TWA 800. Swissair 111.
The unfortunate roll call of recent aircraft accidents blamed on explosions or fires suspected to have been triggered by electrical wiring arcing is all too familiar. And the potential for additional incidents may be even more sobering. Safety reports show numerous, non-fatal incidents of smoke in the cockpit and electrical faults attributed to wire arcing—a phenomenon that cannot be arrested by current circuit breaker technology.
Navy statistics show 64 inflight electrical fires between July 1995 and December 1997. "Of those, 80 to 90% would have been prevented by arc-fault circuit protection," according to Charles Singer, lead project engineer for arc-fault circuit-breaker development with the Naval Air Systems Command (NAVAIR, Patuxent River, MD). On the civil side, Federal Aviation Administration (FAA) data from 1980 through July 1998 show 622 reports of smoke in the cockpit or cabin.
In less than 0.1 sec, arc-faulting damages this 115V, 400 Hz aircraft wiring.
But work underway at Eaton Corp's Aerospace Controls Division (Sarasota, FL) and Hendry Telephone Products (Goleta, CA) could change that. Both companies are working under a NAVAIR and FAA contract to develop arc-fault circuit breakers by 2002. They will be demonstrating prototype units later this year. The companies will furnish arc-fault circuit breakers for flight testing at the end of their individual contracts, Eaton in December 2001 and Hendry a year later. The program is also working with the SAE AE81 committee on circuit protection so that a commercial specification also comes out of the program in 2002, allowing other companies to compete for supplying future military and commercial aircraft needs.
Conventional circuit breakers are more like resettable fuses. They are based on the effect of a current surge in a resistance generating heat, as occurs with a completely shorted circuit, to deflect a bi-metallic element, tripping a spring-loaded latch. "Arc faults don't trip such thermal breakers since their root-mean-square value is not enough," says Jim McCormick, engineer and new product manager for Eaton Corp. He notes that arc faults typically last under 0.1 seconds and may jump between wires in a bundle or between a wire and aircraft structure.
Electromagnetic protection is not normally used on aircraft because of potential nuisance tripping during load start ups and variable power quality. There is also the possibility of EMI taking a circuit offline.
Nature of the beast. Many aircraft are being flown beyond the lifetimes that their designers envisioned. Thus, factors such as mechanical stresses on electrical wiring (from repeated loads, maintenance, or accidental damage), insulation degradation (due to environmental factors), and thermal cycling all contribute towards greater potential for aircraft wiring arcing. While inspection can detect damage, most wiring is not in direct view, as with the wires within a bundle.
New materials can mitigate such damage effects, but the use of such modern wiring is most cost effective in new aircraft installations. Existing air fleets need a solution that works but offers a more economical option to implement than complete retrofitting. And even new, robust insulation, if subject to mechanical damage or very long-term degradation, could benefit from some means of backup protection.
Both Hendry and Eaton have previous experience in arc-fault protection for residential and commercial use. In fact, under new National Electric Code (NEC) regulations, arc-fault circuit breakers will be required for bedrooms in all new home construction starting in 2002. And the White House has set up a research group to investigate aging wiring issues across the electrical infrastructure.
Challenges. According to NAVAIR's Singer, the biggest design hurdles include reliable detection of arc faults without false tripping. Aircraft electrical systems are subject to numerous normal transients as equipment is brought on and off line, which may trigger false activation of an arc-fault circuit breaker. And particularly in military use, there may be a high EMI environment from other systems or adjacent aircraft.
Conventional circuit breakers rely on bimetallic elements that trip due to ohmic heating (top). Arc-fault interrupters must work at lower power levels without false activations, and thus depend on processing algorithms to match arcing event waveforms to trigger a circuit interruption.
As one part of the effort, NAVAIR determined normal current waveforms for various aircraft systems and conditions and furnished these to Eaton and Hendry to add to their existing corporate databases. Eaton's McCormick says microelectronics in the arc fault circuit interrupter "will look at signatures—levels and timing—of the current waveform." Michael Walz, electronics senior engineer at Eaton, adds, "The signature analysis algorithms use frequency and time domain filtering to extract an arcing-fault signature characteristic. The characteristics are then examined to determine the magnitude and duration of the fault and decide if the circuit breaker should be opened." If the logic circuit determines an arc fault exists (which is also done by integrating the signature over time to discriminate between normal currents and a sputtering arc), a signal is sent to trip the circuit breaker built into the device—all while the overall energy still is below the normal trip level of the breaker for thermal (short circuit) overloads. McCormick highlights the company-developed algorithms for doing this and is quick to point out that should the arc-fault electronics fail, the protection level would drop to that which currently exists with thermal breakers.
The next challenge is device size. While arc-fault protection does exist commercially, engineers are faced with packing electronics and bi-metallics into the smaller available volumes in aircraft wiring panels (see figure). At Eaton, designers cut volume by resorting to helically wrapping the thermal trip element. This has been superseded by more of an M-shaped bi-metallic. Hendry could not be reached for specifics on its design approach.
Singer notes that the aircraft environment produces some challenging operational conditions. Aircraft circuits operate at higher frequency (400 vs. 60 Hz) than domestic wiring, which produces a higher dielectric heating rate in wiring. There are also dips and surges that no utility usually sees. The operational environment can range from -55 to 120C, whereas home protection only has to function from 0 to 40C.
McCormick, of Eaton, adds that making the devices low cost is also a goal. One example: Eaton is looking at metal injection molding to control part geometries, and reduce machining costs and parts count. He adds that other issues being investigated include: how do you tell if a trip is from an arc fault or thermal trip; device resetting; powering breaker electronics; and communicating fault occurrence.
While electronics can thwart arcing from wires, new insulation materials can form a first line of defense, not only by reducing arcing occurrences but by inherently strengthening aircraft wiring as well. Toward this end, W.L. Gore & Associates (Newark, DL) has developed High Strength Toughened Fluoropolymer (HSTF) insulation.
According to Product Specialist Paul Hubis, HSTF doesn't have the creep (cold flow), abrasion (cut-through), or hydrolysis (water absorption) typical of PTFE insulation. He says tensile strength is about 20 kpsi, up from 3-7 kpsi of other fluoropolymers. "Cold flow governs the separation of wire insulation under pressure," Hubis notes, which is present in crossing wires in bundles, leading to flaking and cracking—eventually reducing separation between conductors, some of which may be at appreciably higher voltages. Hydrolysis is particularly a problem in naval applications where salt water fog and high temperatures can accelerate insulation breakdown. The highly inert HSTF can be used from -40 to 260C.
Test results furnished by Gore show needle-abrasion test resistance of HSTF-jacketed 20 gauge wiring nearly 20 times that of conventional fluoropolymer. Also, creep testing at room temperature using a 4-lb load on a 0.060-inch blade placed on wire with 0.004 inch of insulation resulted in a virtually instant contact of the blade with the core wire for PTFE jacketed wire. For the comparable HSTF insulated wire, the blade took about seven days to reach about 75% of the insulation depth before the test was stopped.