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schu(at)schu.net
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 Posted: Wed Dec 30, 2009 11:46 pm Post subject: Questions about circuit protection. |
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List,
I'm finally closing in on completing my wiring diagram. I have the
basics roughed in but wanted to get some review on it and also ask some
specific questions about circuit protection:
1. Do my wire sizes look sane? I read though Bob's documentation on
wire sizes and I think I have it mostly figured out, but wanted to
double check.
2. I want to have an avionics master switch, but also an e-bus. Since
the only 3 things I'm running on my e-bus are avionics, I decided to
drive the ebus from the avionics bus though a diode. The goal is to be
able to turn on the avionics master, then the ebus alternate feed. If I
loose my master contactor or the avionics master switch, then my ebus
will continue to work. I think I have eliminated all single points of
failure while maintaining an avionics master. Can someone take a look
and make sure I'm not missing something obvious?
3. My drawing is missing all of the circuit protection except for the
ANL, and 5 amp breaker for the field coil. I want to add breakers for
the rest, but I don't think I need a breaker for each device as that
will get real expensive. Can I group up some of the lighting on a
single breaker? Also, what about using a breaker for the entire e-bus
then omitting the breakers for the devices on the ebus? The lines will
be real short and it eliminates the single point of failure (breaker.)
Also, what about fuselinks? I understand them to be like a fuse that
blows extremely slowly, but I'm not fully understanding when and where
to use them.
Any thoughts or suggestions would be super!!
schu
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nuckolls.bob(at)aeroelect
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| Posted: Fri Jan 01, 2010 5:31 pm Post subject: Questions about circuit protection. |
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At 01:38 AM 12/31/2009, you wrote:
List,
I'm finally closing in on completing my wiring diagram. I have the
basics roughed in but wanted to get some review on it and also ask some
specific questions about circuit protection:
1. Do my wire sizes look sane? I read though Bob's documentation on
wire sizes and I think I have it mostly figured out, but wanted to
double check.
Keep in mind that the "ratings" for how wire is used
in airplanes is conservative to the extreme. A wire
is not in danger of heating to copper-melting temperatures
should you exceed the "rating" by say TWICE or event 4X.
See: http://aeroelectric.com/Pictures/Wire/22AWG_20A.pdf
that 22AWG wire in the foreground has been carrying 20Amps
until the insulation temperature was seen to stabilize at
about 112C . . . the wires INSULATION is RATED for 150C.
So even at 4X the current we normally rate the wire for
in aircraft bundles, it's not in danger of smoking it's
insulation . . . and WAAaaaayyy too cold to melt the
wire.
Now, the voltage drop in this wire at 20A is terrible.
Unless the wire run were limited to a few inches, we'd
choose to upsize the wire if only for that reason.
2. I want to have an avionics master switch, but also an e-bus. Since
the only 3 things I'm running on my e-bus are avionics . . .
Why not simply add a switch in series with your
e-bus normal feed path diode and label it
"Avionics Master". Then ditch the avionics bus
and run your avionics of concern along with
endurance necessities from the e-bus.
I decided to
drive the ebus from the avionics bus though a diode. The goal is to be
able to turn on the avionics master, then the ebus alternate feed. If I
loose my master contactor or the avionics master switch, then my ebus
will continue to work. I think I have eliminated all single points of
failure while maintaining an avionics master. Can someone take a look
and make sure I'm not missing something obvious?
As suggested in my post of a few minutes ago, what operational
problems do you perceive with Z-11 as published?
3. My drawing is missing all of the circuit protection except for the
ANL, and 5 amp breaker for the field coil. I want to add breakers for
the rest, but I don't think I need a breaker for each device as that
will get real expensive.
As describe in
http://www.aeroelectric.com/articles/fuseorcb.html
http://www.aeroelectric.com/articles/fusvbkr2.html
Fuses are 1:1 interchangeable with breakers
for their intended purpose . . . keep
wires from burning and keep faults in one
feeder from propagating to other segments
of the whole system.
Why would you want one set of busses with
breakers and yet other busses with fuses?
Can I group up some of the lighting on a
single breaker? Also, what about using a breaker for the entire e-bus
then omitting the breakers for the devices on the ebus? The lines will
be real short and it eliminates the single point of failure (breaker.)
Also, what about fuselinks? I understand them to be like a fuse that
blows extremely slowly, but I'm not fully understanding when and where
to use them.
It's not clear that you've latched onto what
fuses and breakers are all about. I'll suggest
that you re-consider fuse blocks for ALL busses
and one fuse per accessory. Quick, light, easy
to install, and inexpensive.
Bob . . .
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schu(at)schu.net
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| Posted: Fri Jan 01, 2010 7:42 pm Post subject: Questions about circuit protection. |
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First, thank you very much for looking at my stuff Bob. I greatly
appreciate it and have donated to keep this list going...
Robert L. Nuckolls, III wrote:
| Quote: |
Keep in mind that the "ratings" for how wire is used
in airplanes is conservative to the extreme. A wire
is not in danger of heating to copper-melting temperatures
should you exceed the "rating" by say TWICE or event 4X.
|
Got it, I'll go back and thin some of it out then double check.
| Quote: | 2. I want to have an avionics master switch, but also an e-bus. Since
the only 3 things I'm running on my e-bus are avionics . . .
Why not simply add a switch in series with your
e-bus normal feed path diode and label it
"Avionics Master". Then ditch the avionics bus
and run your avionics of concern along with
endurance necessities from the e-bus.
|
It is my understanding that the purpose of the e-bus is to be able to
open the battery contactor and instantly have the system load drop to
whatever is on the e-bus making it easier and quicker for the pilot to
shed unneeded load in the event of an alternator failure.
To that end I put only required avionics on the e-bus and avionics that
I don't absolutely need to have on an avionics bus thinking that this
was in line with the design goals of using an e-bus.
| Quote: |
As suggested in my post of a few minutes ago, what operational
problems do you perceive with Z-11 as published?
|
I read the prior post and see your point about cut-and-pasting elements
of various diagrams together which results in a more complex electrical
system, but in the case of Z-13/8, the drawing lacks an avionics bus
which means that everything plugged into the e-bus or main bus will be
powered on when the master contactor closes, and will remain on while
the starter is operating.
My drawing mirrors Z-13/8 except for adding an avionics bus and a
switch, which adds complexity, but in return, my avionics can be powered
off during start, and turning off the avionics master but leaving the
e-bus on instantly sheds load that isn't absolutely necessary. This
change does not add a single point of failure for critical avionics,
however it does add a point of failure for non-critical avionics (switch.)
So my question is this: Given that simple is always
cheaper/lighter/more reliable, is it worth the weight/cost/complexity to
add a bus and a switch so that I can keep my avionics off during start,
and be able to instantly shed all non-critical loads? I thought it was,
but it seems like you disagree.
Also, what are others doing here? Are they simply using the built in
power switch to turn stuff off at start (if the component has a built in
switch.)
| Quote: | Can I group up some of the lighting on a
single breaker? Also, what about using a breaker for the entire e-bus
then omitting the breakers for the devices on the ebus? The lines will
be real short and it eliminates the single point of failure (breaker.)
Also, what about fuselinks? I understand them to be like a fuse that
blows extremely slowly, but I'm not fully understanding when and where
to use them.
It's not clear that you've latched onto what
fuses and breakers are all about. I'll suggest
that you re-consider fuse blocks for ALL busses
and one fuse per accessory. Quick, light, easy
to install, and inexpensive.
|
I know that fuses/breakers/fuselinks protect the wiring, what I was
missing was the part about problems with one component spreading to
others instead of being isolated. I think I will just get fuse blocks
per your recommendation for the reasons you mention, but also because
problems with a fuse can be remedied in the air quickly and easily.
Thanks again,
schu
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sat Jan 02, 2010 10:09 am Post subject: Questions about circuit protection. |
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| Quote: | >
> Why not simply add a switch in series with your
> e-bus normal feed path diode and label it
> "Avionics Master". Then ditch the avionics bus
> and run your avionics of concern along with
> endurance necessities from the e-bus.
It is my understanding that the purpose of the e-bus is to be able to
open the battery contactor and instantly have the system load drop to
whatever is on the e-bus making it easier and quicker for the pilot to
shed unneeded load in the event of an alternator failure.
|
No. The E-bus is where you power things that are
part of your Plan-B for sustained flight battery
only. YOU decide what the E-hours are. If you plan
to maintain the battery such that E-power-hours
is equal to or less than battery capacity, great.
If some smaller performance value meets your design
goals, great. But DECIDE what those design goals
are and craft the system to match.
| Quote: | To that end I put only required avionics on the e-bus and avionics that
I don't absolutely need to have on an avionics bus thinking that this
was in line with the design goals of using an e-bus.
|
Since you have subscribed to the ill-conceived notion
of an 'avionics bus' then the simple solution is
to COMBINED the functionality of the A-bus
with the E-Bus and add the switch in series with
the normal feedpath diode. You don't need to add
a special bus to coddle radios that don't need
coddling.
| Quote: | So my question is this: Given that simple is always
cheaper/lighter/more reliable, is it worth the weight/cost/complexity to
add a bus and a switch so that I can keep my avionics off during start,
and be able to instantly shed all non-critical loads? I thought it was,
but it seems like you disagree.
|
As described above, the only increase in complexity
to meet your design goals for an A-bus is make the
E-bus double up in that capacity. Alternatively,
you can abandon the legacy prophylactic for an
A-bus and associated master switch. See:
| Quote: | Also, what are others doing here? Are they simply using the built in
power switch to turn stuff off at start (if the component has a built in
switch.)
|
The avionics master switch was never really necessary
for the reasons imagined at the time it was created.
I remember. I was working as a tech writer at Cessna
when the AV master was conceived. I wrote sections
of maintenance manuals that spoke to the perceived hoards
of hazard lurked upon the bus waiting to pounce on
fragile radios.
See:
http://www.aeroelectric.com/articles/avmaster.pdf
http://www.aeroelectric.com/articles/spike.pdf
http://www.aeroelectric.com/articles/Philosophy/Whats_all_this_DO160_Stuff_Anyhow.pdf
Today, we know better. There's no value to be added
by dedicating a specially protected bus to power
radios.
| Quote: | I know that fuses/breakers/fuselinks protect the wiring, what I was
missing was the part about problems with one component spreading to
others instead of being isolated. I think I will just get fuse blocks
per your recommendation for the reasons you mention, but also because
problems with a fuse can be remedied in the air quickly and easily.
|
As Bob suggested in his earlier post, you might
like to review the philosophy of breakers vs.
fuses and the value of crafting a system where
there are no designed in nuisance trips of
fuses and no single accessory is "critical"
. . . i.e. a failure tolerant system
Fuse blocks can be tucked away out of sight,
out of reach and out of mind. You need only
ONE breaker and that only if you're using
crowbar ov protection. Otherwise, it can ALL
be out of reach fuses.
Bob . . .
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schu(at)schu.net
Guest
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| Posted: Sat Jan 02, 2010 11:12 am Post subject: Questions about circuit protection. |
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Robert L. Nuckolls, III wrote:
| Quote: |
Since you have subscribed to the ill-conceived notion
of an 'avionics bus' then the simple solution is
to COMBINED the functionality of the A-bus
with the E-Bus and add the switch in series with
the normal feedpath diode. You don't need to add
a special bus to coddle radios that don't need
coddling.
|
Thanks for taking the time to write all this out, I see where you are
coming from now.
schu
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tim2542(at)sbcglobal.net
Guest
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| Posted: Sat Jan 02, 2010 1:02 pm Post subject: Questions about circuit protection. |
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Hi Bob;
I think some of us may be dealing with conflicting information from two
respected sources, yourself and in some cases the manufacturers. Garmin and
Grand Rapids for example specify their equipment to be off during starter
engagement, the Grand Rapids units do not have an on/off switch and as we
have recently learned the GTX 327 may not really be off just because you
selected off, and the use of a "A" bus is mentioned in the install manual.
So in Matt's defense it may not be his following an "ill conceived notion"
as you mentioned, but a desire to serve two masters.
Tim Andres
--
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Mon Jan 04, 2010 10:49 am Post subject: Questions about circuit protection. |
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At 03:01 PM 1/2/2010, you wrote:
| Quote: |
Hi Bob;
I think some of us may be dealing with conflicting information from two
respected sources, yourself and in some cases the manufacturers. Garmin and
Grand Rapids for example specify their equipment to be off during starter
engagement, the Grand Rapids units do not have an on/off switch and as we
have recently learned the GTX 327 may not really be off just because you
selected off, and the use of a "A" bus is mentioned in the install manual.
So in Matt's defense it may not be his following an "ill conceived notion"
as you mentioned, but a desire to serve two masters.
|
It's an unfortunate condition of our culture that
so many exceedingly talented and capable suppliers
of products are so ignorant of the environment to
which they market.
There's a mountain of analysis, laboratory and field
testing that promulgated DO-160 and Mil-Std-704 along
with a host of other design guides for DC electrical
systems. At the same time, the "starter spike"
bug-a-boo is one of those deeply held beliefs that
is simply not supported by data.
Nevertheless, manufacturers of devices with
transistors in them seem to embrace some sort
of fragility in their own products after they
spent buckets of money to certify them into
the type-certificated aircraft environment.
I have designed dozens of products and put my
hands on hundreds more that were just as complex
and potentially 'fragile' as the panel mounted
radios . . . yet NONE of these manufacturers
suggest that the pilot pull a breaker on the
device while cranking the engine. Somehow, as
soon as the electro-whizzy is mounted to the
panel where the pilot can see it . . . it's
suddenly worthy of special protection from a
risk that (1) doesn't exist and (2) the product
has been demonstrated to withstand even if it
did exist.
I'll invite anyone on the list to contact the
manufacturer of any product where the operating
manual calls for turning it off during cranking.
Ask them to identify the waveform, magnitude
and duration of any cranking transient that
exceeds their DO-160 certification testing.
I've done this many times over 30 years. I've
never had a lucid defense of the idea. In a few
cases (King and Terra) the guy said, "yeah,
it's all B.S. . . . but we've been doing it
for decades and nobody wants to change it.
So I leave it up to you. If adding a switch to
the normal feed path diode makes you feel
better, by all means do it. It doesn't add
risk because the alternate feed path switch
backs it up.
Bob . . .
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schu(at)schu.net
Guest
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| Posted: Mon Jan 04, 2010 11:48 am Post subject: Questions about circuit protection. |
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Robert L. Nuckolls, III wrote:
| Quote: | I'll invite anyone on the list to contact the
manufacturer of any product where the operating
manual calls for turning it off during cranking.
Ask them to identify the waveform, magnitude
and duration of any cranking transient that
exceeds their DO-160 certification testing.
|
Are the experimental avionics from Advanced Flight Systems, TruTrak
Flight Systems, Dynon Avionics, and Grand Rapids certified to DO-160?
I'm running the AFS box and don't see anything in the manual about
starting, but I do see this:
"All aircraft must have protection diodes installed on their Master
Relay, Starter Relay and any other large relay. If your aircraft does
not have the protection diode on the Master Relay your electrical buss
will experience a voltage spike of 500+ Volts every time you turn off
the master switch. If your EFIS or Engine Monitor is wired directly to
the electrical buss it will be the device that absorbs the voltage spike
and will eventually fail. All users must verify that they have the
protection diodes installed before powering the EFIS or Engine Monitor."
schu
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Mon Jan 04, 2010 4:28 pm Post subject: Questions about circuit protection. |
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| Quote: |
Are the experimental avionics from Advanced Flight Systems, TruTrak
Flight Systems, Dynon Avionics, and Grand Rapids certified to DO-160?
|
Don't know if they're "certified" . . . i.e. have conducted
formal testing for which a report is produced.
In any case, they should certainly be DESIGNED to DO-160
limits and capabilities. First, because its a good thing
to do and secondly, because it's an easy thing to do.
| Quote: | I'm running the AFS box and don't see anything in the manual about
starting, but I do see this:
"All aircraft must have protection diodes installed on their Master
Relay, Starter Relay and any other large relay. If your aircraft does
not have the protection diode on the Master Relay your electrical buss
will experience a voltage spike of 500+ Volts every time you turn off
the master switch. If your EFIS or Engine Monitor is wired directly to
the electrical buss it will be the device that absorbs the voltage spike
and will eventually fail. All users must verify that they have the
protection diodes installed before powering the EFIS or Engine Monitor."
|
This is hogwash. When a relay's coil is un-suppressed, the
energy stored on the coil's magnetic core is dissipated
in the highest impedance portion of the loop. The bus
structure's impedance is a tiny fraction of that which
appears across the spreading contacts of the controlling
switch or relay. Further, consider that while the spike
from a coil collapse can be large, as an energy source
its ability to transfer energy is limited to the same
current that energizes the relay. For example, suppose
you DID have a 500v battery that was absolutely limited
to 1 amp of output current. Now, connect that battery
across the bus that's got several amps of load already
present along with a battery and several capacitors
inside sundry appliances. Folks like to cite that
500v spike without telling you that its current delivery
is limited to 1A. I.e. it's trivial to all devices except
the controlling switch.
This is stone simple to demonstrate in the lab, on an
airplane or in any dc powered vehicle. Who ever wrote
those words was simply repeating something they'd been
misled into writing.
Bob . . .
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