Simulation of AC Arc Faults in Aircraft Electrical Networks — Critical Loads — Critical Ignition Energies

Abstract

The problem of arc tracking in cable bundles of aircraft electrical networks is known for more than 20 years now. Recently this phenomenon increasingly became a focus of the public as examinations of several aircraft accidents did not exclude the possibility of cable arc tracking as a possible reason. Conventional aircraft circuit breakers are not capable to detect this fault, either due to short current flows or too low amplitudes. Thus, for quite some time now, the industry strengthens their efforts to develop equipment in order to detect and eliminate arc faults prior to surrounding material catching fire. A test equipment providing a better and quicker optimisation of algorithms detecting arc fault signals by means of real and synthetic load signals is described. Arc fault signals can be simulated to test AFCIs (Arc Fault Circuit Interrupters) and real arc fault tests at different frequencies may be performed. Tests to determine the ignition energies of different cable insulation materials and thermal insulation materials have been carried out. The electrical energy dissipated at an arc can be described by the following formula: W(t) = ∫ b a ub(t) . i(t) dt = Rb . ∫ b a i(t) 2 dt Where ub is the arc voltage, Rb is the arc resistance and i(t) is the arc current. The integration boundaries are determined by the time of current flow. The right side expression of the equitation is valid supposing a constant arc resistance, which is a simplifying assumption neglecting the non-linear behaviour during ignition and extinction phase. An AFCI is only able to detect the arc current directly, but cannot detect the arc voltage. Thus only the so called i2t-value, which is proportional to the energy, can be used. If and how this value could be correlated to the energy dissipated at the location of an arc fault is described in this paper as well as the question is discussed, how this value might be used for arc fault detection.