A-C and D-C Short-Circuit Tests on Aircraft Cable

Higher-voltage d-c and 400-cycle a-c electric systems are being developed for large aircraft with increased electrical load in order to obtain lower conductor weights than would be required with the present 28.5-volt system. In view of the apparent freedom from sustained faults in low-voltage d-c systems despite damage from gunfire, the use of protective devices to isolate faulted cables has been considered unnecessary. The need for protective devices on the higher-voltage systems will depend on the relative behavior of faults and the required clearing time. A laboratory investigation based on four representative types of cable fault was made to determine (1) reasons for the apparent freedom from sustained faults in present 28.5-volt systems, and (2) comparative fault-clearing ability on the higher-voltage systems. The tests show (1) that freedom from sustained faults on 28.5-volt systems can be attributed mainly to fraying of faulted cables and the high level of fault current at most locations, and (2) that ability to clear is not materially different at the higher voltages if as much fault current is available. Current levels were determined, as a function of cable size, above which faults caused by gunfire can be expected to burn clear without serious fire hazard. Where available fault currents are below these values protective devices may be required to eliminate fire hazard. For 400-cycle systems protective devices may be required to clear faults with minimum generation in time to prevent loss of synchronism or stalling of motors.