An evaluation of diagnostic techniques relevant to arcing fault current interrupters for direct current power systems in future aircraft

Arc fault circuit interrupters (AFCI) are a relatively new development in the area of circuit protection. Typical circuit breakers are designed to detect an overload current in the protected circuit and trip open within a specified period of time. The AFCI is a device that detects and interrupts an arcing fault when it occurs. An arc fault is a dangerous situation with very high temperatures in the arc. These high temperatures are sufficient to ignite wire insulation and other combustibles in the vicinity of the arc. Standard circuit breakers and ground fault circuit breakers generally will not trip in the event of an arc fault since there is sufficiently high impedance in the circuit to limit current below the trip level of its characteristic curve. Numerous arc faults that have been observed on aging aircraft, due to failing insulation, have spurred an interest in AFCIs for both commercial and military aircraft. Such AFCIs assess the AC current waveform to distinguish that an arc is occurring in the circuit. When this occurs, the AFCI will open to mitigate damage to the wiring system. In the AC case, the opening device will generally take advantage of the natural current zero in the AC wave form to clear the circuit. Protective devices for DC systems cannot exploit this characteristic. In addition, the charge transport properties (conductivity) of the DC arc are affected by the gas density (i.e., altitude) surrounding the arc. This paper presents experimental results intended to help evaluate the potential applicability of the typical diagnostics used in AC AFCIs as a valid means of detecting a series DC arc. This is particularly relevant to future aircraft power systems employing 270 Volt DC distribution. Also, these DC arcs have been characterized at low atmospheric pressures to simulate high altitude operation