History of Electrical Systems and Cables

Insulators Home > Book Reference Info > History of Electrical Systems and Cables

Information from:

See the Timeline of Related Developments for additional information.

This information comes from

The 'Acknowledgements' states:

[...] Illuminations have been generously provided by industrial concerns as the individual acknowledgements will indicate, and special thanks are due to the assistance given by Dr. Follett, the Director of the Science Museum, and his staff, one of whom, Miss Weston, has made many valuable suggestions. The collection of historical material and confirmation of dates have been greatly facilitated by the willing help given by the Librarian and Staff of the Institution of Electrical Engineers. In particular, Mr. H. Lansley has been tireless in his responses to my many calls on him. Friends in the Supply Industry have also been most helpful in producing information on early power stations, much of the information being channelled through Mr. P. A. Lingard of the London Electricity Board. [...] The many cases of help received from the light current, heavy current, and instrument sides of the elctrical industry and from the electrical press are too numerous to permit a mention by name, but to all I express my most grateful thanks.

The 'Preface' states:

[...] In the introduction to his History of the Institution of Electrical Engineers, Mr. Rollo Appleyard, writing of the evolution of the Institution from the former Society of Telegraph Engineers and Electricians, says, "It has stood at the confluence of the streams of academic and practical knowledge where for fifty years it has directed and safeguarded electrical progress." [...] The history of such a subject as electrical engineering does not follow a simple straightforward list of dates. The contributions of one man may continue over many decades during which period others pick up the same threads to weave completely different patterns. [...]

The following information is excerpted from this book.

Origins

[...] It is safe to assume that the first electrical effects to be noticed by man were the lightning flash and the aurora borealis. Their existence called for no deliberate act on his part, and they occurred long before he had produced electrical charges himself, either fortuitously or by design. At first they and their effects were completely out of man's control, though several thousand years ago damage to buildings by lightning strokes was prevented by the nature of their construction. Several famous historic buildings, including the Temple of Juno and Solomon's Temple, had their roofs covered with metallic points - sword blades or sharp ornamental objects - with resulting immunity to damage by lightning. [...]

It was not until the great Benjamin Franklin, American writer, philosopher, and statesman, became interested in electrical phenomena, that the idea of deliberate protection of buildings emerged. After his famous and extremely dangerous experiemnts of collecting electric charges by sending kites up into thunderclouds, Franklin, in 1750, conceived the idea of a lightning conductor, and in his Poor Richards Almanac for 1753, he puts forward the proposal for the protection of buildings 'from mischief by thunder and lightning'. [...]

The first half of the eighteenth century saw many discoveries and applications of electricity. Outstandin among these was the principle of conduction and insulation enunciated by Stephen Grey in 1720. By suspending a hempen line on silken threads, he transmitted electric charges hundreds of feet. When metallic wire was substituted for the hempen cord, circuits up to several miles were made to carry the charge.

The French scientist Dufay seems to have been the first to have established the idea that electricity appeared in two distinct forms, vitreous and resinous, the former produced on glass, and certain other materials, and the latter on amber, silk, paper, etc. He also observed that each repels its own kind and attracts the other. [...]

The English scientist Dr. Watson made many experiemtns with the Leyden jar, established the idea of two coatings separated by the dielectric, and spoke of 'plus' and 'minus' electricity. With others, he also made long circuits up to several miles, and discharged the Leyden jar through them. [...]

Birth of the Electric Telegraph

[...] The employment of magnetic and electrical effects for the transmission of intelligence had been predicted long before the advent of the steady current had provided a sound foundation for practical application. [...]

During the early half of the eighteenth century, a good deal of experimenting was carried out by Stephen Gray, a Charterhouse pensioner, while Dr. Watson, a member of a Royal Society Committee, brought to light the essential difference between conductors and insulators. It was discovered that a damp hempen cord suspended by silk loops could be made to transmit a static charge from a Leyden jar. With a metallic wire so supported, the results were still more effective, and a circuit was set up in July 1747 across the Thames over old Westminster Bridge. The circuit was completed through the body of an assistant, who held the far end of the wire in one hand, and with the other touched the water with a metal rod. His reaction completely vindicated the result which had been anticipated. Other experiments followed in various parts of London, and the length of the circuit increased to several thousand feet. Franklin in 1748 in Philadelphia, and De Luc in 1749 in Switzerland, extended the range but remarkable as it seems today, there was no suggestion of the discoveries being applied to telegraphy. [...]

A rather fantastic suggestion was made in 1795 by a Spaniard, Salva, who employed twenty-two pairs of wires for a twenty-two letter alphabet. At the receiving end, each pair was held by a man who called out his letter when he felt the shock. In due course, Salva resorted to simpler detectors in the form of tinfoil plates and reduced the number of wires. [...]

Francis Ronalds moved to the house now known as Kelmscott House in Upper Mall, Hammersmith, and it was here that he constructed the telegraph system through which his name became so famous. The best description of his achievement is contained in a small book which he published in 1823 and reprinted in 1871, from which it is clear that his first concern was whether the electric fluid in its static form could be made to travel over long distances without undue delay. For this purpose, in 1816 he set up two wooden structures in his garden, twenty yards apart, between which he strung iron wire backwards and forwards, forming a continuous length of more than eight miles. The wire was insulated at the 37 hooks on each of the 19 bars at both ends by silken loops, and the two ends brought out to two pith-ball electrometers. [...]

The event which, more than any other, brought the telegraph to the notice of the public, was the arrest in 1842 of the murderer Tawell at Paddington Station. Mr. John Tawell, a respectable resident of Berkhamsted, was known by the police to be in the habit of visiting a woman, Sarah Hart, at her home near Slough. Early one morning she was found dead, and a man had been seen leaving her home in rather suspicious circumstances some time before. On enquiry at the Great Western station, they learned that a man answering to their description had just gone off by the slow train to London. "But," said the inspector, "why not try this new telegraph?" At once the message was sent indicating that the man wanted was dressed as a Quaker with a brown coat reaching almost to his feet. There seems ot have been some delay, owing to the fact that the five-needle instrument had no signal for Q, and KWAKER was not at once understood. When Tawll stepped out of the train at Paddington, however, he was shadowed to a New Road omnibus and ultimately arrested. In due course, he was tried for the murder and executed. The excitement, first over the manner of his arrest, and secondly through startling revelations of his past career, seems to have done as much, if not more, for establishing the telegraph of Cooke and Wheatstone, than all their technical and business ability. Joint stock companies were set up to exploit the invention for commercial purposes - The Electric Telegraph company, which paid Cooke and Wheatstone £33,000 for their invention; the Magnetic Telegraph Company, and others - so that by 1868, over 16,000 miles of telegraph line had been erected. By 1870, the systems had been incorporated into one national undertaking, and taken over by the Post Office.

The early lines were crude in the extreme, and the five wires for the Euston-Chalk Farm circuit were carried in longitudinal grooves cut in the top and sloping sides of wooden bearers. In consequence, electrical failures were common. One of these, on the Fenchurch Street - Blackwell line, proved, according to Fleming, the direct cause of a great improvement in the needle system. There out of the five wires broke down, and the telegraph clerks devised a code which enabled them to continue working with only two needles in use. Finally it was found that one needle was all that was required. When the movement of the needle was restricted by stops on the dial, a convention of 'dots' to the left and 'dashes' to the right was adopted, so that various combinations of dots and dashes gave all the letters and figures required. [...]

Morse was fortunate in enlisting [...] a young man, Alfred Vail, of a mechanical turn of mind, who not only co-operated in the design and in making equipment, but, what was even more vital at this time, secured financial backing. On 3 October 1837, Morse obtained his patent, and Vail improved the printer so that it produced clear dots and dashes. The Morse Code, so well known by the name of the inventor, seems really to have been the work of Vail, for we read "Vail tried to compute the relative frequency of all the letters, in order to arrange his alphabet; but a happy idea enabled him to save his time. He went to the office of the local newspaper, and found the result he wanted in the type-cases of the compositors." Thus was established the Morse Code, which has survived for so long and in so many forms of signalling.

After four years of political string-pulling, frustration, and living on the verge of starvation, Morse had his system approved by Congress, and he received an appointment with a salary of $2,500 a month to superintent the erection of a line connecting Baltimore and Washington. Serious difficulties arose with underground conductors, but a conductor on poles was carried through, and on 23 May 1843, messages were transmitted by Morse from the Capitol and received by Vail at Baltimore. The line was opened on 1 April 1845 as a public service, and after the Postmaster-General of the day had declined to purchase the invention for $100,000, Morse proceeded to secure the support of private enterprise. The Western Union amalgamated some of the earlier participants in the venture, and went ahead in stretching a network of circuits over the United States, the most dramatic of which was the New York to San Francisco line completed in 1861. [...]

Electricity Supply

[...] In the year 1883, an enterprise was started in the West End of London which, although originally intended as a local private lighting installation, ultimately developed into an outstanding example of public electricity supply, and one of great technical and engineering interest. As such, the Grosvenor Gallery installation in New Bond Street will always rank as a pioneer in public supply systems. [...] The lighting of the gallery was by arc lamps on a series circuit with an automatic regulator maintaining a steady current of ten amperes, and gave such satisfaction that requests for supply soon began to come in from neighbouring residents and shopkeepers. Under a new Act, the Grosvenor Gallery Company sought and obtained the permission of the local Authority to run overhead lines across the roofs of houses, carrying the high tension current. Each consumer was provided with a small transformer, the primary windings of several being connected in series on the 2,000-volt supply circuit. THe voltage was later raised to 2,400. [...]

Within a very few months, the system stretched from the Thames to Regents Park, and from Knightsbridge to the Law Courts. Hundreds of iron posts fixed on the roofs of houses carried rubber insulated conductors supported on steel suspension wires by leather thongs. [...]

The release of capital by the 1888 Amending Act resulted in the inauguration of many new supply systems both in London and in the Provinces. By the end of the century, there were something like thirty power stations in London alone, under the control of sixteen undertakings, most of them private companies. [...] Having no power to open up the streets for underground mains, [companies] carried their distribution circuits on overhead lines erected over the roofs of houses. [...]

Alternating Current

The phenomenal growth of electrical engineering during the last two decades of the nineteenth century, which had followed the introduction of the incandescent lamp, left the supply industry in a curious state of uncertainty on an important and much debated issue. [...] Namely, should the current, when generated mechanically, be continuous or alternating? The experts, ranging themselves into two opposing camps, debated the question hotly for many years. As we know today, the exponents of the alternating system ultimately held the field, and development followed this course. The direct current enthusiasts, however, made such progress over the years in carrying their ideas into effect that even now (1960), over half a century later, there are still vestiges of D.C. sections remaining, though they are rapidly approaching extinction. [...]

The direct current engineers, from the very beginning, appreciated that the extension of a system beyond the immediate surroundings of the generating station raised the question of economy in cost of the mains and the control of the drop in voltage at the end of long sections with increasing load. They saw that to overcome this difficulty, they would have to raise the voltage beyond that at which the consumers' lamps would be connected, and Crompton, Hammond, and others introduced three, four, and even five-wire systems whereby the dynamos on a 100-volt system could generate at 200, 300, or 400 volts. [...] Double-wound dynamos with the separate windings brought to two separate commutators were supplied at the high voltage side direct from the central station, and gave the consumers' voltage on the second commutators. High voltage D.C. transmission with voltages up to 1,500 was installed in this way at Chelsea (1889) and Lambeth (1896), and at several provincial towns from 1884 onwards. [...]

The Paddington scheme installed for the Great Western Railway by the Telegraph Construction and Maintenance Company provided for 4,100 incandescent lamps and 100 arc lamps, and covered an area of some 70 acres. This installation had gone into operation on 21 April 1886, and was destined to give satisfactory service for twenty years. [...]

As J. E. H. Gordon's installation [at Paddington] ran satisfactorily until 1907, when it was superseded by a completely new power system at Park Royal, it may be considered as a landmark in the development of alternating current, although he himself lost faith in A.C. and immediately joined the newly formed Whitehall Electric Supply Co., registered in 1887 with a capital of £200,000, to design and install a direct current station at Whitehall Court. [...]

As such voltages with alternating current on this scale were new and their practicability uncertain, Ferranti built an experimental transformer to step up the 2,400 volts at Grosvenor Gallery to 10,000, and gave a public demonstration. There were, of course, no suitable instruments for measuring such a voltage, but he connected a hundred 100-volt lamps in series, and showed that it was a feasible proposition. [...]

So far as the transmission of the current to London was concerned, the amended Lighting Act of 1888 posed certain practical problems. For instance, to open up streets over the distance of nearly five miles would involve many local authorities whose consent might be difficult to obtain or would at least result in delays and extra cost. The railways, on the other hand, were run on private property and soon provided the required rights of way for carrying the high voltage cables to London and across the bridges to Charing Cross, Cannon Street, and Blackfriars. [...]

The earliest example of three-phase generation and transmission in Britain was in the Wood Lane station, constructed as a joint enterprise by the Kensington Court and Notting Hill Companies, and put into operation in October 1900. The designers wished to adopt a voltage of 6,600 volts, but in spite of the existence of the Deptford 10,000-volt single-phase system, the authorities would only sanction 5,000 volts, and the system ran at this voltage for nearly forty years, when it was raised to 6,600 volts in accordance with other systems. [...]

Power and Traction

Much ingenuity was displayed in the method of carrying current to the moving tramcar. The overhead trolley-wire was an obvious engineering solution, but although it was employed in 1883 at Portrush in Ireland and at Richmond in Virginia, opposition arose in other and older towns. Boston, Massachusetts was able to take 9,000 horses off the streets, but opposed the unsightly trolley wire. The result was that several alternative systems were tried out. [...] Neither system survived, however, and the overhead trolley wire operating at about 600 volts became standard practice. The return circuit was usually along the bonded running rail, though double-trolley wires were used at times to prevent interference. [...]

Considering the choice between A.C. and D.C. in the motor field, it is not surprising that in the development of electric traction, what might almost be styled a second 'battle of the systems' should have been fought. Indeed, it still continues, for countries even take sides in the battle between A.C. and D.C. traction, and in this country the choice has several times been the subject of public inquiry.

Electric traction first appeared in Britain on 3 August 1883, when a quarter-mile length of railway was opened for traffic at Brighton by Magnus Volk. The current was supplised from a third rail at 140 volts D.C. A few weeks later, the famous Portrush line in Northern Ireland began to operate at 550 volts D.C., carried by an overhead trolley wire. [...]

In 1890, the City and South London tube was opened, employing electric locomotives, and the Cnetral London tube between the Bank and Shepherd's Bush followed within a few years. Both were D.C. systems at 500 volts, supplied from a third rail.

The City and South London system saw the first application of direct drive from the motors on to the wheel axles. [...] The current at 500 volts was supplied through a third rail of steel channel, supported on glass insulators from transverse wooden sleepers, and picked up by cast iron slippers. [...]

Other sections of electrified track appeared: Bow to Upminster in 1905, Paddington to Westbourne Park in 1906, Lancaster to Heysham in 1908. The London ones, influenced no doubt by the underground examples, kept to D.C., but for the Heysham line, single-phase 6.6 kV at 25 cycles, carried on an overhead trolley wire, was adopted.

Wide differences of opinion arose at to voltage and system, and from 1916, 1500 volts D.C. on an overhead wire became increasingly popular. [...] In 1956, the Transportation Commission decided on a 25 kV 50 cycles single-phase system as national standard. [...]

On the A.C. side, different practices developed in different countries. In 1930, for instance, the three-phase system disappeared finally from the Swiss Federal Railways, and was replaced by single-phase 15,000-volt current at a frequency of 16 2/3 cycles per second. About the same time, there was one scheme in the United States employing 11,000 volts at 25 cycles on the trolley line, with 300 h.p. A.C. motor, and another operating at 3,000 volts D.C. [...]

The Turbine Era

[...] Between the two world wars, the great reorganization of the electric supply industry in this country [Britain], which led to the construction of the 'grid', was formulated and carreid out. In 1917, a government committee recommended that all supply undertakings should be brought under one central authority, and in 1919 the Electricity Supply Act set up the Electrical Commissioners, who started work in 1920. Districts were organized under the title Joint Electricity Authority, but were often under suspicion from the undertakings and were not completely effective, so that in 1925, reconsideration of the situation by the Weir Committee led to the formation of the Central Electricity Board. Its main functions were the construction of a nation-wide transmission network, the adoption of selected generating stations, and the standardization of frequency. At the time, there were 17 different frequencies in use, and 80 undertakings operating on other than 50 cycles.

This development was embodied in the important 1926 Electricity Supply Act, and the now well-known steel transmission towers soon began to appear. A standard transmission voltage of 132,000 volts was adopted, and the three-phase circuits were designed to carry 50,000 kVA on steel-coated aluminium conductors 0.77 in. in diameter, having the equivalent copper section of 0.175 sq. in. The towers were designed for single and double three-phase circuits. In explaining in 1927 the eight-year programme for constructing the Grid, Sir Archibald Page said, "The Grid will go a long way towards ensuring the universal availability of electric power throughout the country. It will bring the cost of production in the majority of supply undertakings down to the figure which has been attained by the few in whose areas electricity is already cheap and abundant. [...]

The continued expansion of the grid and the increased scale on which current is generated and distributed, has naturally had considerable influence on the development of both transformers and switchgear. Larger and larger transformers have been constructed. One example is the 110,000 kVA at Battersea B, first installed for 80,000 kVA and modified for the higher output which converts from 11,000 volts to 66,000, and has forced oil cooling. Another is the 75,000 kVA transformer at Dalmarnoch, which is air cooled, while at Barking there are two, each of 93,750 kVA capacity. During the past few months, a British transformer of 200,000 kVA has been supplied to the United States. Tappings on the windings of high voltage transformers for voltage adjustment on load have developed rapidly and are now in common use. Protection of transformers against damage through system surges becomes increasingly important on large networks, and this has resulted in much greater reinforcing of end turns to withstand steep wave fronts. But possibly the most spectacular improvement in the modern transformer is in the efficiency achieved. Values of 99 per cent are now cmmon, and in the very large sizes 99.5 per cent is frequently achieved. [...]

The function of opening and closing a circuit for the purpose of normal control has usually been separated from that of opening the circuit in an emergency brought about by short-circuit or overload. In this case, the situation has usually been met by inserting a fusible link or cut-out in the circuit. As loads increased, however, the two fucntions were combined in one device known as a circuit breaker, in which a main switch was released by an electromagnetic trip. The range of equipment thus stretches from the simple tumbler or press-button switch with a separate simple wire fuse, to the high-voltage oil-filled or air-blast circuit breaker in the switch yard of a large power station.

The first power stations were controlled by open knife switches on slate or marble panel switchboards, and as currents grew in magnitude, circuit breakers with magnetic blowouts to extinguish the arc were added. Well before 1900, the switchboard had taken its place as a vital component of a power station, and improvements were introduced both into the arrangement of circuits and the design of individual switches to reduce the risk of failure and to ensure the continuity of supply. A major modification was in the adoption of the dead front, in which no live metal was accessible, a condition which became essential on the adoption of alternating current and higher voltages. At one stage in this development, remote mechanical control of the switches was resorted to, the switches themselves being housed in brickwork cubicles a short distance behind the control board. [...]

About 1940, it became evident that from a national point of view, the continued expansion of the British grid required a still higher voltage than 132,000, and in 1952 the Electricity Authority started on the construction of a super grid to operate at 275,000 volts. The two main objects were to interconnect the major groups of generating stations, and to connect new generating plant located on the coalfields to bulk supply points in areas where there was a deficiency of fuel. In addition, the superimposed grid would provide additional safeguards in times of breakdown or other emergency, and so make possible an appreciable reduction in the provision of standby plant. One line of towers on this super grid can carry over 500,000 kW on each of two circuits, and already a considerable part of the scheme is in operation.

Fig. 29. British Power Grid. These three maps illustrate the intensive development in a highly electrified country. The entire country is covered by a three-phase 132 kV system on which a 275 kV was subsequently superimposed. (Acknowledgement to The Electricity Council.)

[...]

The Early Telephone

[...] Early in 1877, the telephone had created a world-wide interest. Speech had been transmitted over the existing telegraph lines, and the time had come for commercializing hte invention. Bell offered pairs of instruments on lease 'for social purposes' at $20 a year, and 'for business purposes' at twice this rent. Very soon the instruments were being produced in a small factory in Boston, Massachusetts, and supplied over a wide area, including New York and London, where Bell read a paper before the Society of Telegraph Engineers. [...]

The idea of a central switchroom by which subscribers could be interconnected was suggested in Octber 1877 by an enterprising journalist in Boston, and quickly took practical shape based on the telegraph system which had already operated to a limited extent in the United States, England, and France. At first a ticket system enabled an operator to receive requests from individual subscribers for a specified line between certain times, and to issue instructions by means of a ticket to another operator, who inserted and withdrew plugs as required.

The first telephone exchange on a commercial basis was installed at New Haven, Connecticut in January 1878. Drop indicators were used with a call bell and two-way lever switches enabled the operator to connect his own telephone to any line and obtain instruction. Another switch enabled him to send out a signal in the form of a loud buzzing sound on the subscriber's receiver. A few months later, an exchange in which cords were used was set up in Chicago. The subscribers' indicators were arranged along a wall, and a series of horizontal metallic rods grouped in pairs as connecting racks. These had clearing-out drop indicators associated with them. [...]

The first telephone exchange in England was established by the Telephone Co., Ltd., at 36 Coleman Street, E.C., in August 1879. [...]

In 1880, the instruments standardized for subscribers' use were Telephone No. 1 and Telephone No. 2. The former, the wall type, was made of wood, having a projecting ebonite mouthpiece on the transmitter, with a tubular receiver and switch hook at the side, and a double magneto bell in front. The desk type, constructed mostly of metal, carried the transmitter with its ebonite mouthpiece on a tubular pillar with the receiver and switch hook on the side, but the bell was accommodated in a separate wooden box. [...]

In 1896, the major trunk lines of the National Telephone Co. were purchased by the Post Office, and at the turn of the century, common battery (CB) working was introduced. Many inventors and many committees contributed on both sides of the Atlantic to this important innovation. In the famous Hayes circuit, based on a patent granted in 1892, the subscriber received current from a large common battery locaed in the exchange and singalled by lifting his receiver from a switch hook. [...]

When the first telephone exchanges were projected, the telegraph system with its ramification of circuits was already established, and it was natural that practice should follow precedent. There was, however, a basic difference in the requirements of the two services; telegraphs were usually long-distance circuits with lines from town to town, whereas in telephony, initially at any rate, the main problem was to connect many subscribers in a congested urban area with one another through a local exchange. Consequently, bare wires strung over rooftops became the usual practice. Baldwin has given an excellent description of the early history of overhead telephone line plant, in which he traces the development from the practice of the telegraph engineer.

Telegraph exchanges were conveniently housed in high buildings surmounted by a derrick from which lines radiated in all directions. The most commonly used wire to start with was bare galvanized iron, No. 12 gauge, with other sizes depending on the whim of the particular engineer. To prevent corrosion in industrial towns and large cities, the wires were often protected with an impregnated cotton covering. Although the high electrical resistance of iron wire was a great disadvantage, it required many years of testing and consideration before its place was taken by copper. Because of their increased tensile strength, phosphor bronze and silicon bronze were favoured by some, but hard-drawn copper held the field until the brittleness of bronze was overcome. Soft copper binding wires were introduced for holding the wire to the insulator of porcelain of various shapes, and gutta-percha covered wire was used for leading into premises. By 1890, silicon bronze had been so much improved that it had been brought into general use for subscribers' circuits throughout Britain in a standard weight of 40 lb. per mile.

The congestion resulting from the large number of wires leaving the structure over an urban telephone exchange soon called for a bunching of the circuits into groups, and this was done by the use of multi-core aerial cables. Early forms consisted of a number of gutta-percha insulated wires, bound together with tape, and associated with a stranded galvanised wire to give supporting strength. Later, rubber was substituted for gutta-percha, which cracked when exposed to air and sunlight. This also enabled more conductors to be accomodated in a cable of the same weight, and standard types were adopted with 26 and 52 pairs. As an example of the magnitude of the task of getting the circuits away from an early telephone exchange, the case of Sheffield may be cited. There, the angle iron derrick erected in 1891 accommodated over 1,000 wires.

A problem which faced the early telephone engineer was the prevention of overhearind between circuits, when so many had to be carried in close proximity to one another. The impracticability of the single-wire earth circuit which had been suitable for telegraphs was soon appreciated, and in 1892, expensive schemes of doubling, or making the circuit 'metallic' as it was called, were carried out. Even then, induction effects were found to be serious, and twists were introduced. The credit for this idea goes to Professor Hughes, who referred at a dinner of the National Telephone Co. in 1895 to his paper in which he had first announced the idea. For long-distance overhead liens, then growing rapidly along the main roads of Britain, the system of twisting a pair between poles and of inserting crosses, was adopted extensively. [...]

The heavy overhead long-distance lines continued to grow in number and size until the First World War but, as even then they failed to meet the demand, more and more cables were laid. For subscribers' lines, 1,000-pair cables with 6 1/2 pound conductors became common. [...]

When Sir William Preece visited the Philadelphian Electrical Exhibition in 1884, the number of telephone stations in the United Kingdom was already 11,000. He found that the comparative figure for the United States was 148,000. He explained the discrepancy to his hosts on the grounds that in London an errand boy cost 2s. 6d. a week, whereas a similar service in New York cost 12s. to 15s. It paid to use the telephone. Today (1960), there are still about nine times as many telephones in the United States as there are in the United Kingdom. [...]

Science in Telecommunication

[...] Michael Idvorsky Pupin [...] spent some time studying at Cambridge and Berlin Universities, [then] returning to Columbia [University], where he was appointed to a chair. It was here that during twenty-five years he made his inventions, the most significant of which, the Pupin Coil, brought him in $1,000,000. The patent was bought by the American Telephone and Telegraph Co.

Thus was laid the foundation stone of 'loading' an electrical transmission line, and the first practical application was in August 1902 when loading coils were inserted in a ten-mile length of telephone cable between New York and Newark, New Jersey. By this modification, the grade of transmission was improved to that obtainable on a five-mile length of unloaded cable. [...]

In the long-distance circuits in England, open lines carried on poles had been strengthened by increasing the size of the copper conductors. In 1895, the limit was reached on the Longon-Leeds-Edinburgh line, when conductors weighing 800 lb. per mile were erected. [...]

After the successful pupinization of the New York-Washington and New York-Boston lines, the American Telephone and Telegraph Co naturally turned its attention to transcontinental possibilities and, by loading and reinforcing overhead lines, a New York-Denver circuit of 2,200 miles was achieved as a first stage. On 25 January 1915, a commercial telephone service was inaugurated between New York and San Francisco, a distance of 3,400 miles over open wires weighting 870 lb. per mile. Today the United States has a vast system comprising some 200 million miles of wire, and practically any two telephones can be interconnected on demand. [...]

Two-wire amplified circuits were at first developed for inland trunk cables in this country, but by 1939 had been replaced by four-wire circuits with losses reduced to practically zero. Not only had the repeater reduced the need for capacity correction in the line by the addition of inductance, but carrier technique, introduced in 1920, had made it possible to transmit more than one conversation over one pair of wires. [...]

By 1937, this new tool had made it possible to carry twelve channels on each pair of a cable. [...]

Measurement Instruments and Standards

[...] At the International Electrical Exposition held in Paris in 1881, much dissatisfaction with the position was expressed by practical electrical engineers, who were still using widely 'Weber' to denote the unit of current, measuring electrical pressure in terms of the equivalent number of Daniell cells, and resistance in terms of miles of telegraph wire; and the new names ampere, volt, and ohm were adopted. [...]

Professional Organizations

[...] For many years before the formation of a professional body for electrical engineering, the pioneers who were laying the basis of the science found a suitable forum in the Royal Society. Many fundamental communications on electrical engineering by Sturgeon, Kelvin, Wheatstone, Hopkinson, and others have appeared in its Transactions.

During the second half of the eighteenth century, small societies were founded by groups of men itnerested more in the development of engineering and technology than in pure science. The Lunar Society, so called because the meetings were arranged at the time of full moon in order that the members could find their way home easily, included many well-known pioneers whose names became famous - Watt, Boulton, Murdock, Priestley, and others. [...]

In 1872, The Telegraphic Journal and Electrical Review was launched, and continued for twenty years when, under Alabaster Gatehouse and Kempe, it became The Electrical Review, aiming rather more at the commercial than the academic side of electrical engineering. Later, in 1891, Lightning appeared under the control of Robert Hammond, who also served as honorary treasurer of the Institution for many years. One of the interesting features of Lightning was the publication of statistics of the industry; subsequently, under the direction of R. W. Hughman, it changed its title to The Electrical Times and has made a considerable impact. [...]

English Social and Historical Background

[...] Soon after its introduction, the telephone assumed an important position in the commercial world. In 1889, the trhree principal companies which had carried through the pioneer work were amalgamated to form the National Telephone Company, with some 28,000 lines. From 1892, the Post Office came into the telephone field, and in 1896 all trunk lines came within official control. The final stage in the development came in 1911, when the Post Office took over the whole telephone system of Britain. [...]

Chronological Table

[very selected excerpts]

1837 - Cooke and Wheatstone first practical electric telegraph, on L. & N.W. Railway, London - Camden Town

1856 - Formation of Atlantic Telegraph Company.

1871 - Society of Telegraph Engineers (later I.E.E.) founded.

1873 - Gramme first transmitted electric power over three-quarters of a mile, at Vienna Exhibition.

1877 - Edison Electric Light Co. formed.

1880 - Society of Telegraph Engineers added 'and Electricians' to its title.

1881 - Edison constructed his first electric power station at Pearl St. N.Y.

1882 - Gaulard and Gibbs arranged transformer in series.

1882 - Tentative electric tramway at Leytonstone, London.

1882 - 35 mile 200 volt A.C. transmission at Munich.

1883 - Step down transformers come into use.

1883 - Charing Cross Electric Supply Company formed.

1889 - Formation of National Telephone Co.

1889 - General Electric Co. formed in England.

1895 - Rotary automatic telephone exchange switch introduced.

1897 - Brown invented oil-immersed circuit breaker.

1900 - Central London Railway opened.

1900 - First public 6,600 volt 3-phase supply, at Newcastle.

1905 - Niagara Falls 60,000 volt transmission over 200 miles.

1905 - First power station system control room in Great Britain at Carville.

1906 - Hewlett introduced high-voltage suspension insulator.

1920 - Emanueli introduced oil-filled cable.

1925 - Barking Power Station opened by King George V.

1929 - First section of the British 132 kV grid operating.

1929 - A.C. Multiplex 12 Channel telegraph system in operation.

1942 - British Electricity Authority planned 275 kV grid.

1946 - Central Electricity Authority connected 180 electricity compaies, 354 municipal undertakings, and 9 joint electricity authorities.

1959 - Kariba hydro-electric shceme on Zambesi, with 100 MW sets and 330 kV lines working.

1961 - Membership of Institution of Electrical Engineers exceeds 48,000.

Illustration Plates

Plate XXIII (a). Power Station Switchboard about 1920. The board intended for low voltage has exposed live metal on slate panels, and the switches are operated manually. (Photo: London Electricity Board) Plate XXIII (b). Control Room in Modern Power Station. This installation at Llynfi, South Wales, is a good example of remote control, which was introduced about the end of the First World War for high-voltage operation. Only low-voltage cicuits are brought into the control room. These bring in instrument readings via instrument transformers, and take out switching instructions to the high-voltage switches. (Photo: English Electric Co. Ltd.)

Plate XXIV (a). Switch Yard in Modern Power Station. The high-voltage circuits are made and broken in a switch yard outside the power station building through circuit breakers operated by low-voltage current. (Photo: Reyrolle & Co.) Plate XXV (a). First Nuclear Power Station, Calder Hall, 1956. This pioneer station had an output of 69 MW divided between two reactors. (Photo: Atomic Energy Authority)

Plate XXXI (c). 25,000 volt A.C. Locomotive, 1960. This locomotive, which develops 3600 horsepower, introduces the most up-to-date ideas for combining the advantages of A.C. transmission with D.C. drive. The power is supplied at 25,000 volts single-phase, by means of overhead trolley wire, and the locomotive carries transformers and rectifiers. Speeds of 100 m.p.h. are obtained. (Photo: Associated Electrical Industries)

Plate XXXIII (c). Early Telephone Switchboard. The first telephone exchange was established by Jones, at New Haven, Connecticut, in 1878. Drop indicators were employed, but there was nothing approaching an operator's cord as developed later. (Photo: Science Museum) Plate XXXIV (a). Manual Telephone Switchboard, 1940-50. While retaining manual operation for many decades, great improvements were introduced in switchboard techniques to meet the increase in number of circuits. In this illustration can be seen the bank of incoming jacks and, above, the subscribers' multiple. The set of plugs and cords assigned to the individual operator is shown low on the right. (Photo: American Telephone & Telegraph Co.) (b) Modern Telephone Exchange, Crossbar Canyon. This illustration gives an idea of the extensive provision in a large exchange which has to be made for making circuit adjustment to suit traffic requirements. (Photo: American Telephone & Telegraph Co.)

Plate XLVI (a). Picture Transmission by Wire. This historic picture transmitted in 1924 indicates the standard of quality obtainable at that time. (Photo: Muirhead & Co. Ltd.)

This information comes from

The 'Foreword' by John Banks states:

Industrial history, especially in its most specialised areas, does not always get the support of the industrialist. As one of the latter and also a professional engineer, I have an ingrained desire to keep records, but this does not ensure that the kind of record provided in this book will result. For that we have to rely on the enthusiasm, almost fanaticism, of the energetic story teller which Robert Black obviously is. [...] This sustained effort has enabled him to write a most comprehensive book, ranging from early telegraph cables to the most recent specialised types of cables such as are required for ships and aircraft. [...]

The 'Preface' states:

The story of the gradual evolution of electric wires and cables, from those used in the early experiments in electrostatic telegraphy to such modern examples as supertension power cables and optical fibre telecommunication cables, is a fascinating one with its full share of drama and suspense. [...] In the chapters which follow, an account is given of the development of the principal types of electric wires and cables, from the earliest times up until about halfway through the present century. The treatment varies in texture and cannot claim to be comprehensive, for each cable installation is a story in itself and availability of space has necessitated selection. Despite this and a confessed parochial interest in two among the many great cable makers that have flourished during the period (Callendars and the B.I. Company, by whose decendant BICC I have been employed for nearly forty years), it is hoped that the reader will share in the fascination and pleasure that building up a cable collection and compiling its history has brought. [...] It was Jim Temple Hazell who first encouraged my interest in the subject, and helped the start of a collection of cables by passing over the duplicate samples from the Hunter-Hazell Collection, now in the Science Museum. [...] The author's thanks are due to Mr. John Banks, Managing Director Group Services, BICC plc., President of the Institution of Electrical Engineers, for permission to proceed with the writing of this history and for providing a Foreword, to his colleagues at Wood Lane for encouragement and advice, and particularly to his fellow library staff, who have suffered from a plethora of cable samples and documents without complaint for a long while. [...]

The 'Acknowledgements' states:

Diagrams and photographs are reproduced by kind permission of the following:
Ferranti Limited (3.2)
Pirelli General Cable Works Limited (9.1)
British Patent Specificication No. 933 of 1851 (5.1)
US Patent Specification 304/539 of 1884 (5.2)
All others, unless otherwise stated, by permission of BICC plc.

The following information is excerpted from this book.

Early Telegraph Cables

In the early years of the eighteenth century, many natural philosophers experimented with electricity, with a view to establishing its applications. Among these was one Stephen Gray, F.R.S., described as a Charterhouse Pensioner, who demonstrated the principles of conduction and insulation in 1730 by suspending a damp hempen line on silk threads. He found that he was able to transmit electrostatic charges for distances of up to several hundred feet. When metallic wire was substituted for the damp hempen cord, the charge could be carried over circuits several miles long.

Six years later, John Woods improved on the system, and in 1747, Sir William Watson, F.R.S. constructed a two-mile line with an earth return. This circuit was set up in the July of that year, and was routed across the River Thames by way of the old Westminster Bridge. In order to demonstrate the effectiveness of the circuit, Sir William completed it through the body of an assistant, who held the far end of the wire in one hand, and with the other touched the water with a metal rod. It was reported that his reaction to the static discharge from a Leyden Jar "completely vindicated the result which had been anticipated".

An interesting development and the first recorded account of the use of paper as an electrical insulating material, was given in a paper presented by Don Francisco Salva (1751-1828) to the Academy of Science in Barcelona in December 1795. In describing his experiments in telegraphy to the Academy, he also suggested the possibility of underground transmission by cable:

"...it appears, little short of impossible to erect and maintain so many wires" (with the electrostatic telegraph system that he was contemplating, Salva would require 22 wires, one for each of the 22 letters of the telegraph alphabet) "for even with the loftiest and most inaccessible supports, boys will manage to injure them, but as it is not necessary to keep them very far apart, they can be rolled together in one strong cable, and placed at a great height. In the first trials made with a cable of this kind, I covered each wire with paper, coated with pitch or some other ideoelectric substance, then tying them together, I bound the whole with more paper, which eventually prevented any lateral escape of the electricity. In practice, the wire cable could be laid in subterranean tubes, which for greater insulation, should be covered with one or two coats of resin."

This grand concept resulted in the laying down of a 26-mile long telegraph line between Madrid and Aranjuex. By means of electrostatic charges, Don Salva managed to transmit effective signals over it. Any underground section would no doubt be out of sight, if not out of reach, of the boys! [...]

Francis Ronalds had set up an eight-mile telegraph circuit in the garden of his home in Hammersmith, now known as Kelmscott House in the Uppoer Mall. The telegraph was described in some detail in a small book, published in 1823 and entitled Description of an Electric Telegraph and of some other Electrical Apparatus. From this it is clear that Ronalds' main interest was to establish whether the electric fluid in its static form could be made to travel over long distances without undue delay. For this purpose, he set up two wooden frames twenty yards apart, between which he strung wire backwards and forwards to form a continuous length of just under eight miles. THe wire passed through loops of silk at each of the 37 hooks on the 19 bars on the two frames. The two ends of the wire were attached to pith ball electrometers. "When the line was charged by connection to a Leyden jar, the electrometers at the near and far ends diverged at exactly the same moment, and on discharge by being touched by the hand, collapsed simultaneously." [...]

[William] Cooke made contact with the Directors of the Company [of the London and Birmingham Railway] and with their engineer Robert Stephenson, initially with the object of interesting them in a fire alarm system based upon his mechanical telegraph and an alarm bell, which he demonstrated to them on 4th July, 1837. They were, however, interested in a means of signalling between Euston and Camden Town, with the object of informing the men operating the winding engine when the train was ready to start.

A further demonstration was arranged for the 9th July, when message sending over a distance was demonstrated to Robert Stephenson and to Robert Creed, the Company Secretary. For this second experiment, Wheatstone had set up a "hastily made telegraph" employing four needles and four wires. The onlookers were impressed, and expressed the wish for the system to be demonstrated over a still greater distance. Accordingly, an extended line was prepared [by Charles Wheatstone] over the route from Euston to Camden Town.

It is known that the installation of telegraph wires over this route included a section at least of the second of the conductor systems described in the patent of 1837: "The long extensions of conducting wires, which may be called telegraph wires, may be lodged in channels formed in wood rails and lines with any suitable resinous matter, with a covering rail of wood over the channels to protect the wires from injury and from damp.... Another form of rail containing distinct channels for the several wires, each of which channels is closed by a fillet of wood driven into it. Such rails may be laid underground when that is more convenient than to place them above ground on posts, and some parts of a long line may be under ground and other parts above ground, and extend along the sides of public roads or railways or otherwise, as is most convenient and suitable to go from one terminus to the other."

As depicted in the patent, the five copper wires were laid in long wooden baulks of trapezoidal cross section, the grooves being plugged with wooden strips and the baulk finally painted with a preservative tar compound (Fig. 1.1). The buried section of the line, in the form of these baulks, was laid in a trench alongside the railway track. The trench was filled in with pitch. The length of the section so buried was just under two miles. [...]

Fig. 1.1. Wheatstone and Cooke telegraph conductors, Euston to Camden Town, 1837

The telegraph was a success. The wood baulk conductor array functioned satisfactorily, despite the fact as may be seen in the lengths that have survived, that the conductors used were not insulated in any specific way other than by the timber and its preservative tar compund. In the original specification, it was suggested that, if necessary, the wires could be covered with thread and varnished. [...]

The Grosvenor Gallery and Deptford

In retrospect, the year 1871 was an important one for the electric cable industry, if such it could be called at that time, bearing in mind its preoccupation with telegraph cables, for it was in this year that Zénobe Gramme (1826-1901) perfected the ring dynamo and a really satisfactory source of electric power became available. [...] The use of arc lamps spread rapidly, particularly with the development of the improved version of the lamp by Paul Jablochkoff. In 1878, these improved lamps, known as Jablochkoff Candles, were used in London for the lightning of the West India Dock, Billingsgate Market, Holborn Viaduct, and parts of the Thames Embankment.

In general, very little is known of the type of cable employed in these installations. Often, earth returns were used in conjunction with a plain copper wire supported on some form of insulator. [...]

It was not until 1911 that a really tough outer sheath was developed for application to rubber cables. This was done by the St. Helens Cable and Rubber Company of Warrington, who, in addition to their cable making activities, had been manufacturing solid rubber tyring by an extrusion process, for the wheels of horse-drawn vehicles, particularly Hansom cabs. From this was derived the term 'cab-tyre' sheathing. Designed to be "of great mechanical strength, water and corrosion proof, flexible, smooth, and 'un-kinkable' ", it was ideally suited for the protective sheathing of cables for such arduous duties as might be experienced in collieries.

In 1886, the India Rubber and Gutta Percha Company produced a cable consisting of a 19/15 gauge tinned copper conductor, insulated with 0.21 in. of pure and vulcanized India rubber, with a double jute braiding as an outer covering. The cable was designed to operate at 2400 volts single phase a.c. This cable was to be used in the now famous Grosvenor Gallery installation. [...]

It all began with the Paris Exhibition of 1882, at which the newly exploited electric light played an important part among the exhibits. The Earl of Crawford, one of the Commissioners appointed by the British Government to visit the Exhibition and report back on the latest developments, on his return suggested to Sir Coutts Lindsay, who was the proprietor of the fashionable Grosvenor Art Gallery in Bond Street, that he should install electric light in the Gallery. Sir Coutts agreed, and in early 1883 a pair of Marshall's semi-portable engines were erected in an out-building behind the Gallery. These were belted to a couple of separately excited Seimen's alternators, generating single phase current at 2000 volts. The plant supplied arc lights in series, and an automatic regulator kept the line current constant at 10 amperes. [...]

Interest in the plant soon spread among the local shopkeepers and residents, and requests for supplies of electricity were met initially by installing individual transformers, or secondary generators as they were then called, in the house of each consumer. The primaries of all the transformers were connected in series with the line, in accordance with the system then recently introduced by Gaulard and Gibbs. The high tension current was transmitted by overhead cables supported from poles on the house tops. This overhead system had the advantage that despite the restrictive provisions of the 1882 Electric Lighting Act, nothing was involved in obtaining permission for the installation than the consent of the house-holders and of the vestries concerned. [...]

The result of all this was that the small installation very soon became seriously overloaded. It was decided therefore to establish a permanent generating station under the Gallery, and on a considerably larger scale. The necessary construction work commenced on December 1884. [...]

Sir Coutts Lindsay then joined the Earl of Crawford and Lord Wantage in forming a small private company, to be called Sir Coutts Lindsay and Company Limited, but which was more famously known as the 'Grosvenor Gallery Electric Lighting Works' or as 'The Grosvenor Gallery Company'.

The new station went into service towards the end of 1885. [...] From the start, a great deal of trouble was experienced, and indeed the installation proved practically unworkable. Numerous complaints were received, arising from poor regulation, loss of supply, overloading, and breakdowns, until the situation fast was becoming desperate. [...]

On the 13th January, 1886, the Directors appointed [Samuel] de Ferranti [as] Chief Engineer of the Company, to be in full charge of the station. In the space of a few months, Ferranti managed to carry out a complete overhaul of the system. The overhead network was re-modelled for parallel working, transformers to his own design were installed in place of the Gaulard and Gibbs series devices, the voltage was increased to 2400 volts by connecting the windings of the Siemen's alternators in series instead of in parallel (each alternator had two sets of 1200 volt windings), and new switchgear was designed and erected. The vulcanized India rubber cable used for the overhead supply network was that supplied by the India Rubber and Gutta Percha Company (Fig. 3.2), and was suspended by leather thongs from a steel catenary wire which ran between suitable iron masts erected on convenient roof tops and a lattice tower on the roof of the Gallery itself. Two cables, spaced 12 inches apart, were used in each connection, being shackled off at the masts. The cable was supported in Johnson and Phillip's 'fluid type insulators'. By 1888, the system had grown to the extent that some four hundred premises were being supplied with electricity.

Fig. 3.2. 2400 volt rubber insulated cable for the Grosvenor Gallery Company's supply system

[...] So successful were the new arrangements, that customers who had been infuriated by the repeated breakdowns which had occurred with the earlier system, had their confidence restored, and the demand for new installations increased rapidly. The Directors decided to form a new company to take over the station, and to extend operations in accordance with some ambitious plans that had been proposed by their Chief Engineer. These were concerned with the setting up of a remote generating station to supply electricity to London on a really large scale. The new company, The London Electric Supply Corporation Limited, was registered on 26th August 1887, with an authorized capital of £1,000,000 in shares of £5 each, of which over half was subscribed. Sebastian de Ferranti was appointed Engineer and Electrician to the new company. [...]

This wide area [along the whole north side of the Thames] was to be supplied with electricity from a generating station located at Deptford on the South Bank of the River Thames, about eight miles from the heart of London. The output of the station was to be such that it would be capable of supplying current for the electric lighting of the whole area at an unprecedented a.c. transmission voltage of 10,000 volts. The location of the generating station on the river, some distance from the town, had the advantage that at Deptford, land was cheap, unlimited supplies of water were readily available, and sea-borne coal could be obtained at low prices. There would be no noise from moving machinery to disturb the slumbers of local residents, and no traffic congestion occasioned by the carriage of coal through the streets to fuel the power station boilers. For a parish of 200,000 lights, it was estimated that the fuel requirements would necessitate the passage of some 200 cart-loads of coal each day.

Work on the proposed new power station at Deptford began during April 1888, under Ferranti's direct supervision. [...]

The erection of the station was rapid, and in October 1888, the press were invited down to view the progress made. [...]

The Electrical Engineer of 27th October 1888, commented: "On Wednesday, the designer of the great Deptford installation was laughingly dubbed the Michael Angelo of that installation, because from first to last, from foundation to top of highest turret, architecture, materials, foundations, and machines, all were specified or designed by one man, and the credit of the success of the really first central station in England will have to be given, without detracting one iota in favour of any other person, to Ferranti... It required not only courage on the part of the engineer, but also a degree of confidence that few men possess in earlier days of industrial development."

At the same time that work commenced on the building of the station, consideration was given to the problem of transmitting power to the capital. Arrangements were made with the railway companies (the London and Brighton; the London, Chatham, and Dover; the Metropolitan; and the South Eastern) for permission to carry the trunk mains along the parapets and bridges of the railways to the distribution sub-stations in London. Six mains, or conductors, were to be provided, two of which would be brought to Cannon Street, two to Blackfriars Bridge, and two to Charing Cross. The use of railway property offered considerable saving, in that it precluded an approach to each parish and vestry for permission to excavate their roadways. [...]

Originally it was intended to transmit the power from Deptford to the Grosvenor Gallery by means of jute insulated cables made by the Fowler Waring Company. This was entirely satisfactory from an electrical point of view, but suffered from the disadvantage that its inflammable nature, in close proximity to passing steam locomotives, gave it a tendency to catch fire. Ferranti therefore decided to use a type of mains that he had specified in his patent application of 1885.

This patent specification had described a complete distribution system for electric lighting. Current from the generating station was to be transformed, or converted as he termed it, down from the supply voltage to 100 volts for supply to customers. The specification also details the type of mains to be employed for the distribution:

"The mains for conveying the current I form of concentric tubes drawn one over the other with insulating material between them."

The insulating material proposed was to be paper saturated with shellac in solution. [...]

The proposal to make this simple concentric design of tubular main did not, however, meet with Post Office approval. In a paper read before the Institution of Electrical Engineers in 1889, W. H. Preece, the Chief Engineer to the Post Office, said:

"It is quite certain that if a single conductor between Deptford and London were subject to rapid alterations under a potential difference of 10,000 volts, the current returning by way of the earth, every telephone circuit in the metropolis would be disturbed, and probably rendered unworkable."

Preece carried out a series of experiments with a lead sheathed concentric cable, and found that when he used the sheath as a return for a.c. at a frequency of 80 cycles/second, near the frequency of a Ferranti alternator, interference in an adjacent telephone line resulted. When, however, he used the outer conductor as the return, all was well. He came to the conclusion that an earthed sheath placed over the concentric main reduced the electrical interference to negligable proportions. Ferranti acted upon this information, and the following, well known, construction of the Ferranti main resulted (Fig. 3.3).

Fig. 3.3 - Ferranti tubular main for 10,000 volts, 1890

A copper tube, 13/16 inch diameter and 20 ft long, was insualted with wax-impregnated paper rolled on spirally, from sheets 20 ft in length and about 36 in. wide, either by hand, or later by special machines which avoided straining the paper. The sheets of paper were overlapped during application, until 0.5 in. of insulation had been applied. THe outer conductor, of the same cross-sectional area as the inner one, and 31/16 in. in diameter, was slipped over the insulated inner conductor, and the combined assembly was drawn through a die in a heavy tube drawbench, to compact the outer conductor down on to the paper insualtion. A further 3/32 in. of waxed paper was then rolled on to the outer conductor, and the whole enclosed in a thin iron tube of 2 3/8 in. diameter. The iron pipe had a braised seam, and the space between the paper and the iron was filled with hot bitumenous compound from a central injection point.

The jointing technique developed for these mains was of considerable importance. Of necessity, joints had to be simple, as in the four seven-mile mains laid between Deptford and the West End, there would have to be over 7000 of them. It is believed that Ferranti got the idea for the joints (Fig. 3.4) from the old wooden water mains, some of which are presevered in the Guildhall Museum, and which consisted of logs with a male cone at one end which was fitted into a female cone in the end of the next log, which in turn had a male cone at its other end.

Fig. 3.4 - Joint for Ferranti tubular main

[...] Before bringing the ends of the main together, an iron sleeve was slipepd over one length. This sleeve was so shaped as nearly to fit the protective iron pipe, but elsewhere was somewhat larger in diameter, and in the enlarged part a sleeve of prepared paper was placed. When the jointing of the outer conductor was complete, it was wrapped with the prepared paper to bring the diameter up to that of the iron pipe, the sleeve was moved into position, and hot wax was forced in through a hole in the sleeve to displace air and to fill up any spaces not occupied by the prepared paper. The ends of the sleeve were fixed down to the protective pipe by corruguations applied by a tool, and the hole through which the wax was introduced was closed by a screw plug.

There was, of course, considerable criticism of the use of a voltage as high as 10,000 votls with these mains, and doubts were cast upon their safety in the event of a fault. Ferranti devised a demonstration to alleviate the doubts and to convince even the most skeptical of their safety. Before a large number of witnesses, Harold Kolle, Ferranti's personal assistant, held an un-insulated cold chisel to the live main with his bare hands, while a colleague with a slege hammer drove it through both conductors. As the chisel cut through the main, the main fuse link cut off the supply without damage to the equipment or injury to the very brave man involved. There is an apocryphal story that Kolle, asked later if he had not been frightened, replied "Frightened? I was scared out of my life. Young Henry had never used a sledge hammer before!". [...]

Originally, it was intended to install twelve 10,000 horse-power generating units, but the complete plans were partially forestalled by the operation of the Electric Lighting Act of 1882, and the subsequent Board of Trade Enquiry held in 1889. These, in effect, split up the area which was to have been supplied from Deptford among a number of smaller companies.

On top of all this, the scheme was beset with a number of disconcerting and almost disastrous incidents. One of the jointers was killed by a passing train during the installation of the mains along the railway embankment. On another occasion, one Joseph Selway was killed in the explosion at Stowage Wharf (the site of the power station) on 9th April 1890. The accident occurred while the works were being inspected by Major Marindin, who headed the Board of Trade Enquiry; a steam pipe on one of the engines burst, and Selway was blown 28 ft by the explosion, and three other men were seriously injured.

A devastating fire occurred at the Grosvenor Gallery while it was being used as a temporary sub-station. A power arc occurred during switching, which set fire to the wooden roof before it could be extinguished. This resulted in the complete destruction of the sub-station, and the loss of supply from the 15th November 1890, when the fire took place, until the following February. During this period, a number of customers were lost, and there was no income, only considerable expenditure. The opportunity was taken to push ahead with what Ferranti referred to as "the permanent work" to an extent which could not otherwise have been possible. The whole system of overhead lines radiating from the Grosvenor Gallery were removed, and replaced by underground cables.

The Board of the Electric Supply Corporation were not, however, very happy with these misfortunes. They had lost a large amount of money, and as the prospects of obtaining the virtual monopoly of the electricity supply for London began to fade, sodid their confidence in their Chief Engineer. At the meeting of the shareholders in March 1892, it was announced that during the year, "the engagement of Mr. de Ferranti has ceased by the effluxion of time".

Although this must have been a severe blow to Ferranti, particularly as he had been accused by one member of the Board as being "sadly lacking in prevision", he was now free to transfer elsewhere his experience, his valuable patents, and his unrivalled knowledge of high voltage technique. He did not have long to wait. The Directors of the newly formed British Insulated Wire Company, having taken the opinion of Mr. W. H. Preece as to the value of the Ferranti patents relating to electric cables, offered him a seat on their board, acquired his patents, and insured his life for the sum of £20,000.

In retrospect, Deptford was a magnificent and daring enterprise. [...]

In the opinion of Colonel Crompton, "electricity supply owes far more to Ferranti than to any other man. All the substantial features of supply are founded on his work at the Grosvenor Gallery and at Deptford".

Electric Lighting Cables

[...] The view has been expressed that had the business of electricity supply been destined to continue on the same limited scale as that of the earlier undertakings, there is little doubt that the direct current system would have been generally adopted.

One of the protagonists of the low voltage d.c. system was Thomas Alva Edison (1847-1931), an inventor with Joseph Wilson Swan of the incandescent filament lamp. Edison was very much aware of the dangers of high voltage electricity, as at that time he was in his laboratory at Menlo Park, New Jersey, with other workers in the field, considering the use of electricity in judicial execution. During those years, the electrical press contained many references to this work, as well as to the development of a 'secret death ray' which was reported to have been applied to a flock of sheep with dire effect. The journals also record the prolonged debate as a result of which the term 'electrocution' entered the language, being thought a suitable word to describe the particular electric shock treatment finally decided upon.

Edison, in a manner not unlike that of Ferranti around the same time, worked out the details of a complete system for the distribution of electricity at low voltage to customers. One problem encountered with low voltages was that of 'copper loss' - the reduction of the supply voltage with the distance of the customer from the generating station. As Edison determined to supply his customers at 100 volts, it is said that he generated at 110 volts to ensure the full voltage reaching the customer. The practice of an additional ten percent has survived to the present day even though it is no longer necessary, and is found in the voltage series of 11 kV, 22 kV, 33 kV, 66 kV, 132 kV, etc. [...]

The mains which he proposed for the commercial distribution of electricity were [...] to consist of two large segmental shaped copper rods (Fig. 4.1), separated from each other by thick cardboard spacers strung together on a jute string and contained in an iron pipe with a screw thread at each end. This pipe was 20 ft in length and was subsequently filled with a bitumen compound by means of a hand operated suction pump. These rigid mains were to be known as 'Edison Tubes', and in a similar manner to the Ferranti mains detailed a year or so later, and which only reached the stage of commercial application in 1889, were of such a length that they could be transported around the average street corner on a horse-drawn cart.

Fig. 4.1 - Edison tube (2-core) from Holborn Viaduct installation, 1882

[...] The installation was started in January 1882, and a central generating station erected in the basement of No. 57, Holburn Viaduct, the London offices of the Edison Company. Mains in the form of Edison tubes were laid in subways under the Viaduct to supply the street lights from Newgate Street to Holborn Circuit, and numerous buildings along that route. [...]

In 1883, Edison adopted the three-wire d.c. system which had been introduced [in] England in the previous year by Dr. John Hopkinson in Patent No. 3576 of 1882. [...]

In adopting the three-wire system, Edison had to modify the arrangement of the conductors in his tube. These now took the form of three solid rods of copper of the same circular cross-section, individually lapped with jute cord in a very open lay (Fig. 4.2). Three such lapped conductors were then bound together in trefoil formation by means of a fourth cord and the whole slipped into the iron pipe. The bitumen compound was introduced as before. Special joints were again designed to overcome expansion, to introduce right-angle bends and to enable service cables to be connected.

Fig. 4.2 - Edison tube (3-core) from Pearl Street installation, New York, 1883

[...]

Paper Insulated Cables

[...] Towards the end of 1892, a horizontal engine was acquired [by the British Insulated Wire Company (B. I. Co.) ] from St. Helens, where it had been used to grind pills. It was set to drive a Brush arc lighting machine. This was to supply sixteen arc lamps mounted on special lamp standards, which were erected chiefly at important corners of the town [Prescot]. These arc lamps were in addition to the twenty 16-candle power incandescent filament lamps which had been mounted in converted gas standards.

The engine and arc lighting machine were appropriately and inevitably named "Joan of Arc", and were said to be a good deal more reliable than either the circuits or the lamps which they supplied. It is recorded that a man was appointed to go round the town after lighting-up time each night, and to apply a vigorous kick to the base of any arc lamp standard in the event of the lamp 'sticking'. [...]

In March 1892, all the four 10,000 volt Deptford mains were put out of commission simultaneously by a fire which broke out in material stored beneath the arches of the South Eastern Railway Company's viaduct near Spa Road, Bermondsey. Despite the fact that the mains were carried along the parapet of the viaduct, the fire was so intense that the exposed section of the mains was completely destroyed. To prevent any similar accidents in the future, it was decided to put this section of the mains underground.

By virtue of Sebastian Ferranti's influence with the London Electric Supply Corporation, an order was placed with the B.I. Wire Company for the supply of two 30-yard experimental lengths of 11,000 volt flexible, paper-insulated concentric cable. These cables, when made, were the first flexible paper-insulated cables to be employed at this voltage anywhere in the world. Their construction followed the now familiar B.I. pattern; the inner conductors were made up of 7/0.064 in. diameter wires, giving a nominal cross-sectional area of 0.0225 sq. in. The outer conductor in one case was formed of round wires, and in the other of flat strips. The experimental cables were put into service in the late spring of 1893, being used to connect a temporary testing station into the existing Deptford Main.

The lengths of experimental cable proved satisfactory, and a further order for 15 miles of 0.25 sq. in. concentric cable, to the London Electric Supply Corporation's own specification, was received in 1896 (Fig. 5.8).

Fig. 5.8 - B.I. Wire Company's flexible 10 kV paper insulated cable used in the Deptford Main, 1896

Two years late[r], in 1898, a 42-mile length of 10 kV flexible concentric cable, with 0.54 sq. in. conductors and capable of carrying 7.5 MVA per phase, was supplied to the Metropolitan Electric Supply Company. This differed from previous cables in that it possessed a segmental inner conductor built up over a rope core (Fig. 5.9).

Fig. 5.9 - B.I. Wire Company's flexible 10 kV concentric paper insulated cable supplied to the Metropolitan Electric Supply Company, 1898

In the last years of the century, some seven miles of three-core, 5000 volt cable was supplied to the Notting Electric Lighting Company and to the Kensington and Knightsbridge Electric Lighting Company. This was to carry power from their new, jointly owned power station at Wood Lane, Hammersmith to the surrounding area. The Wood Lane Power Station started operation in October 1900, and is of historical importance because it provides the earliest example of high voltage three-phase generation and transmission in this country. It is now part of the laboratories of BICC Research and Engineering Limited. [...]

Three-Phase Cables

[...] It was in 1883 that Tesla constructed his first commutator-less motor. In the following year, he emigrated to the United States and in due course became an American Citizen by naturalization. In 1887, he formed the Tesla Electric Light Company and was free to develop into a commercial form many of his earlier ideas. In addition to the discovery of the rotating field in motors, Tesla introduced the use of polyphase systems of alternating current and invented the induction motor.

Following Tesla's pioneer work, polyphase transmission was introduced into North America by the General Electric Company in 1893. Three-phase current was used for the lighting of the Chicago World's Fair which took place in that year. This was not, however, the first application of a polyphase system. Pride of place must go to the overhead transmission line from Lauffen to Frankfurt, a distance of over 100 miles, in connection with the lighting of the 1891 Frankfurt Exposition. The circuit operated at a voltage of 30 kV. [...] Other early examples of three-phase transmission were the Niagara project in 1895, and its adoption by the Kensington and Notting Hill Electric Lighting Companies in 1899.

For three-phase transmission by cable, a three-core construction was desirable. In the early days of single phase alternating systems, the concentric structure was adopted by the B.I. Wire Company as eminently suitable, and was derived from the Hopkinson 3-wire d.c. system. But the logical extension to a triple concentric for three-phase currents, though producing an aesthetically attractive design of cable, gave rise to a considerable loss in flexibility. This was not the case with a multi-core cable in which the separate cores were cabled together, the spaces between the cores (if circular cores were used) filled with additional jute or paper fillers, and the assembly held together with a paper belt of sufficient thickness to provide additional insulation between the cores and the sheath.

The 'clover leaf' cable in which shaped cores replaced the round ones was a substantially improved design introduced by the B.I. Wire Company under the influence of the Ferranti patents. It enabled the dimension of the cable to be reduced for a given cross-sectional area of conductor and given supply voltage. [...]

An early example of this 'clover leaf' design was the three-core 5.25 kV paper insulated cable supplied for Wood Lane Power Sation in Hammersmith (Fig. 6.1) [...] which was the first in this country to generate a three-phase supply.

Fig. 6.1 - Three-core 5.2 kV clover leaf cable for Wood Lane Power Station, Hammersmith, 1900 (B.I. Company)

[...] At some time between 1900 and 1902, the Montreal Light, Heat, and Power Company installed a three to four mile length of three-core paper insulated cable on their Chambly circuits. The cables were commissioned in 1902 at 25,000 volts with a grounded neutral, and were still in service in 1949. The cable was made by the National Conduit and Cable Company of New Jersey, who had acquired the Norwich Insulated Wire Company in 1891. The National Conduit and Cable Company later became the present Anaconda Wire and Cable Company. [...]

The Thury Continuous Current System

Despite the fact that by the year 1900, high voltage alternating current transmisson and distribution was firmly established and three-phase distribution was being adopted in the United States, in Europe, and in the United Kingdom on an ever-increasing scale, there was still interest in the high voltage direct or continuous current system. This has been pioneered by Professor Thury on the Continent since the first installation, under his direction, in Italy in 1889 by the Society Acquedotto de Ferrari-Gallieri. This initial installation comprised a 75 mile circuit capable of transmitting some 630 kW at a 'pressure' of 14,000 volts. The introduction of high voltage d.c. as opposed to low voltage d.c., supported by Edison and others slightly earlier in the era, was to culminate in the Moutiers-Lyon System, erected in 1906.

In a paper presented before the Institution of Electrical Engineers in March 1907, J. S. Highfield, then a member of the Council and later to become President, gave a comprehensive account of the Thury System and compared it with the corresponding three-phase a.c. system. From the results of many tests, he concluded that a direct current pressure of at least twice as great as an alternating current pressure may be used on the same overhead line insulators and on the same underground cable. The direct current system worked on the equivalent of a three-wire system, the middle point of the system being connected to earth so that one line was operating at a pressure above earth and the other at a pressure below. In this way, and using the same insulation, the effective direct current pressure could be doubled. [...]

Protective Systems and Limitations of Solid Type Cables

[...] Solid type cables with screened cores proved eminently satisfactory at 33 kV, and were used, despite the fact that they were very much more expensive than overhead lines, to run power transmission systems underground in the vicinityof large towns. As the demands upon the supply system increased, higher and higher transmission voltages were adopted. With overhead lines, no serious problems were introduced, but with underground cables the use of voltages of the order of from 40 to 60 kV imposed a severe strain upon them. A strain for which they were not designed. Another limitation was the fact that cables, by virtue of their large capacitance, took the full force of any voltage surges induced in the overhead by lightning, and the resulting induced over-voltages resulted in frequent breakdowns. [...]

Telephone Cables

Following the invention of the telephone by Alexander Graham Bell, who patented his invention in 1876, the first telephone company in the United Kingdom was formed in 1878. This was registered on the 14th June as the Telephone Company Limited (Bell's Patents). The choice of telephone cables was initially limited to the already available gutta percha insulated wires which had been used so successfully for telegraph purposes from about 1848 onwards, but while gutta percha possessed good insulating properties, particularly under moist conditions, the dielectric constant of the material, 4.43, resulting in some distortion of the speech waveform, and in consequence, bare wires supported on insulators from poles were found to be superior. A speech-carrying wave is the resultant of the simultaneous transmission of a great many intermittent trains of waves of frequencies varying from about 50 Hz to 3000 Hz. Because of the complexity of the waveform, any considerable length of gutta percha insulated wire in the circuit would mutilate and distort it to such an extent that the reproduction of intelligible speech would be prevented. Short lengths of such insulated wire could be used where underground transmission was essential. [...]

As mentioned above, the use of telegraph cables for telephony could only be regarded as expident in cases where undergrounding was imperative. The distortion of speech due to the high permittivity of the dielectric could be overcome by the use of bare wires and, in consequence, the early growth of the telephone service in this country was provided for almost exclusively by the use of overhead wires, and this, together with the early electric light distribution system which was similarly disposed, did nothing to enhance the skyline in the larger towns and cities. This was not only the situation in the United Kingdom; in America also, overhead wiring was a problem, as is evidenced by the contribution of W. H. Preece (later Sir William Preece) to a discussion on Underground wires, the subject of a paper presented by W. M. Callender in America in 1884.

Mr. Preece observed: "When I was here, seven years ago, I saw your corporation spending heaps of money in beautifying your streets and in the erection of beautiful buildings, and then allowing them to be disfigured with those hideous posts overhead with wires strung on them. The only advantage that I have yet observed with those overhead wires has been that in hot weather in some pace they form very convenient shade. Beyond that I know of no earthly reason why they should be put up." [...]

With the emergence of the dry core telephone cable (Fig. 14.3), there was very little change in the design of telephone cables for many years. Problems there were with the exclusion of moisture from the cores, and this was achieved by lead sheathing, with the design of satisfactory terminations and with the prevention of the phenomenon of cross-talk.

Fig. 14.3 - Dry paper insulated quad trunk telephone cable (BICC)

[...] It was soon realised that the transmission qualities of a cable could be improved consideraby by one or both of two methods: by introducing an inductance in series with the line, or by amplifying the signals (attenuated electromagnetic waves) at intervals along the cable. These two techniques are more generally referred to as 'loading' and as 'repeaters'. [...]

Initially, the carrier current system could only be used with opern wire telephone lines, since the air insulation enabled the carrying of very high frequencies. With the development of low permittivity insulation employing polystyrene or polyethylene spacers within a metallic tube, it was not long before the transmission of large blocks of tleephone traffic over multi-core coaxial cable become the established practice. The technique ensured the viability of the submarine telephone cable, which would otherwise have been uneconomic unless a large number of channels could be provided on a single coaxial cable (Fig. 14.4).

Fig. 14.4 - Composite telephone cable with two sealed tube coaxial pairs and paper quads (Siemens), 1951

[...]

Colliery Cables

In a modern colliery, a very wide range of electric cables are employed, for power supply in the shafts and roadways, as trailing cables for coal cutters, as pliable armoured conveyor cables, as drill cables, remote control cables, cables for the various electronic systems employed for monitoring, signalling, and telecommunications, as shot firing cables, and as specially flexible cords for the electricity supply to the lamps on miners' caps.

The conditions to which this range of cables can be exposed may be arduous, and in consquence the cables must possess very special properties, which include fire resistance, weather and water resistance, toughness to withstand contact with rough and abrasive objects, and in addition, considerable flexibility. The installation of long vertical lengths also determines certain of the cable characteristics. For example, oil-impregnated paper cables had to be designed with non-draining impregnating compounds in order to preclude undue ionization as the cable drained under electrical load and gravity.

Above all, electrical installations underground had to be so designed that they were flameproof, and any ignition of the explosive gases which might seep into the mine workings, by sparks or arcs occurring during switching or coupling of cables, had to be precluded or rendered harmless.

It is generally held that electricity was first used to displace pit ponies from their underground haulage activities at the time when Callenders' vulcanized bitumen insulated cables were introduced in a pioneer colliery electrification scheme, at Abercanaid near Merthry Tydfil in 1891. Early VB insulated cables had stranded copper conductors and were finished by being taped or braided, but for colliery work, double rope armouring was introduced at an early date to give protection without impairing the flexibility of the cable. [...]

PVC insulated power cables were offered by the manufacturers for operation at voltages up to 3.3 kV.

By 1969, catalogues of Mining Cables were giving details of power cables with stranded copper or solid aluminium conductors, with extruded PVC insulation in conformity with a British Standard. The cable cores were identified by colour coding: two-core cable, having red and black coloured cores; three-core, [having] red, yellow, and blue; and four-core, [having] red, yellow, blue, and black. [...]

The principal characteristics required of trailing cables are those of flexibility and resistance to abrasion. More recently, fire resistance has also assumed an important role. From the earliest days of colliery cables, vulcanized rubber proved to be the ideal insulation, both electrically and mechanically. With the development of the tough cabtyre sheathed cable, and the practicality of incorporating flexible steel wire armouring, there has been little change in the design of the various types of trailing cables over the years.

Coal cutter cables

Coal cutter cables normally possessed three or five cores, and operated at 660 volts. The cores were colour coded: red, white, and blue, together with in the five-core cable, green for earth and black for a pilot core. The cores were laid, very much after the manner adopted with the old vulcanized bitumen cables, in a specially shaped centre filler or cradle, and enclosed in an inner sheath, ribbed internally to fill the interstices between the cores. Over this inner sheath could be applied a screen of tinned copper wires (applied spirally), followed by an outer sheath of fire-resisting polychloroprene compound. This construction was known as the collectively screened design, an alternative to which was the individual screening of the separate cores.

Pliable armoured conveyor cables

The construction of the conveyor cables was similar to that of the coal cutter cables, save that the collective screen was replaced with a pliable armour of galvanized steel strands applied spirally, and the cable was sheathed overall with a fire-resisting polychloroprene compound. Multi-core cables with three, four, and five cores were usual.

Drill cables

Drill cables normally operated at 250 volts, and were similar in construction to smaller versions of unscreened coal cutter cables. The cores were insulated with vulcanized rubber and, again, the sheath was of polycholoroprene. To achieve additional flexibility, the cores in their cradle could be assembled with a short lay.

Remote control cables

These small cables were two and three core versions of pliable armoured trailing cables, and were intended to operate at voltages of the order of 30 volts, for power supply to remote control circuits. Their construction was similar in principle to that of a conventional trailing cable, and comprised two or three cores of equal size, each insulated with vulcanized rubber, twisted together and sheathed with a tough rubber compound, and armoured with pliable galvanized steel strands applied spirally. The cables were sheathed with polychloroprene.

Shot firing cables

These were small cables, having conductors of tinned copper wire (4/0.018 in.), insulated with vulcanized rubber, and taped. Two cores, one red and the other black, were twisted together with jute wormings, and braided with either yellow cotton or with cotton with a black compound (bitumen) finish.

Miscellaneous cables

In addition to the various types of cables which have been described, a large number of conventional type cables were and are employed in collieries. These range from multi-pair telephone cables and signalling cables, to the different types of cable necessary for the complex controlling and signalling systems occasioned by the new techniques of production, and the various management control systems which are being introduced gradually into the industry. These involve electronic sensing and monitoring systems, data processing, and recording.

[...]

This information comes from

The 'Preface' states:

This volume contains a collection of circuit diagrams, representing more or less completely all branches of electrical engineering, with the exception of telephony and telegraphy. [...] The circuit connections are drawn so as to be self-explanatory, thus reducing the necessary text to a minimum.

(Additional information from this book is located on the Transformers and Power Equipment page.)

The following information is excerpted from this book.

[...]

Fig. 422 - Standard methods of joining electrical conductors.

Fig. 422 - Standard methods of joining electrical conductors.

[...]

This information comes from

(Additional information from this book is located on the Power Systems page.)

The following information is excerpted from this book.

Cable Systems

[...]

Open Wire

This mode was used extensively in the past. Although it has now been replaced in most applications, it is still quite often used for primary power distribution over large areas where conditions are suitable.

Open-wire construction consists of uninsulated conductors on insulators which are mounted on poles or structures. The conductor may be bare, or it may have a covering for protection from corrosion or abrasion.

The attractive features of this method are its low initial cost and the fact that damage can be detected and repaired quickly. On the other hand, the uninsulated conductors are a safety hazard and are also highly susceptible to mechanical damage and electrical outage from birds, animals, lightning, etc. There is increased hazard where crane or boom truck use may be involved. In some areas, contamination on insulators and conductor corrosion can result in high maintenance costs.

Due to the large conductor spacing, open-wire circuits have a higher reactance which results in a higher voltage drop. This problem is reduced with high voltage and higher power-factor circuits.

Exposed open-wire circuits are more susceptible to outages from lightning than other modes. The effects may be minimized though, by the use of overhead ground wires and lightning arresters.

Aerial Cable

[...] Aerial cables may be either self-supporting or messenger-supported. They may be attached to pole lines or structures. Self-supporting aerial cables have high tensile strength for this application. Cables may be messenger-supported either by spirally wrapping a steel band around the cables and the messenger, or by pulling the cable into rings suspended from the messenger. The spiral wrap method is used for factory-assembled cable, while both methods are used for field assembly. A variety of spinning heads is [are] available for application of the spiral wire banding in the field.

Self-supporting cable is suitable for only relatively short spans. Messenger-supported cable can span large distances, dependent on the weight of the cable and the tensile strength of the messenger. [...]

Spacer cable is a type of electric supply-line construction that consists of an assembly of one or more covered conductors separated from each other and supported from a messenger, by insulating spacers. [...] Uniform line electrical characteristics are obtained through the balanced geometric positioning of the conductors with respect to each other, by the use of plastic or ceramic spacers located at regular intervals along the line. Low terminating costs are obtained because the conductors are unshielded.

[...]

 

Insulators Home > Book Reference Info > History of Electrical Systems and Cables

Contact: A.C. Walker