Today's Message Index:
----------------------
1. 05:18 AM - Re: RV10 Single Batt + Dual Alternator (supik)
2. 10:40 AM - Re: Re: RV10 Single Batt + Dual Alternator (Robert L. Nuckolls, III)
3. 10:53 AM - Re: Minimum length of fusible link wire segment (Pat Little)
4. 04:15 PM - Re: Minimum length of fusible link wire segment (Robert L. Nuckolls, III)
5. 05:59 PM - Re: RV10 Single Batt + Dual Alternator (supik)
Message 1
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| Subject: | Re: RV10 Single Batt + Dual Alternator |
Here is my calculation to avoid a situation when the AVIONICS ESS CB trips before
the equipment on this bus does. Taking care about the AVIONX ESS BUS CB in
series:
The total PEAK DRAW on the AVIONX BUS is 9.04 Amps.
The biggest Delta PEAK DRAW vs CB protection is on the GTN COM connector: 5.98
Amp
When I sum up the Total BUS Peak Draw (9.04A) + Top Delta Peak of the GTN COM connection
(assume this connection is shorted): TOTAL PEAK with fault on the GTN
COM side equals 15.02 Amp
Adding 20% reserve - I would need 18.8 Amp minimum CB rating for the AVIONX ESS
bus. Thus a 20Amp CB is enough.
Is my approach correct?
--------
Igor
RV10 in progress
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=497159#497159
Attachments:
http://forums.matronics.com//files/avionx_ess_bus_cb_117.jpg
Message 2
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| Subject: | Re: RV10 Single Batt + Dual Alternator |
At 07:16 AM 7/4/2020, you wrote:
>
>Here is my calculation to avoid a situation when the AVIONICS ESS CB
>trips before the equipment on this bus does. Taking care about the
>AVIONX ESS BUS CB in series:
>
>The total PEAK DRAW on the AVIONX BUS is 9.04 Amps.
>
>The biggest Delta PEAK DRAW vs CB protection is on the GTN COM
>connector: 5.98 Amp
>
>When I sum up the Total BUS Peak Draw (9.04A) + Top Delta Peak of
>the GTN COM connection (assume this connection is shorted): TOTAL
>PEAK with fault on the GTN COM side equals 15.02 Amp
>
>Adding 20% reserve - I would need 18.8 Amp minimum CB rating for the
>AVIONX ESS bus. Thus a 20Amp CB is enough.
>
>Is my approach correct?
Was unable to open your .jpg . . . "file not found"
you have circuit breakers in your distribution
bus feeders?
Bob . . .
Message 3
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| Subject: | Re: Minimum length of fusible link wire segment |
Thanks for this, Bob. That such a little blob of solder can have such a big
effect is fascinating.
Pat
On Tue, Jun 30, 2020 at 4:06 PM Robert L. Nuckolls, III <
nuckolls.bob@aeroelectric.com> wrote:
> At 07:18 PM 6/19/2020, you wrote:
>
> Bob, in a recent post you state that there is a 9-inch rule of
> thumb minimum length for fusible link wire segments. Could you
> explain the physics behind that, please?
>
> Pat
>
>
> Sorry to take so long on this . . . it took a few
> miles of 'asphalt engineering' to figure out how best
> to explain it. I THINK I've got a way . . . let's
> give it a try . . .
>
> Fusible link performance, indeed performance of any
> fusible circuit protective device is tightly bound
> to the fact that most electrical conductors have
> a positive temperature coefficient of resistance.
> This means, it's resistance rises with temperature
> of the conductor.
>
> The rate of temperature rises is a
> function of power (watts) dissipated in the
> conductor mass which is the product of current
> (amps) times resistance (ohms). Consider a piece
> of wire, any gage, hanging out in space with some
> current flowing through it.
>
> Here's a 22AWG wire that has been happily
> carrying 20A for some time. Note that I
> attached a thermocouple to the sample wire
> out in the middle of the rather significant
> free span. Suppose I conducted this experiment
> with, say a 1" piece of wire? How might we
> guess that center-span's rate-of-rise would
> differ for the two cases?
>
> http://aeroelectric.com/Pictures/Wire/22AWG_20A.pdf
>
> It's intuitive that heat generated in the
> wire is conducted to the ends. Rate of rise
> and temp equilibrium is influenced by sinking
> effects of the terminals and radiation into
> surrounding air. It's easy to deduce that a
> short piece of wire is more resistant to
> fusing than a longer one.
>
> As a practical matter, the opening of an
> electrically fusible segment tends to 'center up'
> on the span. This is seen in the pictures of fuses
> attached. As temperature begins to rise, heat
> is generated all across the span and conducted
> in both directions from any single point. The
> CENTER of the span is least able to reject heat
> to it's adjacent mass . . . it's getting warmed
> up from both directions and also rising the fastest
> due to effects of positive temperature coefficient.
>
> Rising resistance increases electrical power dissipated
> at that location more rapidly than anywhere else along
> the span. Hence the temperature rise is regenerative
> . . . the hotter it gets, the faster it warms up.
>
> How long does this take? Complex question depending
> on a constellation of conditions. Fuse and
> breaker manufacturers work diligently to achieve
> predictable performance in their products.
>
> Here's a well written piece that explains the
> thermal fiddling necessary to achieve predictable
> i.e. fast versus slow blowing characteristic in fuses.
> How about that little fly-spec of heat sink material
> in the slow blo cartridge fuse?
>
> https://tinyurl.com/ybkndmrc
>
> It easy to see that control of the environment
> surrounding the fusible event is critical.
>
> A fusible link is in the 'HULK' family
> of protective devices. Like its cousins,
> the ANL, MANL and similar 'current limiters',
> none are suited to the protection of the
> more pedestrian appliance feeders. The
> fusible link is SLOW and intended to clear
> major faults (HUNDREDS+ AMPS) in the system
> bus structure while maintaining a robust
> indifference to rather severe transient overload.
>
> Unlike our little plastic ATC friends that
> can be 'hammered' into lower operating
> currents by repeated excursions close to
> but still short of their ratings.
>
> Getting back to your question: Would a 6"
> piece of fusible link fail to function?
> How about a 1" piece? Probably not. They're
> still the weakest link in the faulted pathway.
>
> I am GUESSING that 9" figure is probably some
> fusible link designer's 'happy place' for repeating
> the in-service design goals he was working with.
> Or perhaps it's a legacy hand-over from an automotive
> industry specification.
>
> In any case, wanting to shorten 'em up a tad is
> not seriously significant to our task. My own
> 'happy place' would be 6". Hope this helps . . .
>
>
> Bob . . .
>
Message 4
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| Subject: | Re: Minimum length of fusible link wire segment |
At 12:48 PM 7/4/2020, you wrote:
>Thanks for this, Bob. That such a little blob of solder can have
>such a big effect is fascinating.
>Pat
Yeah . . . we don't know how 'big' big is but
intuitively we know that the effect is not
zero.
Similarly, the term SLOW BLOW for describing
fuse behavior is also non-quantified. Generally
speaking, we understand that selecting
a 'slow blow' version of the same kind of
fuse is something to try if we think that
we're seeing nuisance trips due to momentary
transients. Those transients may not be
very intense or long . . . perhaps just
repetitive. We studied the effects of
thermal 'hammering' in the 811HB
incident.
There are literally hundreds of variation
on the details of fuse behaviors. Our
favorite ATC plastics are fairly robust and
when used in automobiles, they are generally
waaayyy bigger than the normal current flow
in the circuit . . . perhaps 2 to 5x the
the normal current flow.
Check out the selection guide chart on
page 8 of this document:
https://tinyurl.com/ydfptnpu
This is a tiny fraction of functionality
to be considered when choosing circuit
protection TIGHTLY tailored to an
application.
Fuses in our cars and houses are NOT
tightly tailored to any application
beyond the protection of wires. For
low risk, convenient operation devoid
of nuisance trips we're encouraged to
take a similar approach to sizing wire
and circuit protection in our airplanes.
While boring holes in the sky, concerns
for keeping power on the far end of
your appliance feeders should occupy
no more of your thoughts than the size
of your prop bolts!
It would be interesting to see the I(squared)R
plots for that fuse with and without the
little glob in the middle. Still more interesting
to see the system requirements that were
satisfied by that final fuse design!
Bob . . .
Message 5
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| Subject: | Re: RV10 Single Batt + Dual Alternator |
nuckolls.bob(at)aeroelect wrote:
> At 07:16 AM 7/4/2020, you wrote:
>
> >
> > Here is my calculation to avoid a situation when the AVIONICS ESS CB trips
before the equipment on this bus does. Taking care about the AVIONX ESS BUS CB
in series:
> >
> > The total PEAK DRAW on the AVIONX BUS is 9.04 Amps.
> >
> > The biggest Delta PEAK DRAW vs CB protection is on the GTN COM connector:
5.98 Amp
> >
> > When I sum up the Total BUS Peak Draw (9.04A) + Top Delta Peak of the GTN
COM connection (assume this connection is shorted): TOTAL PEAK with fault on the
GTN COM side equals 15.02 Amp
> >
> > Adding 20% reserve - I would need 18.8 Amp minimum CB rating for the AVIONX
ESS bus. Thus a 20Amp CB is enough.
> >
> > Is my approach correct?
>
> Was unable to open your .jpg . . . "file not found"
>
> you have circuit breakers in your distribution
> bus feeders?
>
>
>
> Bob . . .
Affirm, all proection on the Avionics Buses are circuit breakers.
--------
Igor
RV10 in progress
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=497164#497164
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