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B 1001
Issue 3, 2.4.63

General Purpose and Description

1. Introduction
Where experience has shown protection to be necessary, apparatus connected to external lines is safeguarded against damage from lightning discharges or from currents resulting from contacts with electricity supply circuits by the lightning protectors, fuses and heat coils described in this Instruction. These protective devices are unaffected by normal speech and signalling currents.

Lightning protectors are provided in certain circumstances to prevent damage to external cables.

The Instructions which indicate the type of protective device (if any) that should be fitted are as follows:-

  1. At subscribers' premises B1002.

  2. At P.B.X.s B1003.

  3. At exchanges B1004.

  4. At D.P.s and terminal poles B1006.

2. Lightning protectors
The function of a lightning protector is to provide a discharge path to earth at a point between line and apparatus (or in some circumstances between overhead line and cable) whenever the circuit is subjected to an abnormally high potential, e.g. due to a lightning discharge. The following properties are desirable in these devices:-

  1. They should efficiently earth the line when its potential above earth reaches a predetermined value. (The present P.O. specification requires operation between 600V and 900V).

  2. They should earth both lines of a circuit simultaneously so as to reduce the risk of acoustic shock.

  3. They should carry heavy discharges without undue heating or arcing.

  4. They should provide high insulation up to the time of breakdown.

  5. They should be self-restoring.

  6. They should be capable of easy removal, examination and replacement.

3. Limitations of P.O. type lightning protectors.

  1. The protectors used by the P.O. will safely discharge to earth the majority of the surges normally induced on overhead lines by lightning discharges. In the case of very heavy surges with direct strokes however, the energy to be discharged to earth is so great that the protector fitting may be destroyed, the line wires and leading-in cables may be fused or even volatilized and the line insulators may he shattered. It is not practicable to avoid such effects.

  2. A lightning protector is not normally called upon to discharge to earth potentials arising from contact with high-voltage power lines as other measures are taken to ensure that such contacts do not occur.

  3. Lightning protectors cannot be relied upon to discharge bath lines of a circuit to earth simultaneously. In so far as it is possible for one line to be discharged through the telephone when the protector breaks down on the other line of the pair, protectors may be said to introduce some risk of acoustic shock.

4. Fuses
The function of a fuse is to disconnect the line should an excessive current flow. The properties desirable are that it should rupture quickly and quietly without scattering of molten metal as soon as sufficiently high current passes which might cause damage to plant and possibly a fire. It should be so designed that any arc which may be set up when the fuse wire melts cannot be maintained by the voltage against which it is desired to guard.

5. In the line fuses used by the P.O. the fuse wire is enclosed in a glass, ceramic or asbestos tube between metal end-caps.

6. In P.O. nomenclature fuses are described by two numbers separated by an oblique stroke. The first number denotes the mechanical features of the fuse and the second indicates approximately the rating current.

7. The 'rating current' is the maximum current the fuse will carry indefinitely without rupture. The 'fusing current' is the minimum current which will cause the fuse to rupture within the time period for which it is designed.

8. Following the introduction of the new protection arrangements described in B1004 separate fuses and heat coils will no longer be fitted at the exchange; these will be replaced by a single basic unit known as a 200mA surge-resisting fuse (see Fig. 1) which combines the functions of the two units. This basic unit is available in several forms, which are described in pars. 20-26, and enable it to be fitted not only on a new M.D.F. but also in Fuse-mountings Nos. 4001 and 4028.

FIG. 1

9. The fuse element of the surge-resisting fuse i.e. the essential operating portion as distinct from the housing, comprises three distinct parts, a helical spring, low melting point solder and fuse wire all enclosed in a glass tube fitted with end-caps. Most of the electrical resistance of the element is in the fuse wire which, with sufficient current flowing through it, develops enough heat to melt the solder which allows the spring to retract and the fuse becomes open-circuited. With large currents the fuse wire will melt. The fuse is rated at 200mA and will rupture within 5-300 seconds when carrying 350mA. Its voltage rating is 250V 50 c/s a.c.

10. Heat coils
The function of a heat coil is to give protection against sustained currents which are smaller than those necessary to rupture the fuse but are of sufficient magnitude to cause damage to exchange apparatus through overheating. The heat coil is essentially a delayed action fuse which, when operated, earths the line wire. The properties desirable properties in a heat coil are:-

  1. It should operate at a current which, although too small to rupture the line fuse, would, if permitted to flow indefinitely, be sufficient to harm the apparatus and possibly cause fire.

  2. It should be mounted so that its operation will connect the line to earth.


11. Protector-electrodes No. 1B (Fig. 2)
These supersede all types of carbon protector for new work and maintenance replacements. They consist of two channel-section brass electrodes separated by a perforated insulating film, the whole being enclosed in a black moulding of high insulation resistance and low inflammability. The flanges of the electrodes project from the moulding and will fit all types of mountings taking channel-section protectors.

The breakdown voltage of Protector-electrodes No. 1B is specified as 600V to 900V D.C. and since the air-gap is sealed this characteristic is maintained even in damp  or dusty situations.  The resistance of the air-gap is high under normal working voltages and it will usually restore after a discharge has occurred, but in the event of a heavy discharge the brass electrodes may weld together and so end the item's useful life.  The breakdown voltage of Protector-electrodes No. 1A is specified as 500V to 750V).

FIG. 2

12. Protector-electrodes No. 1C
These are identical in appearance with Protector-electrodes No. 1B except that the moulding material is in blue. The air-gap is, however, increased and the resultant operating voltage range is 1100V to 1700V. These protectors should be used only on the instructions of the Eng. Dept. (Cn1/2).

13. Protector-electrodes, Dummy, No. 1
These have the same physical dimensions as Protector-electrodes No. 1B, but are made wholly of moulded plastic.


14. At exchanges and large P.B.X.s installed before January 1960, fuses rated at 1.5A are used which rupture within 30 seconds of the current reaching 3.3A. These rating and fusing currents are higher than seem necessary considering the working currents of the circuits involved but were adopted to avoid frequent fuse failures due to lightning surges. Heat coils are always used in conjunction with these fuses.

The various types of fuse are described in pars. 15 to 19.

15. Fuses No. 1/1.5 (Fig. 3)
The fuse wire is inserted in a small-bore ceramic (Porcelain) tube fitted with end-caps. These fuses are fitted in Fuse-mountings No. 4001.

FIG. 3
FUSE No. 1/1.5

16. Fuses No. 1A/1.5
These are similar to Fuses No. 1/1.5 but are fitted with wire 'tails' for use on high grade circuits (see TELEPHONES, General, Z 5006).

17. Fuses No. 2/1.5 (Fig. 4)
The fuse wire is enclosed in a glass tube in. external diameter. The fuse is provided with copper 'spade' tags at its ends and is held in its mountings by screws 2 & 3/16in. between centres. It is used on Fuse-mountings No. 4002. A new type fuse has not been designed to replace this fuse.

FIG. 4
FUSE No. 2/1.5

18. Fuses No. 28/1.5 (Fig. 5)
The fuse wire is enclosed in an asbestos tube held between flat body pieces of pressboard or Bakelite with copper end-caps which grip the fuse, asbestos tube and pressboard/Bakelite. It is used in Fuse-mountings No. 4028.

FIG. 5
FUSE No. 28/1.5

19. Fuses No. 28A/1.5 are similar to Fuses No. 28/1.5 but are fitted with wire 'tails' for use on high grade circuits.



20. Fuses No. 1B/0.2 (Fig. 6)
This is the new standard fuse for Fuse-mounting No. 4001, Fuses No. 62A/0.2 which are described in par. 24 being obsolete.

FIG. 6
FUSE No. 1B/0.2

21. Fuses No. 1C/0.2
These are similar to Fuses No. IB/0.2 but are fitted with wire 'tails' for use on high-grade circuits terminated on Fuse-mountings No. 4001.

22. Fuses No. 28B/0.2 (Fig. 7)
This fuse, which is designed to fit into Fuse-mounting No. 4028, is for use at exchanges using these mountings and which have changed to the new protection policy which commenced in January 1960. These fuses supersede Fuses No. 62B/0.2 described in par. 25.

FIG. 7
FUSE No. 28B/0.2

23. Fuses No. 28C/0.2
These are similar to Fuses No. 28B/0.2 but are fitted with wire 'tails' for use on high grade circuits terminated on Fuse-mountings No. 4028.

24. Fuses No. 62A/0.2 (Fig. 8)
The basic unit in these fuses is mounted in a pressboard tube having end-caps suitable for fitting into the Fuse-mounting No. 4001. It is for use at exchanges which have been changed to the new policy of protection. This fuse is superseded by the Fuse No. 1B/0.2.

FIG. 8
FUSE No. 62A/0.2

25. Fuses No. 62B/0.2 (Fig. 9)
This fuse, which has wire 'tails', is identical to that used in the Fuse No. 62A/0.2, but is fitted in flat pieces of pressboard with end-caps which will fit into the Fuse-mounting No. 4028. It is used at exchanges which have been changed to the new policy of protection. This fuse is obsolete and is superseded by the Fuse No. 28B/0.2.

FIG. 9
FUSE No. 62B/0.2

26. Fuses No. 64/0.2 (Fig. 10)
This is the new standard fuse designed to fit into Fuse-mounting No. 8064 at exchanges equipped with rack-type M.D.F.s.

FIG. 10
FUSE No. 64/0.2


27. Coils, Heat, No. 1 (Figs. 11 and 12)
These were fitted on the M.D.F. at exchanges installed before January 1960. The coil is wound on a thin copper tube and has a resistance of 5.1 ohms, maximum. The copper tube is mounted on a pin screwed into a plastic thimble which carries on its end a brass end-cap insulated from the pin. One end of the coil is threaded through the plastic thimble and is attached to the brass end-cap; the other end of the coil is soldered to the copper tube which is in turn soldered to the pin at the same point through a V-shaped slot in the middle of the tube. A special solder having a low melting point is used. The heat coil will carry a current of 300mA for three hours without operating but will operate with 550mA within 3½ minutes. It is held in its mounting (Fig. 12) under pressure of the outer springs, the circuit through the coil being from the fixed inner spring, through the contact between that spring and the copper tube, the coil wound on the tube, the brass cap and thence to the outer springs.

Under operating conditions, the heat generated in the coil causes the solder to soften and this allows the body of the heat coil, under the pressure of the outer springs, to move inwards. The brass pin presses the travelling spring against the earthed contact stud and directly earths the line. The fault current then no longer passes through the coil and the solder cools and resets. If, as a result of operation of the heat coil, the current from the external line rises in magnitude sufficiently, the fuse ruptures.

FIG. 11

FIG. 12

28. Coils, Heat, Dummy B (Fig. 13a)
These are small metal stampings designed to fit in place of Coils, Heat, No. 1 on Protectors, H.C. & Test, 40B (see par. 30).

The two projections at the ends are of different size and care must be taken to ensure that the smaller projection is inserted into the spring nearest to the protector-electrode with the convex side towards the front.

29. Coils, Heat, Dummy, No. 1 (Fig. 13b)
These supersede Coils, Heat, Dummy, B and are designed to fit horizontally in the Protectors, H.C. & Test, 40B. They are intended to be inserted using Pliers, Wiring, No. 2 and have guides pressed into the body to prevent rotation when held by the pliers.

FIG. 13


30. Protectors, H.C. & Test, 40B (Fig. 14)
This unit provides heat coils, lightning protectors and test facilities for 40 wires and is used on M.D.F.s installed before January 1960. The test facilities are obtained by means of the external springs which allow disconnexion of the line without removal of the heat coils.

On rack-type M.D.F.s installed at exchanges with the new standard of protection, Protectors, H.C. & Test, 4013 are replaced by Jacks, Test, No. 33/1A which provide test facilities only and do not house protection items.

FIG. 14

31. Protectors, H.C. & Test, 40B Dummy
These are the same as Protectors, H.C. & Test, 40B except that they are supplied and equipped throughout with dummy heat coils and dummy protectors. They are intended for use on M.D.F.s of the former standard which are being extended and re-equipped to the new standard of protection.

32. Protectors and Fuses No. 1 2/2 OM (Fig. 15)
This unit was developed for use in subscribers' premises and supersedes both Protectors and Fuses 2/2 O.F. and Protectors and Fuses 201 O.F. The unit consists of two plastic mouldings (base and cover) and accommodates two Protector-electrodes No. 1B and two Fuses, Dummy, No. 37.  The unit is supplied complete with Fuses, Dummy, No. 37 but without the Protector-electrodes No. 1B which should be obtained separately if required.

FIG. 15

32*. Protectors and Fuses No. 1 2/2 O.F.
Fitted to single lines this is similar to Protectors and Fuses No. 1 2/2 OM except the unit has two Fuses No. 37/3 instead of dummy fuses.  The 2.5A fuses are hexangular in shape and have blade type connectors.  These were superseded by Protectors and Fuses No. 1 2/2 OM.


33. Protectors, Heats Coil and Fuses 0/10, 0/14 and 0/20
These are iron frames and covers designed to accommodate the number shown, in the denominator of the part number, of Protectors and Fuses No 1 2/2. If minus Cover (a Rate Book item). The units should be used where a number of lines require protection at the same point.  These frames were originally designed for the Protectors, Heat Coils and Fuses No. 201 and/or Block Terminal No. 5.

Drawing - EC1101.

Protectors, Heat Coils and Fuses 0/10 - Cover removed
Block Terminal No. 5 is shown to the left and Protectors, Heat Coils and Fuses No. 201 to the right

Protectors, Heat Coils and Fuses 0/10 - Cover fitted

Protectors, Heat Coils and Fuses 0/20

33*. Protectors, Heat Coil, & Fuse 2/2
Used on single line installations this unit includes Protectors (2 x Carbon, Protector Nos. 6, 7, 9, 10 or 11), Heat Coils (2 x Coils, Heat, "A", green) and fuses (2 x Fuses No. 1/2).  The base is made of Porcelain and the cover is metal with an insulated surface on the inside.  Later versions had a plastic cover.  Superseded by Protectors and Fuses No. 1 2/2 OF.  Introduced late 1920's.

A Protectors, Heat Coil, & Fuse 2/2  Flameproof was also available.  This was a Protectors, Heat Coil, & Fuse 2/2 installed in a cast iron enclosure. 

Drawing - EC1101 (1939).


33*. Protectors, Heat Coil and Fuses No. 201
These are a single line unit, comprising two protectors and two fuses, that can be fitted onto the Protectors and Fuses 0/10, 0/14 and 0/20.  These are superseded by Protectors and Fuses No. 1 2/2 OM.

Drawing - EC1101.

FIG. 15*

34. Protectors, H.C. & F. 40/40, 40/80 and 80/80
These include Protectors, H.C. & Test, 40B to the capacity indicated by the numerator of the item reference number and Fuse-mountings No. 4002 to the capacity indicated by the denominator of the item number, with provision for a cross-connecting jumper field between the two main items of equipment.  These units are used on certain P.B.X. installations and employ Coils, Heat, No. 1 and Fuses No. 2/1.5.  All the components are mounted on a backplate which is covered with a lid.  Approximate size is 12" across x 15" high x 8" deep for the 40/40 Mark 235, 14.5" across for the 40/40 Mark 236 and 25.5" across for the 40/80 & 80/80 Mark 236.

Normally fitted with Protectors 'D', Heat Coils 'A' and Fuses No. 2/2.

Drawings - 1732 and EC999 both dated 1910.

35. Fuse-mounting No. 4001 (Fig. 16)
This comprises a mild-steel framework which carries clips for 40 Fuses No. 1/1.5.  Alternate fuses are tilted in opposite directions to facilitate removal and replacement.  The mounting is superseded by the Fuse-mounting No. 8064 for new work.

FIG. 16

36. Fuse-mounting No. 4002
This fuse mounting, used on early Frames M.D., also forms an integral part of Protectors, H.C. & F. 40/40, 40/80 and 80/80 and is fitted with Fuses No. 2/1.5. Where the latter need to be replaced by dummy fuses, short lengths of juniper wire should be used and secured under the screws provided to retain the fuses. Provision is made for the mounting to receive 20 Labels No. 55 for circuit designation.

37. Fuse-mounting No. 4028 (Fig. 17)
This consists of a mild-steel framework with two hinged sides and it is arranged to accommodate 40 Fuses No. 28/1.5. The unit is designed for fixing to M.D.F.s and is normally supplied without a cover. When the mounting is used to terminate junction and trunk cables, a Cover, Fuse-mounting, No. 4028 should be provided. This mounting is superseded by Fuse-mounting No. 8064 for new work.


FIG. 17

38. Fuse-mounting No. 8064
This has a capacity for 80 Fuses No. 64/0.2 and the 40 circuit unit is built up from 20 individual two-circuit assemblies which are interlocked by moulded dowels and clamped by two long bolts. The fuse-mounting is made up in two forms which differ only in the supporting metalwork and are as follows:-

  1. Fuse-mounting No. 8064A - The supporting metalwork for this unit is provided with a triangular tail piece similar to that of Fuse-mounting No. 4001 and 4028 (see Figs. 16 and 17), and is suitable for mounting on existing M.D.F.s.

  2. Fuse-mounting No. 8064/B - This item, which is shown in Figs. 18 and 19, is arranged for mounting on the new rack-type M.D.F.

Fig. 18 shows the face of the mounting equipped with two Fuses No. 64, two Fuses, Dummy, No. 64 and a Cover No. 64 which is used for identification of circuits.

FIG. 18

FIG. 19


39. To implement the new protection policy of fitting lightning protectors at the most effective point, the Protector, Unit Pole Top 15 Pair and the Protector, Insert for Insulators were introduced for use at the junction of overhead lines with other plant.

40. Protector, Unit Pole Top 15 Pair (Fig. 20)
This unit is used to replace the block terminal at D.P.s and terminal poles where more than six circuits require protection. The unit provides terminations for the cable pairs and the leads from open wires and also has mountings for Protector-electrodes No. 1B to be associated with each wire. Installation of the unit is described in LINES, Overhead, C3011.  This device would be earthed.

FIG. 20

41. Protectors, Insert for Insulators (Fig. 21)
This is designed to fit into the cavity in the top of an Insulator No. 16, Black or White. A specially deep cover, a Cover, Protector for Insulators No. 16, Black or White, is provided to give the necessary clearance. Screwed terminals are provided, one each for the line wire, the lead wire and the earth wire. The Protector-electrode No. 1B is supported in two flat springs, one attached to the line terminal and the other to the earth terminal. Installation of Protectors, Insert for Insulators is described in LINES, Overhead, G3008.

Drawing - CN175.

FIG. 21

42. Protectors, Unit, 50 Pair (Fig. 22)
This unit is designed for installation in cabinets and pillars and is intended for use for where a large number of pairs require lightning protection. It consists of five 10-pair strips each of which accommodates 20 Protector-electrodes No. 1B thus providing protection for up to 50 pairs. The protector-electrodes are fitted between supporting springs and the central common earth bar. The tail cable is fitted locally to the unit (see LINES, Underground, L3121) and teed in at the cable joint where the protection is required. The present policy is that polythene aerial cable is treated as open wire for lightning protection purposes; protection should therefore be applied where polythene aerial cable joins lead cable or enters the underground network. This is where the Protectors, Unit, 50 Pair will mostly be used.

FIG. 22


B 1006
Issue 3, 20.11.61

Lightning Protection

1. General
The new protection policy as described in B1004 requires that where lightning arrestors (Protector-electrodes No. 1B) are required on external lines, they should be connected at the top of D.P. and terminal poles.

This Instruction states the policy for provision of lightning arrestors and lists the methods to be adopted.

2. Policy at D.P.s
Lightning protection is required only on circuits with more than four open-wire spans, a drop wire lead to a subscriber's premises being counted as one span. The methods of providing the arrestors are given in par. 4.

3. Policy where open wires extend MU and CJ cable pairs
Lightning protection should be fitted at the terminal poles of all overhead circuits, irrespective of length, where these are connected to MU and CJ cable pairs. The methods of providing the arrestors are given in par. 4.

4. Methods of providing lightning arrestors
These depend on the number required and are as follows:-

  1. Where six or fewer circuits require protection, a Protector, Insert for Insulators should be fitted to each line wire. Installation of the Protector, Insert for Insulators is described in LINES, Overhead, G3008.

  2. Where more than six circuits require protection, a Protector Unit, Pole Top, 15 pair should be fitted. The unit should be fully equipped with Protector-electrodes No. 1B. The installation of the unit is described in LINES, Overhead, G3005. The Protector Unit, Pole Top, 15 pair, which is a proprietary item catering for 16 lines, will be used for an interim period.

  3. Where more than 15 pairs of conductors require lightning protection at a D.P. or terminal pole, each group, or part group of up to 15 pairs, should generally be treated as a separate 15-pair group and provided with a Protector Unit, Pole Top. If, however, a part group contains six or less circuits then Protectors, Insert for Insulators may be used to protect those circuits.

  4. Where a non-metallic-sheathed aerial cable with more than four overhead spans is connected to an underground cable of any type or to a metallic-sheathed aerial cable, then a Protector Unit, Pole Top should be fitted at the point of interconnexion. If the cables to be interconnected have more than 15 pairs, the case should be referred to the Eng. Dept. (Cn2/2) via the R.D., (Eng. Bch.).

The extent to which future development should be taken into account when deciding whether (a) or (b) applies should be determined locally. When method (a) has been chosen, it should not normally be changed.

5. D.P.s damaged frequently by lightning
At present general retrospective action is not being taken to apply the new protection policy. If a cable serving a D.P. is known, however, to be particularly vulnerable to lightning damage, protection may be provided in accordance with par. 4. Where this is done, the protection arrangements on the M.D.F. involved should remain unchanged.

6. Fault reports
Any case of lightning damage to cables should be reported by completing the general section and part A of form A 248 for CL, or form A 298 for MU and CJ.

Early Protectors
Extracts from Technical Instruction III, Part 2 (1925)


46. Protectors, including Fuses and Heat Coils, where necessary, are fitted in telegraph circuits to safeguard the office plant, but it should be realized that they are not designed to withstand high pressures. To eliminate fire risk high pressure power systems should be guarded in such a manner that direct contact with telegraph plant is impossible. Special attention should also be given to guarding medium pressure systems where the resistance in the telegraph conductor between the point of possible contact and the office is low, say, less than 10 ohms, as there is risk of arcing where large currents are involved. The function of a lightning protector is to discharge to earth any, current induced by lightning in telegraph wires before the potential rises to a high value. Lightning protectors usually Consist of two electrodes, one connected to line and the other to earth; the electrodes being separated by a small spark gap which breaks down at comparatively low: voltage, say, 300 to 350 Volts.

Protectors of old type including the National Telephone Company's pattern arresters with serrated edges, should be retained in position until they have developed definite faults or until alterations of plant are made which render it desirable to install modern types.

Spark Gaps.
47. Spark gaps of lightning protectors fall into three main classes:-

  1. Open spark gap - air at atmospheric pressure.

  2. "Vacuum" protectors - air or other gas at very low pressure.

  3. Metallic oxides.

A spark gap in air at atmospheric pressure should not exceed a few mils for the protector to operate at 300 to 350 volts. With rigidly, fixed electrodes very accurate machining would be necessary to produce such a spark gap. The common practice, therefore, is to make one or both electrodes adjustable, or to separate the electrodes by a perforated insulating sheet of the requisite thickness, the electrodes being pressed together against the separator by means of a spring or screw. Where this practice is not adopted a longer spark gap, usually 15 or 16 mils, is employed with the result that the break-down pressure is; considerably higher than 350 volts.

The length of spark which can be produced by a, given potential difference is much greater in a gas at very low pressure than in air at atmospheric pressure. The electrodes of vacuum protectors designed to operate at 300 volts can therefore be placed further apart than is possible with open spark gap protectors. The advantages of a longer spark gap and the exclusion of dust and moisture are obtained by enclosing the protector in a glass vacuum tube. By this means insulation under normal conditions is increased and the frequency of  faults reduced. The cost and size of vacuum protectors considerably restrict their use. Vacuum protectors frequently become defective with age, owing to a slight increase in gas pressure, the spark gap then requires a very large increase in the electrical pressure before it breaks down. Increase in gas pressure may be due to imperfect sealing of, the tube or to exudation of gases occluded in the substance of the electrodes.

Certain metallic oxides which are non-conducting under small electric stresses, break down under heavy stresses and this property has been used in some types of lightning protector designed for protecting power circuits. Protectors of this type have not been used by the Post Office.

48. Electrodes
Electrodes may be of metal or carbon. Fixed electrodes are usually of metal, which is mechanically stronger than carbon and can be machined with precision. Carbon possesses several advantages over metal, however, being chemically inert at ordinary temperatures it is free from corrosion trouble and its high temperature of volatilisation reduces the arcing tendency between electrodes. A comparatively heavy discharge can be carried for a short time without appreciably damaging the carbon.

49. Protector "A" (Figure 7)
In this protector the electrodes are circular metal discs, two inches in diameter separated by a perforated mica disc. The electrodes are pressed together against the separator by means of a central screw which also serves as a terminal for connecting the line wire to the protector. The protector is reasonably efficient, but has been largely superseded by D and G protectors which are of more convenient dimensions.

Protector "A"

Protector "B" (Figure 8)
50. Protector B is similar to Protector A, except that there are two line electrodes, one on each side of the earth plate, to provide for the protection of two lines.

Protector "B"

51. Protector "C" (Figure 9)
In this protector the electrodes are circular carbon discs 1 & 3/8inches in diameter. The line carbon is mounted on a metal back-plate with a projecting pin which fits into a hole in a connection plate or bar. The other electrode is held in position by a spring clamp which is connected to earth. The separator is a mica sheet with holes to form the spark gap. The separator is slightly larger in diameter than the electrodes in order to minimise leakage over the edges.

The carbons are called Carbon, Protector No's 1 and 2 and the separator is a Mica for Carbon 'C'.

Protectors C are used for protecting short multiple wire submarine, or under-river cables. (See Protectors and Fuses, Para. 58). They are also extensively used in Box, Pole Test E, for the protection of underground cables connected to open wires, but in this case, the carbon with metal back-plate is made the earth electrode.

Protector C is an efficient lightning arrester, but should be fitted in a dry situation to ensure satisfactory insulation between the electrodes. It has been superseded as an office protector by Protector D.

Protector "C"

52. Protector "D" (Figure 10)
This Protector will hold two sets of electrodes.  It consist of a porcelain base with cover.  The electrodes are small carbon blocks with flat oblong faces 1¼ inch by 3/8 inch and are called Carbons, Protector No's 10 (outer carbon) and 11 (inner carbon). A Mica No. 2 is placed between the two carbons.  A fusible plug is cast in a recess in the earth electrode so that a heavy discharge melts the fusible plug and puts the line and earth electrodes in metallic contact. The face of the line carbon is recessed and the outer edges chamfered to reduce leakage.

The separator is a mica sheet, three to five mils thick, containing three it inch holes as spark gaps. The electrodes are held in position between a line spring and an earth strip, the backs of the carbons being slotted to fit the spring and strip.

Protectors D are efficient, cheap and take up little space, but must be fitted in dry situations.

Carbon and Mica Drawing - 62948.

Protector "D" Carbons


Protector "D" 2/2
This is based on the later style of protector mounting and has provision for two electrodes.  It is used on customers premises and is provided with a cover.

Drawing - EC1101.

Protector "D" 2/2

53. Protector "F" (Figure 11)
The outer or line electrode is a copper cylinder 3 & 11/32 inches long and 1 & 15/16 inches internal diameter. The inner or earth electrode is a cylindrical copper strip recessed 1/64 inch below the surface of an ebonite insulator on which it is mounted. When the outer cylinder is slipped over the insulator and held in position by two screws, a 1/64 inch spark gap of very large area is formed between the electrodes.

Protector F is fitted on the pole arm close to the terminal and leading-in insulator. It is used for coastguard or other circuits in isolated and exposed situations in addition to the ordinary carbon protectors inside the building.

Protector "F"

54. Protector "G" (Figure 12)
The electrodes are carbon blocks enclosed in a glass vacuum tube; the overall length of the protector is 2½ inches and diameter ¾ inch. The protector can be fitted in Box, Pole Test E by means of Clips, Protector, G1 and G2, whilst wire extensions can be fitted locally when it is necessary to fit this type of protector in Pole Test Boxes A or B. Protector G is used in test huts and old pattern pole test boxes where, owing to dampness, a protector with open spark gap cannot be employed.

Protector "G"

55. Protector "H" (Figure 13)
This protector is similar to G with special end clips (Lugs) for attachment to the terminals of transformers fitted in pole boxes.

G and H protectors do not require to be tested periodically, but a number become faulty with age. If trouble is experienced on a circuit and there is reason to suspect the efficiency of a protector of this type a new one should be fitted and the suspected protector returned for test. Where it is necessary to test protectors G or H, Boards, Vacuum Protector, Test, consisting of an Induction Coil No. 5, cylindrical condenser and press key, should be employed. The spark gap across the secondary winding of the induction coil should be adjusted to 1/8 inch. If the contact breaker on the primary side is properly adjusted, the coil should work with one Leclanché cell. If the coil is adjusted to give a good spark across the inch gape a single protector connected between the secondary terminals should glow when the key is depressed. If there is no glow, or if sparking is observed between the protector electrodes the protector is faulty. If Boards, Vacuum Protector, Test are not held in a District, suspected Protectors should be returned to the Post Office Stores Department.

Protector "H"

56. Tablet Protector (Figure 14)
Tablet Protectors are used for the protection of main submarine cables and the designation includes a plate protector, a bobbin protector and a one-ampere fuse.

The electrodes of the plate protector are large brass plates, pressed together against a perforated mica separator.

The large electrode area of this protector enables heavy charges to be quickly dissipated.

The bobbin protector consists of a number of turns of fine silk insulated wire wound on a brass bobbin. The conductor of the silk-covered wire forms part of the line circuit and the bobbin constitutes the earth electrode. Lightning surges break down the insulation of the covered wire and discharge to earth. The fuse is fitted as a final protection against heavy discharges. The open wire is connected to the line electrode of the plate protector and the cable is connected to the fuse with the bobbin protector intermediate. The bobbin protector also acts as an inductance, but it is desirable to introduce additional inductance by spiralling the external wire connections from the bobbin protector to the plate protector and fuse (see Figure 14). Care should be taken to ensure that independent earth connections are made to each tablet protector where two or more are fitted together so that alterations to one protector will not affect the others.

Instructions for testing cable protector (T.E. 61) should be fixed in a prominent position in cable test huts. Spare bobbin protectors should be kept in the hut as the insulation of the silk wire is easily broken down by a heavy discharge.

Tablet Protector

57. Vacuum Protectors (Figure 15)
Vacuum Protectors were formerly fitted in pole test boxes and test huts in damp situations. The electrodes were metal and enclosed in a glass tube exhausted and hermetically sealed.

Vacuum Protector

58. Protectors and Fuses (Figure 16)
Protectors and Fuses consist of a wall case fitted with Protectors C, Inductance Bobbins and Fuses No. 1/2, for protection of short multiple submarine and river cables. The open wires are connected direct to the Protectors and the cable leads to the fuses.

Inductance bobbins are inserted between the protectors and the fuses to choke back lightning discharges from the fuses and cable. With this arrangement, the fuse will be blown by heavy discharges which might otherwise damage the cable, but not by normal discharges against which the protector affords adequate protection, frequent interruption to service is thereby avoided.

Available in variants 8/8, 16/16, 24/24 and 32/32.

Protectors and Fuses 8/8

59. Protectors, Fuse and Test (Figure 17)
Protectors, Fuse and Test, No. 1601, are fitted in Box, Pole Test G. Two metal electrodes with knife edges 1/8 inch apart are provided for each conductor. The earth electrode is a central plate 3/32 inch thick fixed between the two knife edges forming 1/64 inch spark gaps to each. The open wire is connected to the outer line electrode which is bridged to the inner line electrode by means of a removable wire link. Each Protector, F. and T. No. 1601, accommodates 16 wires and is provided with an iron cover as a protection from dust and moisture.

The base plate is iron and has dimensions of 9½ inches by 5½ inches.  Comprises of sixteen Fuse No. 2/2, eight Labels No. 59 and sixteen links.

If open wires are not endangered by an overhead power circuit, the fuses in Protector, F. and T. No. 1601 are replaced by No. 20 tinned copper wire. In view of the large spark gap it is desirable to introduce inductance into the underground leads to choke back lightning discharges. Iron bobbins known as Reels, Coil, Inductance G, upon which the leads are wound are provided for the purpose.

Protectors, Fuse and Test No. 1601

60. Protectors for Power Circuit Dangers
Protectors, consisting of Fuses and Heat Coils are fitted in all telegraph conductors endangered by power circuits, whether guarded or not, to eliminate fire risk and prevent damage to apparatus or important cables.

A fuse should blow and disconnect the line immediately a relatively heavy current flows; a heat coil in operating should earth or disconnect the line before the cumulative heating effect of a current can cause damage.

Fuses inserted in lines are frequently blown during lightning storms thus causing more or less prolonged interruption of service. Therefore, although fuses do afford additional protection from lightning, they should not be fitted specially for that purpose except in the case of submarine or estuary cables.



The "rating current" of a fuse is the maximum current it will carry indefinitely without blowing. This value is approximately one-half the "fusing current" i.e., the minimum current which will cause the fuse to blow almost instantaneously. In practice, a fuse does not blow instantaneously, a time element is always involved so that the current actually flowing in the conductor at the instant a fuse is blown may be many times the normal fusing current.

Fire conditions may arise, even at medium voltages, if the current is sufficiently large to cause arcing when the fuse is blown.
A fuse, to be satisfactory, should blow quickly and quietly so that the current cannot rise to very high values and should be so designed that an arc, if set up, cannot be maintained; scattering of particles of molten metal should also be prevented.

The working currents in telegraph circuits are small and a fuse which blows at one ampere would provide an ample margin of safety. On circuits exposed to lightning, however, (unless the length of leading-in cable is sufficient to absorb lightning surges) there would be excessive interruptions of service if one ampere fuses were fitted, and therefore a fuse which blows at not less than two amperes is desirable.

Types of Fuses
In Post Office nomenclature fuses are described by two numbers separated by an oblique stroke. The first number designates the mechanical features of the fuse and the second represents approximately the rating current.

FUSE, 1/1 (Figure 18)
The fuse wire is phosphor bronze, the rating current being 1/2 amperes, and the fusing current one ampere. The fuse wire is enclosed in a glass tube 1/4 inch external diameter with copper end caps which fit into spring clips. The overall length is 2 1/8 inches. This fuse is used for the protection of submarine cables.

FUSE, 1/2 (Figure 18)
The original type was made with a glass tube similar as regards mechanical features to fuse 1/1 but fusing with a current of about three amperes. It was fitted in Protectors H.C. and F, at subscribers' premises and small offices, in Fuse Insulators No. 14 and fuse mountings 4001.

Owing to cases of serious arcing having occurred with this fuse in fuse mounting 4001 when overhead wires have fallen in contact with tramway trolley wires in the neighbourhood of the office the fuse has been redesigned, the fuse wire being enclosed in a small bore unglazed porcelain tube fitted with caps without vent holes. This is known as Fuse, 1/2 (Porcelain) and will eventually supersede Fuse) 1/2 (Glass) in all cases; but until stocks of the older form have been cleared the latter should continue to be employed except in cases where overhead wires are endangered by tramway trolley circuits within five miles of the exchange or office, and the fuse is fitted in a fuse. mounting 4001.

FUSE, 2/2 (Figure 18)
he fuse wire is phosphor bronze enclosed in a glass tube 1/8 inch external diameter, and the fusing current is 3½ amperes. Copper tags at the ends of the fuse are held by screws 2 & 3/16  inches between the centres. It is used in exchanges where Fuse Mounting 4002 is fitted, and on Protector F and T, 1601, in pole test boxes "G" for the protection of underground cables.

Fuses, 1/1, 1/2, 2/1, 2/2

FUSE, 10/2 (Figure 19)
This is an enclosed fuse. The tube is fibre and the filling material is soap-stone. The fusing current is 31 amperes. The fuse is fixed to the mounting clips by means of a threaded bolt and nut extensions of the fuse wire. The overall length of fuse is 4 1/16 inches.

The fuse mountings on a number of the ex-National Telephone Company's exchanges installed by the Western Electric Company are designed to take Fuses, 10/2, hut this type of fuse mounting, with the fuse, is becoming obsolete.

FUSE, 11/2 (Figure 19)
This is a glass tube fuse with wire extensions for equipping fuse mounting 4003, which is an obsolescent type. The fusing current is 4 amperes and the overall length of fuse wire 2¾ inches.

FUSE N.T.6 (Figure 19)
This fuse carries 3 amperes indefinitely, and fuses at 4 amperes in 30 seconds. It is enclosed in a thick-walled glass tube 3 inches long. The fuse wire has substantial wire extensions for connection to overhead lines.

Fuses, 10/2, 11/2, N.T. 6

Heat Coils
Heat Coils are provided to protect internal plant from damage by currents which are too small to blow the fuses in circuit, but which, if allowed to continue for a lengthened period might damage apparatus or originate a fire.

When used in conjunction with a fuse it is preferable to arrange that the heat coil when operated will earth the line; but in many of the small protector fittings . operation of the heat coil only disconnects the line.

The heat coil is held in position on the mountings between line and instrument springs, and operates by the cumulative heating effect of the current in passing through an enclosed coil of fine wire melting the solder fastening a pin in the centre of the coil bobbin.

Types of Heat Coils
Two types of heat coil are in use: (i) Heat Coils, A, in which the pin is in tension, and (ii) Heat Coils, B, in which the pin is under compression (Figure 20).

Heat Coils, "A" and "B"

The A type, when operated, disconnects the instrument from the line, and the B type, when operated, earths the line spring without disconnecting the circuit.

In the fitting used with the A type the pin when released is pulled out of the heat coil by the instrument spring, and the instrument is thereby disconnected from the line. When this fitting is used on main frames arrangement is sometimes made for the movement of the instrument spring also to effect a contact between a light spring connected to the line and an earth bar, thus earthing the line (Figure 21). In other cases it is arranged that the operation of the heat coil shall disconnect the internal lines from the external lines, and in addition the movement of the internal line spring operates an alarm bell circuit by forcing a supplementary spring into contact with a terminal.

In the fitting used with the B type, the pin, when released is pushed through the coil. The pin bears on a light spring attached to the line spring, and carries this light spring forward until it makes contact with an earth bar.

Heat coil, A, Green, has a brass cover enamelled green. It has a resistance of 4.5 to 5.5 ohms, and operates in 15 to 60 seconds, with a current of 1/2 of an ampere. Heat Coil, B, has a red fibre cover. It has a resistance of 3.5 to 4.1 ohms, and operates in 210 seconds with 1/2 of an ampere.

Modern protective equipment fittings for more than 20 wires are designed to take heat coils B. Fittings for 20 wires or less are designed to take heat coils A.

Protectors "D" and Heat Coils "A"
Section of Strip, showing one coil normal, and one after operating

Protector and (Break) Heat Coil Mounting

68. Insulator Fuses
Originally it was the practice to fit fuses in overhead lines close to the point at which they were endangered by a power conductor. This practice has been discontinued in consequence of the increase in the number of power systems, resulting in a multiplicity of fuses often in positions very inconvenient for replacement. Every effort should be made to eliminate line fuses except where they are fitted in pole test boxes or test huts for the protection of cables in accordance with standard instruction.

The National Telephone Company pattern of insulator fuse, Fuse N.T. 6 (Figure 19) could be fitted on any double groove insulator. Post Office practice was to fit a line fuse in Fuse Mounting, 105, (Figure 24) which is inserted in Insulator No. 14. This fitting equipped with a dummy fuse is still used at testing points in main lines. The latest supplies of Mounting, 105 have Phosphor bronze clips instead of Copper which is found to be too soft. Where Mountings, 105 are permanently retained at testing points on main lines, old pattern mountings with copper clips should be examined and replaced if necessary by the new mountings.

Fuse, Mounting No. 105


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Last revised: January 22, 2024