Testing high-tension impregnated-paper-insulated, lead-covered cable

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The increase in voltage of cables has necessitated that the tests to assure satisfactory cable be more adequate than as standardized at present. This has resulted in an intensive study of the tests previously standardized, development of new tests, and the design and manufacture of suitable testing equipment to meet the new testing requirements. Measurements are made upon cables to determine the following properties of the insulation: Insulation Resistance Dielectric Strength Dielectric Power Loss and Power Factor Capacitance Ability to Withstand Bending Insulation Resistance: This measurement is being made in the same way on cables of all voltage rating. The results of the test on high-tension cables are of doubtful value as a criterion of the suitability of cable for use. Continued study of this measurement should be made. Dielectric Strength: Suitable testing equipment for satisfying the requirements of the increased voltages in dielectric strength tests has been made available. This includes sine-wave generators, adequate testing transformers, appropriate cable testing terminals. Data is given from which conclusions are drawn as to the magnitude and duration of test voltages. The adequacy of these values will become known through experience. The need for field testing is shown. Dielectric Power Loss and Power Factor: The tendency is to extend the measurement of dielectric power factor to include each reel length to be shipped. The Schering Bridge for making such measurements is described. The need for standardizing the testing procedure for power factor measurements is shown. Testing Installed Cable: The study of so-called “current-time curves” for rating installed cable should be continued. Preliminary measurements made at high frequencies as a means of rating installed cable did not show the results to be immediately usable. Testing With Direct Current: Data is given to show the d-c. to a-c.-ratio of breakdown voltage of some samples of 12 kv.-3-conductor. cable. Tests indicate that the d-c.-to a-c.-ratio will depend upon many conditions such as nature and structure of the material, thickness of the material, temperature of the material, shape and size of electrodes, and rate of application of the applied potential.