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Circuit breakers (CB's)


There are two common misbeliefs associated with aircraft circuit breakers: One relates to resetting a tripped CB, the other refers to why the CB is there in the first place.

Resetting a tripped CB

What do you do, when during the pre-flight you notice, that one CB is “popped out”?

What do you do, if the same happens in flight?

A good operator will provide you with guidance on both aspects. If there is no such guidance, my personal recommendation for the pre-flight case is to check the tech-log and to advise a licensed maintenance person. You should never reset a CB (even on ground), unless you have clearly identified the cause for it’s tripping. A quick glance at the associated equipment is not a very thorough check. The worst thing you can do, is pushing it in and take off shortly afterwards…

A tripped CB should not be taken lightly and usually has a story behind it…

As a reminder, the typical failure rate of an aerospace CB for “undue tripping” (tripping when no overload condition exists), lies in the order of 1E-6/hr [5]. If you do not like math, ask yourself: How many times did you win the lottery?

In flight…

While it was common thinking in the 70’s and 80’s that one reset is "ok", today’s approach is different. Both the EASA and FAA have published guidance material for flight crews on how to deal with tripped circuit breakers.

Bottom line is: Leave them alone!

After several incidents and accidents involving different aircraft types and operating environments, the airworthiness authorities clearly recommend to refrain from resetting tripped CB’s unless the AFM would require such an action or there is simply no other option in order to ensure flight safety [2] [3].

Purpose of the CB

According to a common misbelief, a CB is there to protect the "end-equipment" that is connected to it. The main purpose is rather to prevent overload damage to the electrical system (power distribution) and to avoid fire hazard caused by overheating wiring [6]. It is quite important to realize, that the selection and sizing of a circuit breaker are primarily governed by the expected load and the associated WIRING amongst other factors, such as bundle layout and altitude variation [1]. This underlines the fact that “looking at the equipment” when investigating the cause for a tripped CB can be absolutely pointless, as the failure could easily be located somewhere in the wiring, and very hard to find indeed.

Another comment referring to the purpose of CB’s: They are designed to work as a protective device, and should not be used as a “switch-replacement” [1] [4].

So think twice before you touch a CB...

Rev/20181007

References

[1] SAE ARP1199 Rev B

[2] EASA SIB 2009-07 https://ad.easa.europa.eu/blob/SIB_200907.pdf/SIB_2009-07_1

[3] FAA SAIB CE-10-11 https://www.faa.gov/aircraft/safety/alerts/SAIB/

[4] CAA VECTOR, Nov/Dec 2001

[5] MIL-HDBK-217F

[6] EASA CS-25 amdt 21

#avionics #EWIS #circuitbreaker #electrics

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