New technology developed to help detect arc-faults
By Joe Escobar
The average age of aircraft in today's fleets is increasing. There are numerous issues pertaining to aging aircraft - one of them being the condition of the wiring. Aircraft wiring can be subjected to moisture, vibration, chafing, and changes in temperature. While other parts of the aircraft are routinely inspected or rebuilt, the wiring tends to remain untouched. Over time, the physical stresses that they are subjected to as well as intrusive inspection methods contribute to the deterioration of the wiring and can set up an ideal situation for electrical arc faults. New technologies are being developed to help reduce the danger of fire caused by the arcing events.
Circuit breaker operation
Conventional circuit breakers work by a mechanical means. Current flowing through the breaker heats a bi-metallic current sensor, causing it to bend. When the amount of this bending exceeds a limit, an armature is released and a spring forces the contacts to open. The load current also flows through a magnetic current sensor that creates a magnetic field that trips the armature faster than the bi-metal strip can respond when very large currents flow through the breaker.
Conventional aircraft circuit breakers, like those used in residential applications, are designed for thermal, or bolted faults. Bolted faults occur when two wires touch each other solidly creating a classic short circuit. In these type faults, the breaker works just fine. When the bi-metallic strips heat up to the specified temperature, the breaker trips.
The problem with conventional circuit breakers pertains to their inability to detect arcing faults. Arcing faults are caused by breaks in wire insulation where microscopic cracks, abrasions, or broken insulation occur. They can also exist when wire is improperly installed or maintained. Because these arcing faults occur so quickly and cause little thermal impact on the bi-metallic element of a conventional circuit breaker, the circuit breakers are not able to detect the problem and break the electrical flow. This can allow the arcing event to continue to occur unnoticed.
Although arcing faults can occur very quickly, their potential for causing significant damage is extreme. The temperatures at the arcing point can reach as high as 6,000 degrees Centigrade - hotter than the surface of the sun. This creates definite potential for fire.
Smart circuit breakersto the rescue
Several companies are working on next generation 'smart' circuit breakers that are able to detect dangerous arcing faults in wiring. The Eaton Corporation and Hendry Telephone Company (both of which have produced industrial arc-fault circuit protection) were both awarded contracts by the Department of the Navy in December 1999 to develop arc fault circuit breakers for aircraft applications. In addition, Phoenix Aviation and Technology has licensed the E-T-A Group to use its technology in the joint development of an intelligent circuit breaker for aircraft.
How they work
Smart circuit breakers incorporate microprocessor-based wire diagnostics within the breaker. These diagnostic boards are able to predict and locate potentially catastrophic arcing in wiring bundles before any short circuits occur. It's like having a miniature computer in the circuit breaker.
In one example of this technology, Eaton's Arc Fault Circuit Interrupter (AFCI) technology utilizes micro-electronics to monitor and analyze a circuit's current waveform. It uses algorithms to look directly for the randomness in the 400Hz current signal to determine if an arcing event in the circuit is occurring. If it determines that an arc fault exists, a signal is sent to the circuit protection device which initiates a sequence that will safely shut down the circuit in question.
These algorithms are also able to prevent nuisance tripping by distinguishing between actual arc faults and normal 'good' arcs such as starting a pump or other electrically driven component. The algorithms can identify the degree of chaos or randomness in the current signal, thereby preventing the chance of nuisance tripping.
There are several added benefits to AFCI circuit breakers:
•They are the same size as current aerospace circuit breakers listed in MS24571 and MS14015.
•There is a separate visible indication of an arc fault versus an over-current fault.
• If the AFCI electronics fail, the breaker will never provide any less protection than a conventional breaker.
• Expected power draw is less than 7 milli-amps per breaker.
With all of the benefits these circuit breakers offer, many aircraft fleets in the near future could be using them. It is a promising technology that can improve the safety of the aircraft we work on.