Abstract: Aircraft have two primary sources of electrical energy. The first is the generator, which converts mechanical energy to electrical energy. The second is the battery, which converts chemical energy to electrical energy. The generator is the main source and the battery is the auxiliary source. In aircraft, the electrical system is primarily an AC system. Aircraft electrical components operate on many different voltages both AC and DC. However, most of the systems are 115 VAC with 400 Hz, 28 VDC and 26 VAC is also used in aircraft for lighting. DC is also supplied from a battery installation. The battery provides 28 VDC. The function of the electrical system is to generate, regulate and distribute electrical power throughout the aircraft. New generation aircraft rely heavily on electrical power because of the wide use of electronic flight instrument systems. For aircraft constant frequency systems 400 Hz has been adopted as the standard. The application of 400 Hz frequency, which is higher than usual frequencies, offer several advantages over 60 Hz - notably in allowing smaller, lighter power supplies to be used for aircraft operations and computer applications.