what’s the different with alt circuit breakers between 5a vs 55a??

If alt field circuit breaker pops out does that mean alternator is not producing electricity? And if al current circuit breaker(55a) pops out does that mean alternator is producing electricity but they are just not carrying electricity to bus?

Am I correct??

  • $\begingroup$ please scroll down to "Advanced Regulating Systems Designs" in the reference for more info on the Voltage Regulator. $\endgroup$ Jan 31, 2022 at 9:06

1 Answer 1


Pretty good reference here on aircraft alternator systems. It appears the field curcuit breaker (5 amp) goes from the bus bar to the alternator to get it to start working. Power into the bus bar is initially supplied by the battery.

Once turning, the alternator can supply all electricity needs of the aircraft, charge the battery, and keep itself running.

The alternator to bus bar breaker (60 amp) would open if excessive current was drawn from the alternator. 60 amp short curcuit would have to be a fairly catastrophic event.

  • $\begingroup$ Thanks for reference. As far as I understand, 5a circuit breaker acts like a overvoltage prevention. Right? when bus require more than 5a to regulator, 5a circuit breaker pops out and disconnect connection between bus and regulator which result in like shutting off Alt master switch. Am I correct?? $\endgroup$
    – Jimmy
    Jan 30, 2022 at 17:50
  • $\begingroup$ @Jimmy. It seems to be on the same circuit as Alt master switch, but the bus provides current, something down the line is drawing too many amps. If this is happening, I would consult a mechanic right away. I believe the 172 can keep running on its magneto even if both alt and batt are off, but the whole bus will be dead. $\endgroup$ Jan 30, 2022 at 22:27
  • 1
    $\begingroup$ @Jimmy The 5A breaker supplies power to the alternator field. Needing a little input power to make a lot of output power is a key difference between alternators and generators, and that’s what you seem to be misunderstanding. $\endgroup$
    – StephenS
    Jan 31, 2022 at 1:02
  • $\begingroup$ @Jimmy, Per the linked document, overvoltage protection is yet another element. Circuit breakers always break on overcurrent. $\endgroup$
    – Jan Hudec
    Jan 31, 2022 at 6:15

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .