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A. Dry Arcs A dry arc test using a guillotine produces a short circuit that may last only a few milliseconds before the prototype SCB is designed to trip. An example of this is seen in Fig. 4, which shows the STDR trace at the exact time the short occurs, with the short clearly visible at 120 ft. The strobe light is the load. The system is not impacted by the noise from the strobe light,and no visible change in the circuit occurred because of running the STDR. Similar results are seen for fluorescent lights and the landing gear light and motor. The reflection is only about 30% of what would be expected for a complete short circuit, most likely because the guillotine (which does not cut all the way through the wires) does not create a complete short. Fig. 5 shows the method that is used to identify the time that the fault occurs, so that all of the data does not need to be analyzed. The trace that is shown in Fig. 4 is a single time step of data. The RMS of this time step is taken and plotted as point 172 (this is the 172nd time step) along with the RMS of all of the other time steps. Each time step requires about 20 ms to store, due to the slow acquisition speed of the parallel port. When implemented on aircraft, this acquisition time could be shortened considerably. The system maintains a “rolling window” that is 200 time steps long. The first time step is taken to be the baseline (averaging several early steps can be used to improve this baseline, but this has not been found to be critical). Subsequent time steps are compared to the baseline and stored only if they are sufficiently different (10% in our case) from the baseline. When the arc occurs, there is a significant change in the signal, indicating an event worth noting. The algorithm for location ofa short circuit can then be as simple as searching for the maximum negative peak when the significant event occurs. Fig. 6 shows similar results when the load is a retractable landing light with its retraction motor. The short circuit is clearly seen at 120 ft, and the change in RMS indicates the fault event. These two plots are good examples of arcs identified and located by the STDR method. Unfortunately, the data collection system (utilizing the parallel port on a PC, and requiring about 20 ms) was not fast enough to catch all of the arcs we tested before current was removed by the SCB (which tripped in about 5 ms in many cases). |
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