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Hi Ian
Like many things in this "hobby" - it depends. I have two of everything and circuit breakers on those circuits. First, if you have the bad fortunate to have your "back up" fail after your primary failed - then whether fuses or Circuit Breakers - you are going down, so 1st establish your best glide speed {:>).
Second, if your primary fails and your backup still works then as you say find a close-by airpatch and set it down to see what is wrong - again, does matter whether fuse or circuit breaker.
Third, if you encounter a "temporary" surge of current (regardless of the cause) your fuse is going - but, you are likely going to be able to reset the circuit breaker. IF one reset of the circuit breaker does not stay (it pops again) then this not a temporary condition - don't push it in again or as you say you may be risking a fire. Hopefully your back up system will get you down safely.
Fourth for some reason people seem to think that fuses and fuse holders can not fail despite (for instance) Paul Conner telling of the number of times he has gone out on service calls only to find a fuse holder oxidized and causing a voltage drop. Only thing he has to do is pull out the fuse and reinsert it (scraping of the oxidation) and its working. And as Paul also pointed out the fuse marked he had for 20 amps had a filament more the size of a 7 1/2 amp fuse. My fuel pump draws around 6- 7 amps and I believe he has a similar one - which if the fuse filament was 7 1/2 amps explains why it ran for a while and then failed. So nothing is perfect or guaranteed {:>)
Here is a hypothetical scenario - lets say your alternator decides to go west in a failure mode that produces 40 volts (yes, there is an overvoltage failure that can produce voltages approach 60 -100 volts). Since you fuel pump will draw the basically the same current (perhaps a bit more with the higher voltage) the power across the fuse (Power = current * Voltage) will jump approx 4-5 fold. (It is the power rating rather than current that causes a fuse to blow by the way). You have just blown several fuses - could be critical ones. Your alternator has died or you have disabled it and you could make it on the battery - but, fuse is blown. Circuit breaker reset and one of your two redundant circuits (perhaps both) may work again. Just a hypothetical scenario.
But, again Ian - not trying to talk you out of fuses - if you feel comfortable with them then you save some $$ and a bit of weight.
SOoooo, the bottom line is go with whatever you feel comfortable with - like I have said - I have nothing against fuses - I used them in my aircraft for non-flight critical components. I sell neither fuses nor circuit breakers.
Best Regards
Ed
----- Original Message ----- From: "Ian Dewhirst" <idewhirst@dewhirst.ca>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Thursday, May 05, 2005 11:52 AM
Subject: [FlyRotary] Re: Mismarked Fuse??
Good Morning Ed,
I have read the contact articles you wrote, one of which detailed your
fault tolerant electrical system, I enjoyed reading them and integrated
several good ideas from you along the way. I agree that six CBs to
support critical flight systems are not going to break the bank or take up
most of the panel in my RV. I still fail to see the utility in CBs; if a
CB pops I expect that most everyone would switch to the backup system and
hit "go to nearest" on their GPS. I can here the reply from the list so
let's take that a step further, CB number two pops, what do you do? If
you push and hold CB number 1 and a fire starts you have a situation that
went from bad to unmanageable, in the immortal words that we read last
week "What are you going to do now?".
Someone recalled a story of a gear motor overheating and popping the CB,
there must be some other way to pump the gear down, why risk the reset,
it's not like you plugged in the vacuum with the kettle on and the breaker
blew at home, there is a good chance that there is a problem. The CB's I
am familiar with offer no safety valve while you are resetting them, push
and hold for a second could equal fire or equal fused contacts which could
equal fire, please correct me if I am wrong as it would likely change my
mind about CB's.
I suppose that you could get a cheap fuse and have a problem as Paul did,
same thing with a breaker. Switching to a backup system should solve the
problem without having to reset a breaker or replace a fuse.
Cheers -- Ian
Homepage: http://www.flyrotary.com/
Archive: http://lancaironline.net/lists/flyrotary/List.html
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