If you open the alternator output breaker while the alternator is
generating power then you get what is known as a "Load Dump".
this will generate a short inductive voltage spike of several hundred
volts followed by a steady +90 volts. This really stresses the diodes
in the alternator.
Most 12 and 24 volt circuit breakers that are used in GA aircraft are
not designed to open 90 volts DC. (There is a big difference between
the AC and DC voltage ratings of circuit breakers, switches and
fuses.) The breaker either burns up due to the constant arc, the
contacts melt and short or the breaker finally opens but the contact
surfaces are damaged.
The more current that is flowing when the breaker opens the worse the
result. Some part of this voltage will feed through the breaker while
it is opening and put a several hundred volt spike into your expensive
radios.

On my C172 when the old tired 60 amp alternator breaker opened one
night it burned up the alternator field, the regulator, the over
voltage warning light, and the over voltage relay. The alternator
stator and diodes were ok. This cost me about $500 because the old
tired 60 amp breaker would open at 35 amps after a few minutes.
Cessna screwed up when they wired the alternator and the regulator on
the "M" model but that is another story.
John



On 20 Oct 2003 16:53:18 -0400, (Frank Ch. Eigler)
wrote:


Hi -

My PA23-250 POH uses strong wording to insist that whenever the
engines are running, the alternator breakers should be on. This
contradicts some checklists for this plane (and those of a few other
single-engine Cessnas) that suggest that those breakers be open during
engine start and after shutdown. On the twin, one checklist includes
an alternator load check during runup that includes opening one then
the other breaker, to ensure that the electrical load can be carried
by either unit - that sounds valuable but appears to go against the POH.

Can someone offer an explanation of the relative wisdom of two
approaches?

- FChE