The 5 amp field breaker in the 1976 Cessna 172M does NOT have the
alternator field current flow through it. It has only the field relay
coil current flow through it which is about 1/10 of an amp.
The alternator field current comes from the output terminal of the
alternator through the field relay and the switching transistor in the
regulator. There is NO fuse or breaker in this circuit unless you
call the 60 amp alternator breaker the field breaker.
Look it up, you will be surprised. The schematic of the regulator is
shown in the Cessna "Alternator charging Systems"Manual. The
airframe wiring is shown in the airframe service manual.
Cessna made a mistake in my opinion.
Ask your self what will happen if the 60 amp breaker opens under
nearly full load. Will the field switching transistor that normally
operates at 14 volts be able to take the 90+ volts without shorting?
Will the over voltage relay be able to break the 90+ volts DC on a
relay that has contact spacing designed to break 14 volts?
If the above two answers are no then you have just applied 90 volts or
more to the field winding and the regulator. I can tell you this it
does not take long to fry the regulator and the field into a crispy
critter once this occurs. I know for sure that if this happens just
after take off you will not be able to come back and land before the
regulator and the alternator is fried.
The 14 volt over voltage light is really bright with 90 volts applied,
but not for long!

When I was looking for the 60 amp breaker the largest I could find in
that case size was 50 amps so I had to buy the Cessna over priced
part. This circuit really needs a 70 amp breaker since the 60 amp
rated alternator will generate more than 60 amps when the windings are
cold.

You are correct that if the breaker popped during landing then there
may be something loose in the wiring or a sharp metal edge cutting
through a wire's insulation.
John Frerichs

On 4 Nov 2003 17:55:59 -0800, (Dan Thomas)
wrote:

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