An improved method of locating small breaches in insulation on electrical wires combines aspects of the prior dielectric withstand voltage (DWV) and time-domain reflectometry (TDR) methods. The method was invented to satisfy a need for reliably and quickly locating insulation defects in spacecraft, aircraft, ships, and other complex systems that contain large amounts of wiring, much of it enclosed in structures that make it difficult to inspect.
In the DWV method, one applies a predetermined potential (usually 1.5 kV DC) to the wiring and notes whether the voltage causes any arcing between the wiring and ground. The DWV method does not provide an indication of the location of the defect (unless, in an exceptional case, the arc happens to be visible). In addition, if there is no electrically conductive component at ground potential within about 0.010 in. (˜ 0.254 mm) of the wire at the location of an insulation defect, then the DWV method does not provide an indication of the defect. Moreover, one does not have the option to raise the potential in an effort to increase the detectability of such a defect because doing so can harm previously undamaged insulation.
In the TDR method as practiced heretofore, one applies a pulse of electricity having an amplitude of <25 V to a wire and measures the round-trip travel time for the reflection of the pulse from a defect. The distance along the wire from the point of application of the pulse to the defect is then calculated as the product of half the round-trip travel time and the characteristic speed of a propagation of an electromagnetic signal in the wire. While the TDR method as practiced heretofore can be used to locate a short or open circuit, it does not ordinarily enable one to locate a small breach in insulation because the pulse voltage is too low to cause arcing and thus too low to induce an impedance discontinuity large enough to generate a measurable reflection.
The present improved method overcomes the weaknesses of both the prior DWV and the prior TDR method. One prepares the system to be tested by filling all or part of the system with a liquid or gas that does not harm the wiring and that is either electrically conductive or undergoes dielectric breakdown (and thereby becomes electrically conductive) at a relatively low applied electric field. For example, if the system to be tested is an aircraft, one can fill the interior of the aircraft with neon, through which arcs can readily develop between wires and metal grounds. This permits arcing to a ground as far as 1.0 in. (≈25.4 mm) from the conductor.
The figure depicts two typical alternative assemblies of equipment that could be used to implement the present method, along with three typical alternative voltage waveforms that could be used in the method. Once the system to be tested has been prepared as described in the preceding paragraph, one of these waveforms is applied to a wire under test. In the case of the first waveform, one superimposes a conventional TDR signal on a gradually increasing voltage until arcing occurs. To make the arcing occur at the identifiable time of one of the TDR pulses (preventing the somewhat random arcing that might otherwise occur) and thereby make it possible to measure the round-trip travel time, (1) the rate of the interval between the TDR pulses is made long enough to encompass any reflections that might occur and (2) the rate of gradual increase of voltage is made such that highest voltage yet reached occurs at the peak of each superimposed TDR pulse.
The second voltage waveform is a staircase function. In this case, the highest voltage yet reached (and thus arcing) always occurs at a rising edge. The third waveform consists solely of TDR pulses, but unlike in conventional TDR, these are high-voltage pulses. In this case, the amplitude of the pulses is increased gradually until they cause arcing.
This work was done by Owen R. Greulich of Ames Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category. Inquiries concerning rights for the commercial use of this invention should be addressed to
the Patent Counsel, Ames Research Center, (650) 604-5104.
Refer to ARC-14612.