a terminal block on poles
carrying power wires in line of route, Cable, Leading-in, 1 pair 12½
Flat should be used. The two wires of the cable should be twisted together and
used as a single conductor and the cable led into the insulator cavity via one
of the cable holes and connected to the H.V. wire by means of an Insert,
Insulator, No. 1.
17. Joints within spans
Joints should not be made within a span of covered wire, especially H.V. wire.
Any wire breakage that may occur must be repaired by replacing the whole span or
by terminating the broken section at the nearest convenient poles and erecting a
new section of wire and cross-connecting it to the existing wire as described in
P.O. ENGINEERING DEPT.
Issue 1, 1.10.37
INSULATED OVERHEAD LINE WIRES
Description and use of "Wire, Insulated, P.B.J."
Brief details of the composition of "P.B.J." (Paper, Braided, Jute) covered
wire, and the circumstances under which it should be used, are outlined in this
Instruction. Particulars of size, weight and ohmic resistance are given in E
3013. The kindred conductor, "Wire, Insulated, J" forms the subject of E 3021.
Description of "Wire, Insulated, P.B.J."
Conductor - One of two materials and several differing gauges of conductor
are used, depending upon the mechanical strength and conductivity required.
Cadmium-Copper is used for wire which weighs 70lb. per mile, and Copper for
those conductors which weigh 150lb., 200lb., 300lb., and 400lb. per mile.
Covering - The conductor is provided with two helical lappings of
oil-impregnated, chemically-pure paper of high insulating properties and which
is also mechanically strong. It is then given a lapping of cotton, and a tight
cotton braiding, both of which have been impregnated with a mixture of Red Lead,
Linseed Oil and Paraffin Wax. Finally, the completed cable is immersed in hot
paraffin wax and wiped so as to leave a fairly smooth and glossy surface.
Characteristics of P.B.J. wire
The P.B.J. covering is intended primarily to provide a dielectric of high
insulation resistance (of the order of 10,000 ohms per mile of single wire). It
also confers protection from chemical corrosion, but offers little resistance
to continued abrasion even if the pressure is reasonably light.
Conditions of use
P.B.J. Covered Wire is used to cross unguarded low- and medium-pressure power
circuits where it has been agreed with the power Undertakers that such a method
of construction will be more acceptable than the provision of some other form of
Selection of conductor
The conductor which is selected must be such as will meet the transmission
requirements of the circuits which are to be carried, as specified in
TRANSMISSION, Telephone, B 3006, B 3501, B 3502.
Carrying capacity of poles, and staying
Before covered wire is erected, it should be ascertained if the carrying
capacity of the existing poles (see C 3001) is likely to be exceeded by the
additional stress to be imposed, and stronger poles should be erected if
necessary. The staying of the line should also be examined, and strengthened as