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The fuse arrangement Lancair uses, which I've been told was recommended by Bob Nuckolls, is a pretty good example of how to use fuses to protect your electrical system. Keep in mind that the main purpose of a fuse or circuit breaker is to isolate a failed device from the rest of the system. If an alternator fails you don't want it to drain your battery. Also, since batteries are capable of supplying large amounts of current (100's of Amps), not breaking the connection between the battery and a shorted alternator could easily result in melted wiring and an electrical fire. Using a fuse, instead of a breaker, on the alternator output makes sense because if your alternator fails shorted you really don't want to try to reconnect it to the system while in flight. As other folks have pointed out the alternator is a current limited device so the fuse isn't going to blow unless there's a catastrophic failure at the alternator.
In general fuses and breakers are there to protect the wiring in the event of a failure of a device. For example, the wiring from a panel mounted breaker to whatever that breaker is feeding is sized to handle the current rating of the breaker. If the downstream device fails such that it draws excessive current the breaker will open before the wire gets hot enough to melt the insulation.
A comment on fuse and breaker costs. Lancair (Aerocraft Part) has good prices on Klixon breakers. On the other hand they are outrageously high on ANL fuses at $12 ea. Stella-Maris (https://www.stellmar.com/) has them for $2.86 ea. They also have a lot of neat electrical accessories.
Tom Gourley
Legacy Builder
----- Original Message ----- From: "Marvin Kaye" <marv@lancaironline.net>
To: "Lancair Mailing List" <lml@lancaironline.net>
Sent: Saturday, June 17, 2006 10:54 PM
Subject: [LML] Re: Avionics transient
Posted for Gary Casey <glcasey@adelphia.net>:
In my opinion it is not advisable to put a pullable breaker in the
alternator output ("B") lead. And yes, if you pull that breaker with the
field being powered the alternator output voltage will go to a very high
level - certainly well over 100 volts (with the engine at high rpm). If the
voltage regulator over-voltage sense lead is connected to the alternator
output it presumably shut itself off, popping the alternator field breaker.
If not and if it is reconnected, though, it may or may not result in an
unacceptable system over-voltage, depending on the capacity of the battery to
absorb the sudden high current. I installed a fuse in the B lead in the
event that a stator winding gets shorted to ground. In the first scenario,
with an alternator charging into a dead battery the B lead breaker should
never blow. The alternator is a current-limited device and can never produce
a higher current than its rating (approximately - the ratings aren't exact).
If your friend had the breaker blow repeatedly it either had too low a
rating or the breaker itself was defective. I recommend a fuse with a rating
at least 10% higher than the rating of the alternator. 20% is better.
Gary Casey
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