New invention detects intermittent electrical faults

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New invention detects intermittent electrical faults

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June 21, 2006 An intermittent fault is the most frustrating problem of all. Most of us who play with Gizmos have experienced it at least once, and some of us in specialist jobs have experienced it many times. One can only imagine the feelings of futility that technicians experience when trying to find an intermittent short circuit in the miles of wiring behind the panels of an aging commercial airliner – or the cost of tracking the potentially dangerous short circuit down. Now Sandia National Laboratories has invented a new rapid technique that may make it financially feasible for airlines to quickly diagnose and repair the hard-to-locate intermittent faults that have plagued the industry and cost millions of dollars in lost revenue due to aircraft downtime. Pulsed Arrested Spark Discharge (PASD) is expected to be on the market by September via licensee Astronics Advanced Electronic Systems, combined with that company’s other patented test methods under the name ArcSafe. Other possible uses eventually envisioned for PASD are as inexpensive tests for the wiring harnesses of passenger cars and new homes. Military tanks and the hard-to-reach wiring behind the steel bulkheads of submarines are also possible candidates.

The commercial product is about the size of a small suitcase. It can be plugged into aircraft-installed wire harnesses, 40 wires at a time, to check for the very small insulation breaks associated with intermittent faults. The challenge to engineers has been how to locate an intermittent wiring fault before — not after — it becomes a problem. Intermittent electrical short circuits in aging commercial airliners range from the trivial to the deadly. They can make cabin lights blink, air conditioning falter, or even cause fatal crashes, as with flights SwissAir 111 or TWA 800.

PASD’s trick is to make the short circuit manifest before it normally would and to do so on the ground so that technicians can fix it. To achieve this, the Sandia method sends a nanosecond pulse of electricity, fiercely propelled by a high voltage, along airplane wiring bundles. The tiny pulse is powerfully driven so that it can jump gaps in slightly frayed insulation but has so little energy that it is harmless.

The process resembles sending a brief trickle of water over a high waterfall. The fluid will certainly fall a long ways very fast, but the tiny amount arriving at the bottom isn’t enough to do damage.

Because the voltage is higher than that normally used in airplanes, the electrical pulse will jump like a rabbit from the smallest wiring insulation fault (which to ordinary instrumentation seems undamaged) either to the bulkhead or to another nearby damaged wire. That spark — like static electricity leaping from hand to doorknob — in effect lights up the invisibly damaged spot like a tracer bullet at night lights up a target. The amount of time it takes for the current to return to its source is analyzed by the automated test-set to tell within inches how far the break is from the test entry point.

“Rather than reacting to a problem, these systems can find a fault before it manifests into a catastrophic event,” says Sandia team leader Larry Schneider. “Rather than ripping apart the fuselage for access to a faulty harness that may run the length of the plane, airline mechanics will be able to use this new tool to efficiently locate and repair the intermittent fault.”

Sporadic short circuits occur where two exposed conductors, or a conductor and aircraft frame, make temporary contact during flight. Vibrations caused by turbulence may cause wires to touch, interrupting power to sensitive electronics and possibly damaging wires. These conditions are tricky to diagnose when the aircraft is stationary (on the ground) because the shorting wires often have shifted back to a non-shorted state. Sometimes these breaks can barely be seen by the naked eye because missing insulation may be the size of a pinhole, or nearly invisible like a fine cut from a razor blade. Traditional wire-test systems have great difficulty finding these faults.

To overcome myriad problems initially besetting the promising technique, the National Nuclear Security Administration and the U.S. Navy supported the research, followed by the Federal Aviation Administration, to the tune of about $2 million. It took two years for Astronics to adapt it to its suite of tools, which were developed over four years of research to locate wire breaches with the potential for electrical shorting.

Says Astronics team leader Mike Ballas, “We really value PASD technology. We licensed it, turned it into a practical portable test unit targeted for the aviation industry to find intermittent faults, and we believe it’s the best way now to do the job. It’s a nice complement to our patented technology.”

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