Thursday, August 24, 2000
Engineers use chips, software, and materials to combat aircraft
fires due to arcing wires
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.
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
on 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. Aerospace Controls Division (Sarasota,
FL). 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 (see sidebar), 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.
In less than 0.1 dec, arc-faulting damages this 115V, 400 Hz
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
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.
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, 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. 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.
In concluding, Singer notes that the program is also working with
aircraft maintainers and the Air Line Pilots Association for input on locating
and using the arc-fault breakers "to avoid any operational problems
Strengthening the shield
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 on the order of 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 (see figure). 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 (see chart).