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vmenkal(at)mac.com
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 Posted: Thu Nov 17, 2011 8:30 pm Post subject: Main panel power supply fuse |
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Hi folks. Appreciate advice on following.
I am finishing up wiring a CH750 with a Rotax 912ULS and a composite design power panel. None of the "typical" wiring diagrams I have show a fuse on the main power supply from the battery (master solenoid) to the panel. I am using a 10 gauge tefzel feed cable which is about 5' long.
Is it not good practice to install a "mega fuse" as close to the master solenoid as possible to prevent fires and to protect this circuit in the event of a short? Ive calculated that a 60 amp fuse will be more than sufficient for all foreseeable loads.
Thanx Vic
Victor Menkal
CH750 Rotax 912ULS-2
Whitehorse, Yukon, Canada
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nuckolls.bob(at)aeroelect
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| Posted: Fri Nov 18, 2011 5:28 pm Post subject: Main panel power supply fuse |
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At 10:26 PM 11/17/2011, you wrote:
Hi folks. Appreciate advice on following.
I am finishing up wiring a CH750 with a Rotax 912ULS and a composite design power panel. None of the "typical" wiring diagrams I have show a fuse on the main power supply from the battery (master solenoid) to the panel. I am using a 10 gauge tefzel feed cable which is about 5' long.
Is it not good practice to install a "mega fuse" as close to the master solenoid as possible to prevent fires and to protect this circuit in the event of a short? Ive calculated that a 60 amp fuse will be more than sufficient for all foreseeable loads.
You won't find circuit protection for that
feeder in a Part 23/25 certificated airplane. The
rationale is illustrated by the following excerpt
from Part 23:
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.
The operative words that speak to your question are in Paragraph
(a)(2). Under what conditions would you expect this wire
to become overloaded to a degree that puts that wire at risk?
To open 60A protection in that feeder you would have to drag the
feeder itself or the bus to ground with a really hard fault. Hard
faults on a skillfully installed wire or bus structure required
a lot of damage to be done . . . like a crash. One assumes that
preparation for hard landing includes killing the master switch.
If the airplane is at risk for suffering that much damage with
no time for pre-landing preparation, post crash fire from hot
wiring is probably the least of your worries.
Bob . . . [quote][b]
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