ELIPPSE wrote:
To those of you who have a circuit breaker between your alternator's
output and your main buss, here's a wonderful way to fry your present
old avionics so you can replace them with new ones. This is a real help
in justifying new avionics to your significant other! If your
alternator field-circuit-breaker is connected to your main buss, then
if your alternator breaker opens, either through an overload or
manually, your main buss voltage will drop. The regulator, sensing
this, will increase the alternator's field current. 'Course, there is
no load on the alternator, so its voltage will rise. Eventually, in a
few milliseconds, the regulator will be pumping maximum current through
the field. Since we drive our alternators at 7000-9000 rpm, the
alternator will put out well in excess of 100 volts. Closing through
the alternator breaker will put this high voltage on your buss. Your
over-voltage protector, if present, will shut off the field supply, but
by the time the field collapses, the damage will have been done. Here's
two things you can do to prevent this and keep your old avionics. Any
time your alternator breaker opens, pull your field breaker before
restoring the alternator's breaker. If you don't want to depend on your
memory in a time of stress, rewire your field circuit breaker to the
alternator side of the alternator breaker. That way the alternator's
field will serve as the alternator load and the regulator will be
sensing the alternator output, thus keeping it in regulation. If you
don't think this scenario could actually take place, I challenge you to
do this test on your plane at cruise rpm with lots of avionics load and
prove me wrong!

This is known as a load dump. Disconnecting your battery from the
alternator can do the same thing. Transients can go as high as 150VDC.
Modern avionics systems are supposed to be designed to withstand this
scenario (per DO-160). If older stuff gets fried, I suspect it must
pre-date the DO-160 standard.