Load ratings of cable — II

Since the author's 1939 paper,
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further data have been obtained regarding the loading of underground power systems having impregnated-paper-insulated lead-covered underground cable. The results of the studies may be summarized as follows: 1. The factors affecting the setting of maximum safe conductor temperatures are so numerous that no fixed value (or values) can be assumed as applicable to all installations of a given design and size of cable in this country. 2. For some cables, the present temperature limits for normal day-in and day-out operation may be safely exceeded, especially for wartime conditions. 3. For emergency operation safe temperatures may be even higher than listed in the previous paper, especially for wartime operation. 4. Operation in wartimes at special temperatures will mean in some cases substantial shortening of the life of underground circuits and an accompanying increase in service interruptions. 5. Wartime increases in usual maximum daily loading and in load factors may cause large increases in duct and copper temperatures, even if past current ratings are maintained. These temperatures will sometimes exceed those in the present standards. 6. The safe temperature for emergency operation, particularly for extra-high-voltage cables, may be limited by the joints. Also, for all kinds of cable sufficient room must be provided to avoid mechanical damage of cable or joints in manholes with cable movements incidental to emergency loads. 7. Cracking of lead sheaths due to reciprocating cable movement into manholes may limit the temperature range for usual daily loading, but has little effect on the safe emergency loading. 8. For three-conductor solid-type cable, the insulation of the shielded type can safely withstand higher temperatures than the belted type, but the reverse is true as to the allowable daily temperature range with regard to its effect on the sheath in manholes. 9. Cable movement increases with length of conduit sections up to about 250 feet but shows little change with further increases in length up to 1,025 feet. 10. Changes in installation methods and the use of new types of repairs may help to mitigate troubles due to sheath cracking. 11. Copper shielding tape in three-conductor cable with relatively thin insulation has little effect in reducing the thermal drop from the conductor to the sheath. 12. In some cases considerable thermal advantage may be gained economically by making relatively deep installations of conduit. 13. In most cases, the maximum conduit temperature of 50 degrees centigrade given in the previous paper may be safely exceeded.