fzzzzt, popped alternator breaker C-172M

wrote in message ...
You need to replace the main 60 amp breaker. This is a thermal type
breaker and as it gets older the contacts build up oxidation and self
heat more reducing the breaker rating.
You are lucky!! When my 60 amp breaker popped one night the load
dump fried the alternator field, the voltage regulator and the over
voltage lamp.
I measured the 60A breaker and it would pop at 35 amps.
Cessna wants lots of money for the 60 amp breaker but it still is lots
cheaper than a new alternator and regulator.
On a C172M the 5 amp field breaker will NOT protect the alternator
field winding since it is not in series with the alternator field. It
protects the over voltage relay that draws only 0.1 amps.

The 60 amp breaker can be bought from someone other than
Cessna, like Aviall, for a lot less cash.
The 5 amp breaker in the 172M feeds the regulator and
therefore the field. We have two of these airplanes. The overvolt
sensor disrupts the feed if the voltage gets too high.
A good way to burn out an alternator field is to turn the
master on when preflighting, then forget to turn it off. The field is
being fed full voltage (since the alternator isn't producing anything)
and gets really hot (since the alternator isn't turning and drawing
cooling air through itself).
If the original poster's breaker popped during a landing, I
might expect to find some loose bit of hardware dancing around behind
the panel and tangling with the bus bar. Wouldn't be the first time.

Dan