Cycling Circuit Breakers Can Improve Safety

Air Safety Week, August 23, 2004

Operators are being encouraged to periodically cycle the circuit breakers on their aircraft. The initiative applies to thermally activated breakers, which function mechanically to protect a circuit from overheating and from arcing damage.

New arc fault circuit breakers (AFCBs) in development will be electronically rather than mechanically activated. The quicker reaction time of AFCBs provides an additional level of protection against overcurrent conditions. In the meantime, with traditional circuit breakers (CBs) installed, and aging in service, researchers have found that their proper functioning can be aided by periodically cycling the mechanism with the power off.

Recent research on age-related degradation has shown that CBs generally hold up well in service, but that periodic cycling of the breakers can help ensure their proper functioning (see ASW, Feb. 10, 2003).

The findings stem from research conducted at the Federal Aviation Administration's William J. Hughes Technical Center in Atlantic City, N.J. More than 300 CBs from high-time aircraft were evaluated. Researchers found that the breakers can trap debris, or corrosion can impede the mechanical functioning of the breaker. As a result, the FAA's report on aging circuit breakers contained a number of recommendations, to include cycling the breakers off and on yearly.

In concert with these research findings, the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC) has recommended periodic cycling of the breakers and that aircraft manufacturers should encourage their customers to do so. However, ATSRAC has recommended cycling at an interval greater than once yearly to more closely accord with operators' heavy maintenance schedules. Airbus is the first to produce guidance emanating from the ATSRAC recommendation. Its June 28 service information letter (SIL) urges operators to cycle their breakers within the next 24 months, and to repeat the process every 36 months thereafter. Boeing [BA] plans to issue a comparable SIL in early 2005 to coincide with the next scheduled update of its maintenance manuals.

Manufacturer Dassault is expected to issue a SIL recommending cycling of the breakers in September or early October.

While it appears that the FAA will encourage other aircraft manufacturers to follow suit, it is not clear whether the FAA will require that such guidance be issued, or that compliance will be made mandatory.

Michael Walz, manager of the FAA Tech Center's aging aircraft electrical systems program, said there are a number of advantages to the program as outlined by Airbus, and in other manufacturers' SILs to come:

* The program has been designed such that the aircraft electrical system does not have to be turned on.

* The cycling will exercise the mechanical innards of the CBs.

* With maintenance technicians physically cycling the breakers, malfunctions are more likely to be identified. This is an important point. Recall the FAA's Aircraft Wiring Practices Job Aid, another outcome of the ATSRAC examination of aircraft electrical systems. This document was produced to help prevent maintenance malpractice regarding electrical wiring (see ASW, July 19). This document also discusses CBs, cautioning that "circuit breaker failures are, for the most part, latent in nature." The Job Aid warns, "So you won't know they have failed until you need them." The implications are potentially dire, reinforcing the merit of cycling the CBs periodically to spot those lurking latent failures.

There is one disadvantage to the CB cycling initiative. "The technicians would have to get into areas of the airplane that they don't normally," Walz explained. That activity creates the potential for inadvertent damage to wiring and connectors. However, on balance, Walz believes the CB cycling effort represents a net benefit for electrical system safety.

'Fused Bombs'

Extrapolate the Job Aid statement that most breaker failures are latent by the large number of CBs in an airplane (not all visible on panels, and some hidden away in nooks and crannies or in modular components themselves) and one can see that by design some "fused bombs" are aboard - in the form of CBs that won't trip when and if required.

The consequence of that failure to function will be, by definition, circuit overload and smoke/fire/system failure.

'Cycle all Breakers'

Maintenance Manual Recommendations, from the Aging Circuit Breaker Study (extracts):

* Provide instruction to cycle all breakers off and on once a year.

* Provide instruction to examine all [back] panels for loose, broken, misapplied circuit breaker wire termination hardware, and require replacement with the correct hardware.

* Provide clear instructions on how to avoid cross-threading screws or thread-stripping breaker terminals and require complete replacement of the breaker when it occurs.

* Provide instruction on how to inspect for overheating and electrical arcing as well as defining what repairs are needed to minimize hotspots and prevent future occurrences.