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rtitsworth
Joined: 16 Jan 2006
Posts: 76
Location: Detroit, Mi
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 Posted: Tue Feb 27, 2007 7:02 am Post subject: Z14 ground circuit question(s) |
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Bob, etal,
I'm building a composite aircraft (Lancair) with a Z-14 (dual alt, dual bat,
dual bus) with batteries in the tail. I originally decided that a shared
common ground wire back to the batteries was "reasonable safe" and an
appropriate compromise on a true dual electric system as there are few
likely failure modes for a well secured #2 Tefzel wire (the big ground
wire).
It "seems" the most likely potential failure is that one of the two end
attach points would fail (disconnect) - at either the large ring terminal
(crimped and soldered to the cable) or the nuts/studs used to connect them
to the ground bus (forest of tabs). While perhaps relatively unlikely, in
either of those failure scenarios, all my other dual bus redundancies are
for naught. To date, my only countermeasure for this risk was (attempted)
good workmanship. However, since neither ends is easily visible it seems I
might always be wondering what if(????).
Questions:
#1) Would it be appropriate to use a nylon lock nut or a steel lock nut to
secure the ground cable ring terminal to the ground stud(s)?
#2) Would it be reasonable to think about a second smaller parallel "backup"
ground wire which would be sized to only carry normal in-flight electrical
loads. Any suggestions on how to design/connect this so it does not try to
accept 1/2 of ground (return) current during engine start? Perhaps a
"backup ground" switch - normally left in the off position. Or perhaps a
settable breaker with one of the little red plastic rings holding it in the
off position.
#3) Would I be better off with two #5 ground cables simply wired in parallel
- ie. creating redundant "ends" for roughly the same current capacity and
weight. If so, how would I know if/when one failed (the smell of the other
during engine start?).
#4) Should I reconsider creating truly separate busses with independent
ground returns?
#5) I also got to looking at the single engine ground strap (ordered from B
& C). If it came loose, both alternators go dead together. I'd still have
~40 amp hours of combined batter juice (when new). However, I'm considering
two engine ground straps, which might also help further reduce any
electrical resistance there during staring.
#6) I'm planning on having a backup Airspeed, Altitude, and electric
Attitude indicator. If I put a small local battery on the attitude
indicator, then perhaps none of all the rest of the electrical equipment is
"essential" anyway. Bring a flashlight with fresh/backup batteries, a
handheld radio and perhaps a handheld GPS and forget all the above.
Sorry for the long message - Comments?
Rick
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klehman(at)albedo.net
Guest
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| Posted: Tue Feb 27, 2007 8:05 am Post subject: Z14 ground circuit question(s) |
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Hi Rick
FWIW I ran a #4 welding cable ground wire from each battery to its own
5/16" brass bolt on a forest of tabs ground plate on the firewall. Each
of those bolts has a ground strap to the engine and I think that two
ground straps is the most important thing to address your concerns.
Ground straps do fail. And of course flexible cable for battery
connections. My batteries are near the firewall so two ground wires was
not a big issue. Even though I only need one ground wire to crank the
engine I would expect to notice one electrical system starting to
fluctuate or something if its ground wire connections started to
degrade. With a long #4 ground wire from each battery I would also
expect to notice slower cranking as one cable won't be doing much, and
in my case one battery won't be doing much either.
Now that it's cold around here, I can report that my two little 8 AH
Dekka batteries crank my subaru just fine at -15C (5 F). One cranks it
briskly in warm weather but I haven't tried that in cold temps.
Ken
rtitsworth wrote:
| Quote: |
Bob, etal,
I'm building a composite aircraft (Lancair) with a Z-14 (dual alt, dual bat,
dual bus) with batteries in the tail. I originally decided that a shared
common ground wire back to the batteries was "reasonable safe" and an
appropriate compromise on a true dual electric system as there are few
likely failure modes for a well secured #2 Tefzel wire (the big ground
wire).
It "seems" the most likely potential failure is that one of the two end
attach points would fail (disconnect) - at either the large ring terminal
(crimped and soldered to the cable) or the nuts/studs used to connect them
to the ground bus (forest of tabs). While perhaps relatively unlikely, in
either of those failure scenarios, all my other dual bus redundancies are
for naught. To date, my only countermeasure for this risk was (attempted)
good workmanship. However, since neither ends is easily visible it seems I
might always be wondering what if(????).
Questions:
#1) Would it be appropriate to use a nylon lock nut or a steel lock nut to
secure the ground cable ring terminal to the ground stud(s)?
#2) Would it be reasonable to think about a second smaller parallel "backup"
ground wire which would be sized to only carry normal in-flight electrical
loads. Any suggestions on how to design/connect this so it does not try to
accept 1/2 of ground (return) current during engine start? Perhaps a
"backup ground" switch - normally left in the off position. Or perhaps a
settable breaker with one of the little red plastic rings holding it in the
off position.
#3) Would I be better off with two #5 ground cables simply wired in parallel
- ie. creating redundant "ends" for roughly the same current capacity and
weight. If so, how would I know if/when one failed (the smell of the other
during engine start?).
#4) Should I reconsider creating truly separate busses with independent
ground returns?
#5) I also got to looking at the single engine ground strap (ordered from B
& C). If it came loose, both alternators go dead together. I'd still have
~40 amp hours of combined batter juice (when new). However, I'm considering
two engine ground straps, which might also help further reduce any
electrical resistance there during staring.
#6) I'm planning on having a backup Airspeed, Altitude, and electric
Attitude indicator. If I put a small local battery on the attitude
indicator, then perhaps none of all the rest of the electrical equipment is
"essential" anyway. Bring a flashlight with fresh/backup batteries, a
handheld radio and perhaps a handheld GPS and forget all the above.
Sorry for the long message - Comments?
Rick
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