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I got the following response from the folks who are helping
me with the wiring of my 4.
This is what he said when I posed some of the issues
regarding the position and wiring of the hydraulic pack.
MDS
--------Mike,
--------I got a little wordy in my reply, but I figured
maybe
--------it might be helpful to others as well. Feel free to
share this
--------with the network. By the way, what group(s) are you
subscribing
--------to on he net? I'd like to tune in.
--------Jim
>>
The hydraulic pump and relays are behind the baggage
bulkhead on my L-235.
It's a very difficult place to get to, so make sure
everything is working
properly prior to installing it there.
--------It's not too bad on the L-4. A little awkward 'cause
you're working
--------through the baggage door and it's hard to get both
hands in there at
--------the same time. We'll make sure everything can be
un-bolted with
--------one hand. It would be easier if you're left handed.
In my case, I built a little "mule" cart with everything
mounted on it,
which I used to cycle the main gear prior to installing the
upper stub-wing
skins. So I knew the pressure switches were set properly
before installing
the unit in the plane. (My pressure switches are mounted on
the pump package.
Re: the 50A relay will in *no way* protect your pump motor
from burning up
if it stays on too long - it will only protect against a
dead short
somewhere. I have measured the current draw of those pumps,
and it is about
12A while the gear is being cycled, then spikes up to about
30A just as the
gear tightens up and stops. What I am saying is that if your
pump runs on,
it will just sit there and draw 10-15 Amps until it burns up
and fills the
cockpit with smoke. That 50A CB will not open under this
scenario.
-------Hmmm, not a pleasent thought. I guess that emphsizes
the importance of
-------having a good annunciator system. We have an amber
annunciator that
-------lights when the pump is on. We could rig a second
(red) one to come on
-------if the pump runs for more than 20 seconds or so. If
it hangs up,
-------just pull the breaker. Of course, using a good relay
will help.
-------According to FAA AC 43.13-1A section 2, switching
devices should be
-------de-rated by a factor of 4 when switching inductive
loads.
-------I am using a relay rated at 100 amps (400 A inrush).
It's made by
-------Stancor, P/N 124-903 available from Allied
electronics and probably
-------Grainger. They are used in electric forklifts and
such. About $35.
-------I'm using a 35 amp breaker and 10 gage on this one. I
used a 50 amp
-------on Steves L-4p (12 volt system) with 6 gage wire.
-------According to the pump manufacturer, the 24v pump will
draw 30 amps at
-------it's internal relief setting of 1400 psi and 25 amps
at the pressure
-------switch setting of 1100 psi. They said the 12 volt
pump draws 45 amps
-------at 1100 psi.
What ever rating you decide to use, the "disconnecting
means" must be
accessible. In my case, I used a 30-A automotive-type
"Maxi-Fuse" from
Digi-Key and located it next to the hydraulic bypass valve
where I can
reach it. I wrapped a tie-wrap around it and left the tail
sticking up to
make it easy to pull the fuse should I ever have to.
-------Why not just use a regular old circuit breaker on the
panel?
***Note on fuse rating. I found the 30-A fuse would blow
after about 5
rapid gear cycles, so I have a 40-A fuse installed right
now. I will
probably leave it that way for my test-flight period, then
switch back to
30-A eventually.***
I'm sure this will spark some controversy, but I wired my
hydraulic pump
*directly* to the battery through the 30-A fuse using #10
wire, and the
control relays to the master bus via a 1-A circuit breaker
mounted on the
panel next to the gear switch. My reasoning is as follows:
1. The fuse should *only* have to be pulled in case of a
confirmed
hydraulic pump run-on *due to a stuck relay*. (Unlikely,
since I used the
Bosch relays designed for inductive loads with MOVs across
the contacts.)
2. It kept those big, noisy, hi-current wires off of the
main buss behind
the panel. The hydraulic motor current doesn't pass thru the
master relay
either.
-------I wired Steve's "down stream" of the master relay,
but used a
-------separate feeder wire to minimize noise.
3. The pump can't come on with the master switch off, unless
a relay
contact fails closed. I suppose there is a "possiblity" that
a contact
could decide to weld itself shut while the plane was
unattended in the
hangar and the pump come on momentarily, but I can't think
of any physical
process that would cause this - earthquake? lightning
strike?. If I'm
missing something here, please let me know.
-------The only problem I can think of is that you have a
hot wire running
-------through the cockpit all the time. I believe part 23
and/or part 25
-------of the regs prohibits this and, although I'm not sure
of the exact
-------rationale, (fire hazard in case of crash?) it seems
reasonable.
-------Of course, we're not REQUIRED to follow those regs in
our homebuilts,
-------but it only seems prudent to give due consideration
to the decades
-------of experience they represent.
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