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SR 111 Investigation Report

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2.13.3  Circuit Breaker Maintenance

As a result, in part, of the CB's inherent reliability, preventive maintenance is rare and largely confined to a general visual inspection and cleaning, as required. When a CB experiences a failure, it is typically confined to one of two modes: the CB either exhibits nuisance trips, or it fails to trip when exposed to an over-current condition. Maintenance action in either case is usually confined to replacement of the faulty CB. While both failure modes are undesirable, the failure of a CB to trip leaves the associated wire or cable unprotected. Analysis of the failed CBs has revealed that, in some instances, long periods of inactivity can cause the CB's trip characteristics to change with time.[118]

According to both the FAA and the Society of Automotive Engineers, this CB aging phenomenon can be prevented by the periodic cycling of the CB mechanism. Despite such recommendations, aircraft maintenance programs do not typically include a requirement to "exercise" CBs on a periodic basis. In its recently published bulletin on the issue of resetting tripped CBs, the FAA made no mention of the adequacy of the various operators' CB preventive maintenance programs. Addressing the practice of periodic CB cycling would help those responsible for CB maintenance programs to ensure that their programs are consistent and optimized to provide maximum CB reliability. There is a need for the aviation industry to identify a "best practices" approach to CB maintenance and to ensure that CB maintenance programs are designed appropriately.

[118]    Society of Automotive Engineers Aerospace Recommended Practice 1199 Rev. B, para 5.7.9.

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Updated: 2003-03-27

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Important Notices