Sender: <marv@lancaironline.net>
To: lml@lancaironline.net
Date: Mon, 19 Jun 2006 12:20:16 -0400
Message-ID: <redirect-1183514@logan.com>
From: "Tom Gourley" <tom.gourley@verizon.net>
Subject: Re: [LML] Re: Avionics transient
Reply-to: "Tom Gourley" <tom.gourley@verizon.net>
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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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