Cable Ducts Research Articles

Chalky Boulder-clay in the North-West District of London

At the commencement of the year 1925 the Post Office authorities, for the purpose of laying down telephone lines, were making a trench from the corner of Rochester Road (Camden Town) north-eastward up the Camden Road and across it into and along Murray Street in the direction of St. Paul's Road, and were laying ducts at the depth of about 10 ft. The distance from the corner of Rochester Road to a point opposite the corner of Murray Street is 160 yards, the direction north-east, with a slight rise in that direction. Previously the London County Council tramway cable ducts had been laid down to the depth of 3 ft. 6 in., but below this level the whole of the ground excavated was in an undisturbed state. The trench was 2 ft. 7 in. wide. A man-hole had been sunk at the corner of Rochester Road, another at the end of the works on the Camden Road, and a third opposite to it at the corner of Murray Street, all to a depth from the surface of more than 12 ft. In fact the Camden Road man-hole was sunk to the depth of 15 ft. 6 in., so as to enable the excavators to tunnel under the road to Murray Street in order that the stream of traffic along the Camden Road might not be disturbed.

Accuracy required in the measurement of dielectric power factor of impregnated paper-insulated cables

This paper deals with the effect of errors in the measurement of power factor upon the usefulness of impregnated paper-insulated cables. It points to error in the knowledge of the thermal properties of the cable and the cable duct. The latter error affects the usefulness of a cable to such an extent as to permit a limited error in the power factor without materially reducing the efficiency of the cable. This limited error defines the required power-factor accuracy. The required power-factor accuracy in general is found to vary directly with the frequency and the specific inductive capacity of the insulation, and to increase with the number of cables in a duct bank and the ratio of E <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /G, where E is the operating voltage of the cable in kilovolts and G is the geometric factor. For very high-voltage single-conductor cables the power-factor accuracy should be within the order of 0.002.

III---Accuracy Required in the Measurement of Dielectric Power Factor of Impregnated Paper-Insulated Cables

This paper deals with the effect of errors in the measurement of power factor upon the usefulness of impregnated paper-insulated cables. It points to error in the knowledge of the thermal properties of the cable and the cable duct. The latter error affects the usefulness of a cable to such an extent as to permit a limited error in the power factor without materially reducing the efficiency of the cable. This limited error defines the required power-factor accuracy. The required power-factor accuracy in general is found to vary directly with the frequency and the specific inductive capacity of the insulation, and to increase with the number of cables in a duct bank and the ratio of E2/G, where E is the operating voltage of the cable in kilovolts and G is the geometric factor. For very high-voltage single-conductor cables the power-factor accuracy should be within the order of 0.002.

Applications and limitations of thermocouples for measuring temperatures

E. D. Tanzer: Not so long ago, a thermocouple was solely a physicists instrument. At the present time, however, it has a wide field of application among the operating companies. At least, we of the Philadelphia Electric Company, during the past three years have used it extensively in field determinations of the operating temperatures of cable duct lines, cable sheaths and transformer manholes, as well as in research problems pertaining to the operation of an extensive underground cable system.

Effect of moisture in the earth on temperature of underground cables

The author describes a permanent installation which was made for supplying moisture to the earth in the neighborhood of underground cables with the object of reducing their temperature. The approximate temperature of the cables is found by taking with a resistance thermometer the temperature of a duct adjacent to the cable which is the source of heat. In uncovering the conduits and exposing them to air, as a remedy for hot spots in the cable, it was found that the adjacent earth was hot and dry so that it crumbled to powder. This suggested opening a ditch in the ground above the conduit and directing a stream of water through it. This was found to lower the temperature immediately several degrees. Where an open conduit was not practical, water was discharged in to a vacant cable duct by means of a hose and this was found to be more effective than the open ditch method. These experiments led to the installation of a line of porous tile duct in the earth above the conduit, surrounded with clean sand. The leakage of water through the pores of this duct has been found very effective in reducing the temperature of the cables. Whenever the temperature of the cables is found by exploring with a resistance thermometer to approach the danger point, water is turned into the porous drain tile, and the temperature is taken on successive days to see whether the desired reduction has been obtained. In this way one or two men, with resistance thermometers attached to long leads, can keep track of and control the temperature of the cables in a large system. No breakdowns of insulation of cables have occurred due to high temperature since the adoption of this method.

Effect of Moisture in the Earth on Temperature of Underground Cables

The author describes a permanent installation which was made for supplying moisture to the earth in the neighborhood of underground cables with the object of reducing their temperature. The approximate temperature of the cables is found by taking with a resistance thermometer the temperature of a duct adjacent to the cable which is the source of heat. In uncovering the conduits and exposing them to air, as a remedy for hot spots in the cable, it was found that the adjacent earth was hot and dry so that it crumbled to powder. This suggested opening a ditch in the ground above the conduit and directing a stream of water through it. This was found to lower the temperature immediately several degrees. Where an open conduit was not practical, water was discharged into a vacant cable duct by means of a hose and this was found to be more effective than the open ditch method. These experiments led to the installation of a line of porous tile duct in the earth above the conduit, surrounded with clean sand. The leakage of water through the pores of this duct has been found very effective in reducing the temperature of the cables. Whenever the temperature of the cables is found by exploring with a resistance thermometer to approach the danger point, water is turned into the porous drain tile, and the temperature is taken on successive days to see whether the desired reduction has been obtained.