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jpx(at)Qenesis.com
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 Posted: Sun Sep 21, 2008 12:30 pm Post subject: Z13/8 Aux Alternator Fuselink |
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There have been discussions here that the B&C SD-8 alternator can
perhaps put out as much as 10A under some conditions.
The wiring shown for it in Z13/8 is 14awg which matches the wire size
charts for 10A.
The Book indicates 4 awg sizes smaller for fuselinks, yet the fuselink
shown for the SD-8 alternator at the main battery contactor is only
20awg.
The same 14awg with 20awg fuselink combination is shown for the
connection from the Endurance bus to the Alternate Feed relay.
Should the fuselinks be 18 awg ?
Thanks !
Jeff Page
Dream Aircraft Tundra #10
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nuckolls.bob(at)cox.net
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| Posted: Sun Sep 21, 2008 7:13 pm Post subject: Z13/8 Aux Alternator Fuselink |
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At 04:28 PM 9/21/2008 -0400, you wrote:
| Quote: |
There have been discussions here that the B&C SD-8 alternator can
perhaps put out as much as 10A under some conditions.
The wiring shown for it in Z13/8 is 14awg which matches the wire size
charts for 10A.
|
Depending on the chart . . . 14AWG is good for 15A in
a 10C rise design . . . see below . . .
| Quote: | The Book indicates 4 awg sizes smaller for fuselinks, yet the fuselink
shown for the SD-8 alternator at the main battery contactor is only
20awg.
The same 14awg with 20awg fuselink combination is shown for the
connection from the Endurance bus to the Alternate Feed relay.
Should the fuselinks be 18 awg ?
|
No, current ratings for any given size of wire is
dependent upon the TEMPERATURE RISE you're willing
to tolerated along with VOLTAGE DROP you're willing
to tolerate. The wire tables like those shown in
http://aeroelectric.com/articles/Rev11/Ch8_R12.pdf
are crafted for a 10C rise above ambient.
In the case of using wire for fusible links, the
fusible part is short . . . so it contributes little
to overall voltage drop. We allow it to run a bit
hotter to keep the wire size as small as practical
(faster acting fuse for hard faults). In the cases
you cited, 20AWG is at least 4 AWG steps smaller than
14, in fact it's 6 steps for an even faster fusing
characteristic. Actually, you COULD use 22AWG for
this fusible link if you wanted. See:
http://aeroelectric.com/Pictures/Wire/22AWG_20A.pdf
In this picture, we see a length of 22AWG wire in
the foreground with a thermocouple attached. The
wire has been carrying 20 amps for the last 30
minutes or so (temperature has stabilized). Note
that the surface temperature of the wire is only
111C . . . we COULD run another 40C hotter without
pushing this wire's insulation. 10A would drop
the rise by about 1/2 down to about 85C. Even
better yet.
Fusible link fabrication is a special case that
ignores (but does not abuse) the rules we use
for sizing wire in bundles to carry power around
the airplane.
Bob . . .
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jpx(at)Qenesis.com
Guest
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| Posted: Mon Sep 22, 2008 6:53 am Post subject: Z13/8 Aux Alternator Fuselink |
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| Quote: | Depending on the chart . . . 14AWG is good for 15A in
a 10C rise design . . . see below . . .
|
I would have expected all the charts to be the same. Measuring
temperature rise versus current should be an easily repeatable
experiment. I suppose the result landed midway between practical
values, so some folks rounded up and others rounded down ?
| Quote: | In the case of using wire for fusible links, the
fusible part is short . . . so it contributes little
to overall voltage drop. We allow it to run a bit
hotter to keep the wire size as small as practical
(faster acting fuse for hard faults). In the cases
you cited, 20AWG is at least 4 AWG steps smaller than
14, in fact it's 6 steps for an even faster fusing
characteristic. Actually, you COULD use 22AWG for
this fusible link if you wanted. See:
|
The Connection suggests 4 awg smaller as a typical size for fuselinks.
Z13/8 is using 6 awg smaller. I understand the tradeoff between a
fuselink that blows quickly when needed versus temperature rise in
normal use. So is 6 awg smaller a better rule of thumb ? Or is there
a particular temperature rise that would be a good point to select
instead ?
Thanks !
Jeff Page
Dream Aircraft Tundra #10
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nuckolls.bob(at)cox.net
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| Posted: Mon Sep 22, 2008 8:16 am Post subject: Z13/8 Aux Alternator Fuselink |
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At 10:51 AM 9/22/2008 -0400, you wrote:
| Quote: |
> Depending on the chart . . . 14AWG is good for 15A in
> a 10C rise design . . . see below . . .
I would have expected all the charts to be the same. Measuring
temperature rise versus current should be an easily repeatable
experiment. I suppose the result landed midway between practical
values, so some folks rounded up and others rounded down ?
|
All charts ARE the same . . . i.e. based on some temperature rise
driven by design goals. In other words, all 10C charts will be
the same, 20C charts will agree, etc.
If you're interested in viewing the full constellation of all
charts, take a peek at the wire sizing exercises described in
http://www.aeroelectric.com/Reference_Docs/FAA/AC43.13-1B_Ch11_Electrical.pdf
| Quote: | > In the case of using wire for fusible links, the
> fusible part is short . . . so it contributes little
> to overall voltage drop. We allow it to run a bit
> hotter to keep the wire size as small as practical
> (faster acting fuse for hard faults). In the cases
> you cited, 20AWG is at least 4 AWG steps smaller than
> 14, in fact it's 6 steps for an even faster fusing
> characteristic. Actually, you COULD use 22AWG for
> this fusible link if you wanted. See:
The Connection suggests 4 awg smaller as a typical size for fuselinks.
Z13/8 is using 6 awg smaller. I understand the tradeoff between a
fuselink that blows quickly when needed versus temperature rise in
normal use. So is 6 awg smaller a better rule of thumb ? Or is there
a particular temperature rise that would be a good point to select
instead ?
|
Your thumb is different than my thumb. Not trying to be
obtuse here. I'm just pointing out the fact that there's a huge
range of combinations that (1) do not offer excessive
voltage drop - depending on your quantification of "excessive"
and (2) do not put the wire's insulation at risk due to
maximum anticipated continuous current, ambient temperature,
and relative insulation from ambient due to bundling.
I believe the major idea here is that it's probably not worth your
productive $time$ to worry about it much. If you're wanting
to acquire a more academic understanding, then the task before
you is significant. You can start with AC43-13 and I can offer
some other references. I made my recommendations knowing that
the combination suggested was well inside the edges of the
voltage-drop/temperature-rise envelope.
Bob . . .
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jpx(at)Qenesis.com
Guest
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| Posted: Wed Sep 24, 2008 5:54 am Post subject: Z13/8 Aux Alternator Fuselink |
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| Quote: | I believe the major idea here is that it's probably not worth your
productive $time$ to worry about it much. If you're wanting
to acquire a more academic understanding, then the task before
you is significant. You can start with AC43-13 and I can offer
some other references. I made my recommendations knowing that
the combination suggested was well inside the edges of the
voltage-drop/temperature-rise envelope.
|
I guess I am looking for some basic approaches so i can choose the
size of a fuselink. The wire charts easily allow me to select a wire
size for currents based on voltage drop and temperature rise. Is
there a chart for fuselinks ? AC43-13 doesn't seem to even mention
the word. The AeroElectric Connection says to use 4 agw smaller for
fuselinks, but this discourse indicates that is incorrect.
Continuing on in the theme, what about protection for the wire that
activates the battery contactor ? The diode will fail open, but what
about a short internal to the contactor due to, say, mechanical
failure ? There would be an unfused 4awg wire feeding huge currents
through the switched wire to ground. Although an unlikely
possiblilty, it would be ugly if it happened. What about a fuselink
on it ? What size ?
Thanks,
Jeff Page
Dream Aircraft Tundra #10
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nuckolls.bob(at)cox.net
Guest
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| Posted: Wed Sep 24, 2008 6:36 am Post subject: Z13/8 Aux Alternator Fuselink |
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At 09:51 AM 9/24/2008 -0400, you wrote:
| Quote: |
> I believe the major idea here is that it's probably not worth your
> productive $time$ to worry about it much. If you're wanting
> to acquire a more academic understanding, then the task before
> you is significant. You can start with AC43-13 and I can offer
> some other references. I made my recommendations knowing that
> the combination suggested was well inside the edges of the
> voltage-drop/temperature-rise envelope.
I guess I am looking for some basic approaches so i can choose the
size of a fuselink. The wire charts easily allow me to select a wire
size for currents based on voltage drop and temperature rise. Is
there a chart for fuselinks ? AC43-13 doesn't seem to even mention
the word. The AeroElectric Connection says to use 4 agw smaller for
fuselinks, but this discourse indicates that is incorrect.
|
And it shouldn't. You need to review the writings I've
posted on fusible links. I've cautioned that they are not
replacements for fuses or circuit breakers. They are
circuit protection with timing constants on the same order
as the ANL current limiters and similar devices used in
the automotive world. I do not recommend their use in any
locations and/or sizes not illustrated in the Z-figures.
You seldom see them incorporated into the Z-figures and there's
a reason for that . . .
| Quote: | Continuing on in the theme, what about protection for the wire that
activates the battery contactor ? The diode will fail open, but what
about a short internal to the contactor due to, say, mechanical
failure ? There would be an unfused 4awg wire feeding huge currents
through the switched wire to ground. Although an unlikely
possiblilty, it would be ugly if it happened. What about a fuselink
on it ? What size ?
|
The Z-figures are direct descendants of system architectures
that go back 70 years or more. They've been crafted to
illustrate architecture and protection philosophies that
have been filtered through the production history of hundreds
of thousands of airplanes. The fact that no "protection" is
shown on any given wire is not accidental. I've never seen
or heard of a coil suppression device failing in service. They
are VERY lightly stressed as-installed. I.e., not worthy of
additional concern for protection against failure.
Adding circuit protection where none is shown/needed/recommended
only increases parts count and drives system reliability down.
Bob . . .
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