Control and protection of aircraft D-C power systems

D-C POWER SYSTEMS on transport aircraft before World War II were small in capacity as compared to the present requirements. Present wide speed-range generators are 12 to 15 times the ratings used on old DC-3´s, and system capacity on new transports may be 30 times greater and capable of sustaining fault currents greater than 5,000 amperes at 30 volts. It is also necessary to consider the increase in the voltage from the smaller system to the larger system. The 12-volt system does not have sufficient potential to maintain an arc or ionize gases so that an arc could be maintained for any length of time, whereas, the 24-volt system does have sufficient potential to maintain an arc or ionize gases. This is a fundamental characteristic of the electric circuit and is quite often forgotten about when voltages greater than the critical arcing values are used. Arcing voltages are in the neighborhood of 18 to 20 volts, depending upon the materials and atmospheric pressures at the electrodes involved. The number of electrically operated devices also has been increased, and many of the electrical requirements which are vital to flight operation are therefore dependent on the continuity of electric power. Study of various means of controlling and protecting electric power systems against failures in the light of these facts has led to some interesting simple developments which are described in this paper.