Electric Cables
Ferranti first referred to paper-insulated concentric mains in his patent (15251 of 1885) for Improved Means of Distributing Electrical Energy, but did not immediately make them. At that time he had 2,400 volts in mind.
For the 10,000-volt transmission from Deptford, after careful appraisal of available cables with the two forms of dielectric then regarded as the most suitable for high voltage, namely rubber and jute, he decided on Fowler-Waring jute-insulated concentric-type cable. With the consent of the railway companies these were run along the parapet walls of the track, but there were many breakdowns due to deterioration of the cables and to their susceptibility to catch fire, causing the London Electric Supply Corporation considerable financial loss.
The problem was difficult and urgent. What was the point in having generating units with their boilers and auxiliaries ready to run at Deptford if there was no reliable means of getting the current to London ?
Ferranti decided to make suitable mains himself and chose wax-impregnated paper as the insulating medium. This was not only a startling innovation but a decision of historical importance, being the start of all paper-insulated high voltage cables in the world.
The work is described in Ferranti's own words in a letter to his son Vincent dated 3 March 1910:
"The sample of main which I sent you was a piece cut from the extra high tension main which I invented in 1888 and of which we made some thirty miles for laying between the Deptford generating station and the various distributing stations in London. The inner conductor consisted of a copper tube, as you will see, and was insulated by means of ordinary brown paper soaked in ozokerite or black earth wax, which was a residue obtained in candle making. The effectiveness of the insulation was the result of the very large number of layers of this wax-coated paper which were interposed between two conductors. The outer copper tube was slipped over the roll of paper and then the whole main was drawn through a die in a heavy tube draw bench and so squeezed very tightly down upon the paper. This made a solid structure from a mechanical point of view and thoroughly protected the paper. The outer conductor was then insulated in the same way and fitted tightly into the outside wrought iron pipe which was used as a means of protection. These mains were made in 20 ft. lengths, and the system of jointing is shown in the accompanying specification. The joint between the inner conductors was effected by pushing a rod up two adjoining ends: the insulation was joined by turning two cones and forcing the cones over each other. The outer conductor and the outer protecting tube were each joined by sleeves which were burnished into position by means of a circular burnishing tool used for rolling parts together. These mains were set to work on 10,000 volts pressure in 1889 and were the first extra high-tension mains which I believe were ever laid underground.
Practically all mains are now insulated with layers of paper wound round and round and most high-tension supplies used for transmission to towns are in the neighbourhood of 10,000 volts, there only being a few supplies using underground mains working on much higher tension.
The outer conductor of the Deptford mains was earthed at Deptford so as to ensure that no part with which one could possibly come into contact would be at much of a pressure above the earth. This provision was strenuously opposed by the Board of Trade in the early days, and it was only by actually doing it in London without their permission that I demonstrated that it was the right thing to do, when the regulations were altered, making the earthing of the exterior conductor in concentric mains compulsory—the exact reverse of the previous regulations.
It is interesting to note that although these mains were made in a great hurry and with somewhat faulty appliances, and with a great want of knowledge, notwithstanding that, at least half the quantity of mains originally laid down are still supplying London on 10,000 volts today, the remaining portion of these same mains being still in use but working at a lower pressure".
Ferranti first referred to paper-insulated concentric mains in his patent (15251 of 1885) for Improved Means of Distributing Electrical Energy, but did not immediately make them. At that time he had 2,400 volts in mind.
For the 10,000-volt transmission from Deptford, after careful appraisal of available cables with the two forms of dielectric then regarded as the most suitable for high voltage, namely rubber and jute, he decided on Fowler-Waring jute-insulated concentric-type cable. With the consent of the railway companies these were run along the parapet walls of the track, but there were many breakdowns due to deterioration of the cables and to their susceptibility to catch fire, causing the London Electric Supply Corporation considerable financial loss.
The problem was difficult and urgent. What was the point in having generating units with their boilers and auxiliaries ready to run at Deptford if there was no reliable means of getting the current to London ?
Ferranti decided to make suitable mains himself and chose wax-impregnated paper as the insulating medium. This was not only a startling innovation but a decision of historical importance, being the start of all paper-insulated high voltage cables in the world.
The work is described in Ferranti's own words in a letter to his son Vincent dated 3 March 1910:
"The sample of main which I sent you was a piece cut from the extra high tension main which I invented in 1888 and of which we made some thirty miles for laying between the Deptford generating station and the various distributing stations in London. The inner conductor consisted of a copper tube, as you will see, and was insulated by means of ordinary brown paper soaked in ozokerite or black earth wax, which was a residue obtained in candle making. The effectiveness of the insulation was the result of the very large number of layers of this wax-coated paper which were interposed between two conductors. The outer copper tube was slipped over the roll of paper and then the whole main was drawn through a die in a heavy tube draw bench and so squeezed very tightly down upon the paper. This made a solid structure from a mechanical point of view and thoroughly protected the paper. The outer conductor was then insulated in the same way and fitted tightly into the outside wrought iron pipe which was used as a means of protection. These mains were made in 20 ft. lengths, and the system of jointing is shown in the accompanying specification. The joint between the inner conductors was effected by pushing a rod up two adjoining ends: the insulation was joined by turning two cones and forcing the cones over each other. The outer conductor and the outer protecting tube were each joined by sleeves which were burnished into position by means of a circular burnishing tool used for rolling parts together. These mains were set to work on 10,000 volts pressure in 1889 and were the first extra high-tension mains which I believe were ever laid underground.
Practically all mains are now insulated with layers of paper wound round and round and most high-tension supplies used for transmission to towns are in the neighbourhood of 10,000 volts, there only being a few supplies using underground mains working on much higher tension.
The outer conductor of the Deptford mains was earthed at Deptford so as to ensure that no part with which one could possibly come into contact would be at much of a pressure above the earth. This provision was strenuously opposed by the Board of Trade in the early days, and it was only by actually doing it in London without their permission that I demonstrated that it was the right thing to do, when the regulations were altered, making the earthing of the exterior conductor in concentric mains compulsory—the exact reverse of the previous regulations.
It is interesting to note that although these mains were made in a great hurry and with somewhat faulty appliances, and with a great want of knowledge, notwithstanding that, at least half the quantity of mains originally laid down are still supplying London on 10,000 volts today, the remaining portion of these same mains being still in use but working at a lower pressure".
Soon after Ferranti's departure from the London Electric Supply Corporation Ltd in 1891 the directors of the newly-formed British Insulated Wire Company, Prescot, took the opinion of W. H. (later Sir William) Preece, F.R.S., Chief Engineer to the Post Office, as to the value of Ferranti's patents, and received a report in favour of their purchase. At a special meeting of the shareholders it was decided to increase the Company's capital by £50,000, and not only to acquire the patents but also to insure Ferranti's life. On completion of the Agreement for Purchase in August 1891, Ferranti joined the Board and it was soon found that his practical knowledge and experience were of the greatest value. In 1893, as the result of Ferranti's considerable influence with his old company, the British Insulated Wire Company obtained an order from the London Electric Supply Corporation for two 30 yd. experimental lengths of 11kV flexible paper-insulated cable. Ferranti had envisaged such cables in his patent 2315 of 1888 and when they were made they formed a landmark in the history of cable development, since they were the first flexible paper-insulated cables to be employed at such high voltage anywhere in the world.
The cables proved to be satisfactory on test, and in 1896 fifteen miles of 11kV cable of the `flat-strip-outer' type, with 0.25 sq. in. conductors, were ordered by the London Electric Supply Corporation for replacements in the Deptford main where it remained in service for over thirty years.
The cables proved to be satisfactory on test, and in 1896 fifteen miles of 11kV cable of the `flat-strip-outer' type, with 0.25 sq. in. conductors, were ordered by the London Electric Supply Corporation for replacements in the Deptford main where it remained in service for over thirty years.