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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.


Issue 2, 30.8.38

Conditions of Use of Various Types

1. General
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.

3. Transmission
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 mile.

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 life-saving posts.

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. copper conductors.

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.

E 3016
Issue 1, 8.4.63

Insulated Line Wires

1. General
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:-

  1. cross over or under power wires working at low or medium voltages (see PROTECTION, Power, D 0016).

  2. 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)

  3. 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).

  4. cross railways other than electrified railways where adjacent power wires have to be crossed or exist as in (a) , (b) and (c).

  5. cross electrified railways operating at voltages up to 750V d.c. with the third or fourth rail system.

  6. 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 system.

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:-

  1. In localities where the life of bare wire is abnormally short due to the corrosive atmosphere.

  2. In coastal districts where bare wire is attacked by salt spray or salt.

  3. As a protection against electrical leakage and momentary contacts for
    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).

  4. 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.

  5. 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.


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 insert.

10. Tensioning
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:-

  1. Loop the spliced eye of the rope over the wire jaws of the ratchets and tongs.

  2. Twist the rope against the lay as at A (Fig. 1) in a clockwise direction thereby separating the strands to receive the wire.

  3. 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 (Fig. 1).

  4. Proceed as in (b) and (c) above until approximately 18 in. of the wire is enclosed by the rope.

  5. 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.

14. Binding-in
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.

Fig. 1

Fig. 2

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.

Fig. 3

Fig. 4

Fig. 5

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 par. 15.

E 3026
Issue 1, 1.10.37


Description and use of "Wire, Insulated, P.B.J."

1. General
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 guarding

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 required.




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Last revised: March 12, 2022