nuckolls.bob(at)aeroelect
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 Posted: Fri Oct 02, 2015 7:45 am Post subject: Grounding Question On dual Buss Dual Battery Aircraft |
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At 09:29 AM 10/2/2015, you wrote:
| Quote: | --> AeroElectric-List message posted by: Kent or Jackie Ashton <kjashton(at)vnet.net>
Ditto on avoiding ground loops (a small difference in ground potential at different ground points that causes a small current loop). In my Cozy, I pulled was puzzled for years by a squirrelly fuel pressure indicator. The FP sender was grounded at the engine, FP gauge grounded up front. It is really a problem with gauging systems that use tiny currents. Probably not so much with big-amp devices. |
Actually, it was the BIG amp devices that generated
the difference in potential between what was expected
to be same-voltage-grounds.
This was a common problem in early LongEz ships
with no starter and a 30-40a alternator. The builder
RATED his ground return from crankcase to battery(-)
for ALTERNATOR currents . . . so 10A wire at 1 milliohm
per foot x 15ft long x 30A would generate as much as
1/2 volt difference between CRANKCASE ground and
BATTERY ground. Hence, engine grounded 'senders' would
produce strange and variable readings as the alternator
loads varied and only read correctly when the alternator
was off. Problem was still there . . . only much smaller
if the builder added a starter and the crankcase ground
was up-sized to 4 or 2AWG.
Seldom a problem on BIG airplanes. A Beechjet measures
about .001 ohm from stem to stern . . . enough to
experience significant differences in 'ground' potentials
during lighting strikes . . . but seldom an issue for
more mundane current flows circulating between system
grounds. This is why potential victims are grounded at
various locations about the airframe . . . thusly . . .
http://tinyurl.com/odvpk6z
. . . but the ground systems in smaller airplanes and
of course composite aircraft are not so robust. Hence
the need for artful grounding architectures.
Bob . . . [quote][b]
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