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icubob(at)newnorth.net
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 Posted: Sat Jun 28, 2008 1:24 pm Post subject: fuse placement |
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hi all . i did most of my wiring 2 yrs. ago and now i am putting everything back together for the last time. my main bus is powered by a wire from the hot side of my starter solenoid where the 4ga. wire from the battery connects. when i put this together i put the inline fuse [20-30 amps i dont remember] on the engine side of the firewall before the wire entered the cabin. this is a vfr airplane that i would have no intention of trying to replace this fuse in flight. i cant imagine it blowing except from a dead short of the wire that it protects. is there a reason, other than convenience, to move the fuse into the cabin while i am reconnecting my wiring? i dont expect it to ever blow.
thanks , bob noffs
[quote][b]
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rshannon
Joined: 05 Sep 2007
Posts: 62
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| Posted: Sat Jun 28, 2008 1:50 pm Post subject: fuse placement |
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Generally you want the fuse close to the supply end. If you're going to fuse the main bus feed, I'd say yours is right.
Ron
On Sat, Jun 28, 2008 at 2:19 PM, bob noffs <icubob(at)newnorth.net (icubob(at)newnorth.net)> wrote:
[quote] hi all . i did most of my wiring 2 yrs. ago and now i am putting everything back together for the last time. my main bus is powered by a wire from the hot side of my starter solenoid where the 4ga. wire from the battery connects. when i put this together i put the inline fuse [20-30 amps i dont remember] on the engine side of the firewall before the wire entered the cabin. this is a vfr airplane that i would have no intention of trying to replace this fuse in flight. i cant imagine it blowing except from a dead short of the wire that it protects. is there a reason, other than convenience, to move the fuse into the cabin while i am reconnecting my wiring? i dont expect it to ever blow.
thanks , bob noffs
[b]
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icubob(at)newnorth.net
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| Posted: Sun Jun 29, 2008 4:47 am Post subject: fuse placement |
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ron,
thanks for the feedback. i guess it was an obvious answer that i took into account 2 yrs ago and now didnt think about it as i was sorting thru my work. surprising how often i have to look at the 100 pics i took of the firewall in '06 before i can reassemble anything!
bob noffs
[quote][b]
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rshannon
Joined: 05 Sep 2007
Posts: 62
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| Posted: Sun Jun 29, 2008 6:03 am Post subject: fuse placement |
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Bob,
Well, at least you have the pictures. The trouble here is that when I'm taking pictures, I often think "Aha, this shot will show this or that brilliantly conceived detail item, and I'll write it up in the description right away." Of course, I don't get around to completing the caption, and a couple weeks later, I've no clue why I even took the picture. 
The answer to fuse location may not always be obvious though. The circuit should be fused "early" so that little if any of the wire is exposed to the possibility of live shorts (from impact, chafe, overload, etc.) occurring before the fuse, leaving a sparking wire end. For example, I have a roughly 6 ft. total run between a low current (10A) external ground power jack and the battery bus (through diode, OVP, etc.) The source of power on that circuit could be either end. Solution? A fuse on each end, one on the battery bus fuse block, and another inline fuse at the jack.
BTW, I was curious why you chose to fuse the main bus feed itself, presumably in addition to fuses/breakers on individual device circuits. The Z figure schematics don't fuse the main bus feed because it would be a single point of failure for everything, unless you also have an E-bus with alternate feed and even then, an EFIS or GPS on the E-bus would "reboot" before you turn on the E-bus alt feed (if they don't have independent backup batteries.) It's a trade off, of course. You have added some fire protection, but at the cost of a single point of failure, no?
Ron
On Sun, Jun 29, 2008 at 5:43 AM, bob noffs <icubob(at)newnorth.net (icubob(at)newnorth.net)> wrote:
[quote] ron,
thanks for the feedback. i guess it was an obvious answer that i took into account 2 yrs ago and now didnt think about it as i was sorting thru my work. surprising how often i have to look at the 100 pics i took of the firewall in '06 before i can reassemble anything!
bob noffs
[b]
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icubob(at)newnorth.net
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| Posted: Sun Jun 29, 2008 8:34 am Post subject: fuse placement |
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hi ron,
yes, i have a single point of failure for my main bus but i thought it more important to protect that fairly hefty wire from shorting. if i lose all elect. to my cabin i guess i will land in the next hour. a shorted wire would really limit my options. i fly only day vfr so this works for me.
i have hundreds of pics where i didnt even have a subject in mind. i just shot. every angle i can. then when i go back to my pics i usually get lucky and find just what i am looking for!
bob noffs
[quote] ---
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trigo(at)mail.telepac.pt
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| Posted: Sun Jun 29, 2008 10:36 am Post subject: fuse placement |
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hi bob
Is it your keyboard that lost its “shift” key, or do you really hate to hit it?
Couldn’t resist J
Carlos
Do not archive
To: aeroelectric-list(at)matronics.com
Subject: Re: AeroElectric-List: fuse placement
hi ron,
yes, i have a single point of failure for my main bus but i thought it more important to protect that fairly hefty wire from shorting. if i lose all elect. to my cabin i guess i will land in the next hour. a shorted wire would really limit my options. i fly only day vfr so this works for me.
i have hundreds of pics where i didnt even have a subject in mind. i just shot. every angle i can. then when i go back to my pics i usually get lucky and find just what i am looking for!
bob noffs
[quote][b]
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icubob(at)newnorth.net
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| Posted: Thu Jul 03, 2008 4:30 am Post subject: fuse placement |
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carlos, i type slow enough with 2 fingers as it is!
i broke my arm a few months ago but i find i can type as fast with my left finger as with my right finger!
bob noffs
[quote] ---
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nuckolls.bob(at)cox.net
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| Posted: Thu Jul 03, 2008 7:20 am Post subject: fuse placement |
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At 04:19 PM 6/28/2008 -0500, you wrote:
| Quote: | hi all . i did most of my wiring 2 yrs. ago and now i am putting
everything back together for the last time. my main bus is powered by a
wire from the hot side of my starter solenoid where the 4ga. wire from the
battery connects. when i put this together i put the inline fuse [20-30
amps i dont remember] on the engine side of the firewall before the wire
entered the cabin. this is a vfr airplane that i would have no intention
of trying to replace this fuse in flight. i cant imagine it blowing except
from a dead short of the wire that it protects. is there a reason, other
than convenience, to move the fuse into the cabin while i am reconnecting
my wiring? i dont expect it to ever blow.
|
Most other designers don't expect bus feeder fuses
to blow either . . . that's why they are left out
of most designs. Check the wiring diagrams for any
light TC aircraft . . . you'll find no fuses or
breakers in bus feeders. This is consistent with
the FARS, specifically:
-----------
Sec. 23.1357 Circuit protective devices.
(a) Protective devices, such as fuses or circuit breakers, must be
installed in all electrical circuits other than--
(1) Main circuits of starter motors used during starting only; and
(2) Circuits in which no hazard is presented by their omission.
(b) A protective device for a circuit essential to flight safety may not be
used to protect any other circuit.
(c) Each resettable circuit protective device ("trip free" device in which
the tripping mechanism cannot be overridden by the operating control) must be
designed so that--
(1) A manual operation is required to restore service after tripping; and
(2) If an overload or circuit fault exists, the device will open the
circuit regardless of the position of the operating control.
(d) If the ability to reset a circuit breaker or replace a fuse is
essential to safety in flight, that circuit breaker or fuse must be so
located and identified that it can be readily reset or replaced in flight.
(e) For fuses identified as replaceable in flight--
(1) There must be one spare of each rating or 50 percent spare fuses of
each rating, whichever is greater; and
(2) The spare fuse(s) must be readily accessible to any required pilot.
-----------
I'll direct your attention to paragraph (a)(2) which speaks to
battery feeders and feeders to main power distribution busses
that are typically the largest wires in the airplane and are
not subject to being "smoked" by downstream faults. You can
comfortably leave out the fuse you've cited.
Bob . . .
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